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Design of Reinforced Concrete Masonry Diaphragm Walls

February 2019

INTRODUCTION

Masonry is a versatile and robust structural system. The available variety of materials, shapes and strengths offers countless opportunities to create many types of masonry elements. Masonry’s versatility offers a continuous spectrum of systems from unreinforced to reinforced or post-tensioned. One example of such versatility is reinforced diaphragm walls. While not specifically mentioned in Building Code Requirements for Masonry Structures (ref. 1), reinforced diaphragm walls can be designed and constructed using criteria in that standard.

Diaphragm walls are cellular walls composed of two wythes of masonry with a large cavity or void; the wythes are bonded together with masonry ribs or crosswalls (see Figure 1). The ribs are connected to the wythes in such a way that the two wythes act compositely, thereby giving a fully composite section. This TEK covers the structural design of reinforced diaphragm walls. See TEK 03-15, Construction of Reinforced Concrete Masonry Diaphragm Walls, (ref. 2) for construction.

Figure 1 shows an example of a diaphragm wall constructed with concrete masonry units and its associated terminology. The reinforced wythes can be fully or partially grouted. The exterior face can be treated as the weathering side of the wall as shown in Figure 1, or a drainage cavity and anchored veneer can be used on the exterior face. The internal cavity (void) of the diaphragm wall is left open for utilities.

Figure 1—Typical Reinforced Diaphragm Wall

ADVANTAGES

Reinforced diaphragm walls present several opportunities for masonry design.

These include:

1. Diaphragm construction can efficiently create strong, stiff walls with individual units bonded together. Consider the economy of building a 24-in. (610-mm) thick wall with two 6 in. (152 mm) wythes and a 12 in. (305 mm) cavity rather than a solid 24 in. (610 mm) wall.

2. Thick diaphragm walls can be designed to span much further horizontally or vertically than single wythe walls or conventional composite walls. It is also possible to make very tall walls by virtue of the large sectional stiffness (ref. 3).

3. The greater thickness of diaphragm walls can also be used to replicate historic walls (buildings of Gothic style, monasteries, etc.) using modern methods.

4. The walls can have exposed, finished surfaces inside and out, and those finishes can be different because they are created by two individual wythes of masonry units.

5. The exterior wythe can be flashed and drained similar to the conventional back-up of an anchored veneer in cavity wall construction as detailed in TEK 19-05A, Flashing Details for Concrete Masonry Walls, or for single wythe walls per TEK 19-02B, Design for Dry SIngle-Wythe Concrete Masonry Walls, (refs. 4, 5).

6. The large interior voids allow for placement of insulation and utilities.

7. These walls can generate significant out-of-plane load capacity while supporting in-plane lateral loads.

8. The two distinct wythes provide a resilient system that can resist debris penetration from a high wind event and also provide great protection to potential blasts. With the high out-of-plane lateral load resistance, these walls can provide a good option for safe rooms or community rooms in tornado and hurricane regions.

HISTORICAL PERSPECTIVE

Unreinforced diaphragm walls have been used in Great Britain for decades. Many have been built using both concrete and clay masonry (Reference 3 provides wall diaphragm design criteria for concrete masonry assemblies used in Great Britain). The philosophy for unreinforced masonry in flexure is that the mortar controls the flexural tensile resistance and the composite of masonry and mortar controls both the compressive and shear stresses.

Valuable characteristics of unreinforced diaphragm walls are that the net section properties are easily calculated and they have a large moment of inertia. Given that they are thick, unreinforced diaphragm walls are effective at resisting out-of-plane loads and are inherently very stiff. However, unreinforced walls often crack before deflections control the performance. To further increase the bending resistance of unreinforced diaphragm walls, many walls in Great Britain have been posttensioned. The post-tensioning tendons are often placed in the void, unbounded and unrestrained, and protected from corrosion.

Unreinforced diaphragm walls have been used for sports halls, swimming pools, theaters, cinemas and other buildings that require tall walls. Other applications include tall freestanding walls, retaining walls, and replicating historical construction.

Figure 2 shows a fire station in Great Britain with posttensioned diaphragm sidewalls (arrows). These walls provide lateral stability for the building in both directions. As with traditional masonry buildings, the sidewalls are shear walls and resist loads acting on the front and rear of the building. In the transverse direction (plane of the overhead doors), the large openings leave short pier sections. Therefore, the diaphragm walls are designed to act as cantilever walls to provide the transverse building stability. This is a unique design solution because most masonry buildings do not depend upon the out-of-plane strength and stiffness of the walls to provide stability against lateral loads. Diaphragm walls, however, can be designed with sufficient thickness to develop the necessary out-of-plane strength and stiffness.

Figure 3 shows a cross-section of a bridge abutment and a photograph of the completed bridge where unreinforced post-tensioned brick diaphragm walls were used. Various bridges also use diaphragm walls for the cantilever wingwalls.

Unreinforced diaphragm walls have not been specifically addressed by name in codes and standards in the United States. Even the definition of a diaphragm wall does not exist. However, Building Code Requirements for Masonry Structures (the MSJC Code) includes design methodologies for unreinforced masonry using allowable stress design and strength design, as well as design criteria for composite assemblies. Therefore, unreinforced diaphragm walls can be designed using the existing standards, despite the fact that there is no specifically stated diaphragm wall criteria.

Figure 2—Fire Station With Diaphragm Wall (courtesy of Malcolm Phipps)

REINFORCED DIAPHRAGM WALLS

Even though unreinforced masonry is possible in areas of the United States, reinforced masonry is more widely adopted. Most regions require reinforcement for commercial masonry construction based upon the International Building Code (IBC) (ref. 6).

The MSJC provides design methodologies for reinforced masonry using allowable stress methods, post-tensioning, and strength design. These provisions can all be applied to reinforced concrete masonry diaphragm walls.

Design Detailing

Regardless of the design method utilized, there are some detailing criteria that apply equally to all reinforced diaphragm walls. These criteria are outlined below.

a) Spacing of Ribs

The ribs of the reinforced diaphragm wall act as webs for out-of-plane loads and connect the wythes structurally to create a composite section.

It is preferable that the ribs be spaced so that the flanges are fully effective in resisting applied loads. This is controlled by MSJC Section 5.1.1.2 which governs wall intersections. For reinforced walls where both flanges experience compression and tension, the MSJC requires the effective flange width on either side of the web to not exceed 6 times the flange thickness or 0.75 times the floor-to-floor height. In addition, the effective flange width must not extend past a control joint.

Therefore, the effective clear spacing between ribs is 12·t_wythe for walls without control joints (6·t_wythe from each rib), and the effective flange width is 12·t_wythe plus t_rib. Figure 4 illustrates how this effective flange width is smaller when a control joint is located at a rib. When placing a control joint between ribs, center the control joint and the effective flange remains 12·t_wythe plus t_rib.

b) Flange Thickness

The masonry unit selected for the flange wythe dictates the flange thickness (t_wythe). To accommodate reinforcement, a 6-in. (152-mm) concrete masonry unit is the smallest practical unit to be used. Larger units can be used to accommodate larger bars and provide larger compression areas.

c) Grouting

The choice of full vs. partial grouting is a function of design:

1. If the compression area required by out-of-plane
design exceeds the face shell thickness of the wythe,
the recommendation is to fully grout the flanges. Alternatively, the designer can use partial grouting and perform a T-beam analysis on the wall.

2. If the compression area does not exceed the face
shell thickness of the wythe, either partial or full
grouting can be used without using the more cumbersome T-beam analysis.

3. The ribs are often fully grouted, but they can also
be designed with partial grouting.

d) Masonry Bond

TMS 402 Section 5.1.1.2.1 requires that intersecting walls be constructed in running bond for composite flanging action to occur. Therefore, reinforced diaphragm walls are always constructed in running bond.

e) Connecting the Ribs to the Wythes

MSJC Section 5.1.1.2.5 requires that the connection of intersecting walls conform to one of the following requirements:

1. At least fifty percent of the masonry units at the interface
must interlock.

2. Walls must be anchored by steel connectors grouted into
the wall and meeting the following requirements:

(a) Minimum size: 1/4 in. x 1-1/2 in. x 28 in. (6.4 x 38.1 x711 mm) including a 2-in. (50.8-mm) long, 90-degree bend at each end to form a U or Z shape.
(b) Maximum spacing: 48 in. (1,219 mm).

3. Intersecting reinforced bond beams must be provided at a maximum spacing of 48 in. (1,219 mm) on center. The minimum area of reinforcement in each bond beam is 0.1 in.2 per ft (211 mm2/m) multiplied by the vertical spacing of the bond beams in feet (meters). Reinforcement is required to be developed on each side of the intersection.

The use of bond beams in requirement 3 above is one
way of handling the interface shear requirement. However,
the equations below can also be used for this purpose:
For allowable stress design:
f_v = V/A_n TMS 402 Section 8.3.5.1.1 (Eqn. 8-21)
where F_v is controlled by Section 8.3.5.1.2.
For strength design, the shear strength, f_v, is controlled by
Sections 8.3.5.1.2 and 8.3.5.1.4.

f) Control (Movement) Joints

CMU-TEC-009-23, Crack Control Strategies for Concrete Masonry Construction (ref. 7) are the industry standards for determining control joint spacing. Both were developed for single wythe walls with and without horizontal reinforcement.

There is no specific research on shrinkage characteristics of reinforced diaphragm walls. The expectation is that the ribs restrain shrinkage movement of the wythes and the resulting spacing of control joints can be increased over what would be expected for a single wythe wall. Until research becomes available, however, the current recommendation is to use the existing industry crack control recommendations to space control joints for reinforced diaphragm walls.

Additional attention must be placed on the size of the corner control joints if the diaphragm walls are used to support out-ofplane loads (see Example 1).

Allowable Stress Design of Reinforced Diaphragm Walls

Reinforced masonry designed using allowable stress design (ASD) methods follows similar guidelines as that used for unreinforced masonry. The maximum wall height is controlled by the loadings and slenderness effects. The slenderness effects are based upon the h/r ratio and prevent the wall from buckling.

The design methodology for reinforced diaphragm walls is similar to reinforced single wythe wall design and is discussed in TEK 14-07C, ASD of Concrete Masonry (2012 IBC & 2011 MSJC) (ref. 8).

Strength Design of Reinforced Diaphragm Walls

The strength design method has no specific limit on h/t. However, it has design criteria that limit service load deflections and ultimate moment capacity for out-of-plane loads. The service load deflections cannot exceed 0.7 percent of the wall height. For a 30-ft (9.1 m) wall, that is 2.5 in. over 30 ft (64 mm over 9.1 m) for a simply supported wall.

There is an axial load capacity limitation when h/t exceeds 30: the factored axial load for these walls must be limited to 5 percent of f’m based upon the gross section properties.

The design methodology is similar to single wythe design and is discussed in CMHA TEK 14-11B, Strength Design of CM Walls for Axial Load & Flexure (ref. 9)

Reinforced Concrete Masonry Diaphragm Walls Using Post-tensioned Masonry Design

Post-tensioned masonry design of diaphragm walls is the same as single wythe design. However, the large void in diaphragm walls provides an opportunity for the tendons to be placed eccentrically as needed for the loadings. Placed inside the void, the tendons are generally unbonded and unrestrained.

Seismic Design

The MSJC Code and ASCE 7 (refs. 1, 11) provide additional criteria for seismic design of walls that need to be considered as for any other masonry wall. This includes the degree of grouting and the inclusion of prescriptive reinforcement.

Figure 3—Cross-Section of Diaphragm Walls for Abutment and Completed Bridge (courtesy of Malcolm Phipps)

Figure 4—Effective Flange Width, beffective

DESIGN EXAMPLE: WINGWALL DESIGN FOR A REINFORCED CONCRETE MASONRY MAINTENANCE STORAGE FACILITY

Figure 5 shows the basic building layout for the design example. The front and rear walls are perforated with 20 ft x 20 ft (6.1 x 6.1 m) overhead doors for vehicle access. Control joints are shown over the door openings; the pier sections are 6 ft (1.8 m) in length. The endwalls have personnel access openings. Because the front and rear walls are perforated, the pier sections may not have sufficient in-plane stiffness and strength. Therefore, the endwalls should be designed to brace the building in both directions.

Although the roof structure is not shown, long-span joists bear on the front and rear sidewalls (i.e., the walls with the large perforations); the endwalls are nonloadbearing. The roof diaphragm would be designed to distribute the frontrear lateral loads to the endwalls, which must be designed as conventional shear walls. Conventional shear wall design is covered by the Masonry Designer’s Guide (ref. 12) and is not covered here.

The roof diaphragm will not be used to brace the side-toside lateral forces. For this example, the out-of-plane design (the large red arrow in Figure 5 depicts the out-of-plane load) will treat the endwalls as diaphragm walls acting as cantilevers to brace the building for the side-to-side lateral loads similar to Figure 2. This decision exempts the roof diaphragm from the strength and stiffness requirements for lateral loads that are perpendicular to the plane of the roof trusses. These requirements are typically met by horizontal braces between roof trusses.

Input:
Location: Coastal US, South Carolina
Loadings: ASCE 7-16, Part 2 for wind design
Masonry Standard: TMS 402, ASD method Because no bracing is used at the top of the wall, component and cladding loads will be used to design the wall.

1. Proposed wall section

Use 6-in. (152-mm) concrete masonry units for wythes and 8-in. (203-mm) for ribs (see Figure 1).

Masonry units: ASTM C90 (ref. 12), f’_m = 2,000 psi (13.8 MPa); unit weight 125 pcf (2,000 kg/m3)
Reinforcement: ASTM A615 (ref. 13), Grade 60
Grout: ASTM C476 (ref. 14), 2,000 psi (13.8 MPa)
Mortar: ASTM C270 (ref. 15), Type S

2. Select control joint spacing

The three possible options are:
a) Using CMU-TEC-009-23 (empirical method), space control joints at the lesser of 1.5h = 45 ft (13.7 m), max 25 ft (7.62 m). The 25 ft (7.62 m) criteria governs. The required horizontal reinforcement in the walls is 0.025 in.2/ft (CMUTEC-009-23, Table 1). This corresponds to two-wire W1.7 TEK 14-24 5 CONCRETE MASONRY & HARDSCAPES ASSOCIATION masonryandhardscapes.org (9 gauge, MW11) joint reinforcement at 16 in. (406 mm) on center vertically over the height of the wall (CMUTEC-009-23, Table 2).

b) Using CMU-TEC-009-23 (alternative engineered method), space control joints at the lesser of 2.5h = 75 ft or 25 ft (7.62 m). Again, the 25 ft (7.62 m) criteria governs. The required horizontal reinforcement in the walls is 0.0007An , which corresponds to 0.064 in2/ft or two-wire W1.7 (9 gauge, MW11) wire joint reinforcement at 24 in. (610 mm) on center vertically over the height of the wall (CMUTEC-009-23, Table 5).

c) Using CMU-TEC-009-23, space control joints at any length provided the horizontal reinforcement in the walls exceeds 0.002An (CMU-TEC-009-23). This corresponds to 0.183 in2/ft or two No. 6 (M#19) reinforcing bars in bond beams at 32 in. (813 mm) on center vertically over the height of the wall (CMU-TEC-009-23, Table 6 for fully grouted walls).

To minimize the possible number of control joints, select option c) with the horizontal bond beams. Provide control joints only at the corners (Figure 5). If the designer chooses to use horizontal joint reinforcement and not bond beams, the maximum control joint spacing would be 25 ft (7.62 m) using either options a) or b).

While the inner wythe will generally be exposed principally to shrinkage with only minor thermal effects, it is common to reinforce both wythes similarly.

3. Determine wind loads

From ASCE 7-16 Part 2, the suction load at the Exterior Zone (5) is calculated as 66.3 psf (3.17 kPa) (see Figure 6). In ASCE 7, wind loads are strength level. Roof dead load is ignored at the nonbearing wall.

4. Determine base of wall loads

V_u = 66.3 psf × 30 = 1,989 lb/ft of wall (29.0 kN/m)
M_u = 66.3 x (30)²/2= 29,835 ft-lb/ft of wall (132 kN-m/m)
V_ser = 0.6 V_u = 1,193 lb/ft of wall (17.4 kN/m)
M_ser = 0.6 M_u = 17,901 ft-lb/ft of wall (79.6 kN-m/m)
Note: 0.6 reduces V_u to ASD per ASCE 7.

5. Determine b_effective

in field of wall (solid region away from openings):
b_effective = 12t_wythe + t_rib = 12(6 in.) + 8 in. = 80 in. (2,032 mm)

6. Determine minimum t_wall to satisfy shear capacity

V_rib = V_ser × 80/12 = 7,953 lb (35.4 kN)
f_v = V_rib/A_rib = 7,953/[(7.63 in.)·t_wall] (TMS 402, Equation 8-21)
F_v ≤ 2 √f’_m γg = 89 psi, assuming M/Vd > 1.0 and γg = 1.0 (MSJC Code, Equation 8-24)
This produces t_wall ≥ 11.7 in. (297 mm)
Checking M/Vd = 17,901/[1,193 x (<1 ft)] = 15.0 > 1.0 OK
Shear is not an issue. The prescriptive requirements for the
intersection of the ribs and flanges are sufficient.

7.Determine minimum t_wall due to moment capacity

Try a rib length of 1.5 courses of concrete masonry.
t_wall = 15.625 in. unit + 0.375 in. mortar joint + 7.625 in. half
unit = 23.63 in. (600 mm)
d = 23.63 in. – (5.63 in./2) = 20.82 in. (529 mm)
Ignoring axial load,
A_s (estimated) = M_ser /(2.16d)
= (17,901/1,000)(2.16 x 20.82 in)(529 Mm)
Try No. 8 at 24 in. o.c. (As = 0.40 in.2/ft) (M#25 at 610 mm o.c.)

8. Determine wall dead load at base of wall

From CMU-TEC-002-23 (ref. 16): wall weight of 125 pcf 6 in. fully grouted concrete masonry = 62 psf (303 kg/m2 )

125 pcf 8 in. fully grouted = 84 psf (411 kg/m² )
Flange load: 2 wythes x 62 psf = 124 psf per ft
Rib load: [23.63 in. – 2(5.63 in.)]/12 x 84 psf/80 in./12 = 13.0 psf/ft of wall

P_DL = (124 + 13.0) x 30 ft = 4,110 lb/ft of wall (60 kN/m)

9. Load combination

0.6 P_DL + 0.6W from ASCE 7-10 for ASD

Note: This one load combination is shown for this example. The designer must check all combinations required by ASCE 7.
P = 0.6P_DL = 0.6 (4,110) = 2,466 lb/ft (36 kN/m)
M = 0.6 M_u, _wind = M_ser = 17,901 ft-lb/ft (79.6 kN-m/m)

10. Determine n

From MSJC Section 4.2.2:
E_s = 29,000,000 psi (200,000 MPa)
E_m = 900f’m = 1,800,000 psi (12,410 MPa) n = Es/Em = 16.1
For A_s = 0. 44 in.² /ft (from 7 above),

nρ = nAs /bd = 16.1(0.4)/12(20.82) = 0.026
If P = 0, k = √ (nρ)2 + 2nρ – nρ = 0.204;
j = 1- (k/3) = 0.932

kd = 4.25 > t_face of 6-in. CMU but less than the wythe thickness. Axial load may increase kd. Therefore, grouting the full
wythe is appropriate.

11. Design for P_DL and M

(see Figure 7)
From statics: P = C – T
M = C x e_m + T (d – t_wall/2)
Per foot: C = 1/2(kd)f_m x 12 in.
f_m = E_m ε_m
T = A_s f_s
f_s = E_s ε_s
e_m = t_wall/2 – kd/3

From strain compatibility: εm/kd = εs(d – kd)
(f_m /Em)/kd = (f_s/E_s)/(d – kd) → f_s = n [(d – kd)/kd] f_m
Therefore, C = 6(kd)f_m
T = 0.4(16.1)((20.82 – kd)/kd)) fb
= 6.44((20.82 – kd)/(kd)) fb
Solving for P = C – T and M = C em + T (d – t_wall/2)
gives kd = 4.65 in. (118 mm) and fb = 498 psi (3.4 MPa)
Checking:
C = 12,449 lb (55 kN)
T = 10,030 lb (44 kN)
P = 2,419 lb (10.7 kN) OK
e_m = t_wall/2 – kd/3 = 10.27 in. (264 mm)
M = C e_m + T (d – t_wall/2)
= 12,449(10.27)/12 + 10,030(20.82 – 23.63/2)/12
= 18,185 ft-lb approx. = M =17,901 ft-lb OK
Check:
f_m = 498 psi < F_b = 0.45 f’_m = 900 psi (6.2 MPa)
OK (TMS 402 8.3.4.2.2)
f_s = 16.1((20.82 – 4.86)/4.86) 417 psi = 22,047 psi (152 MPa)
f_s < F_s = 32,000 psi (221 MPa) OK (MSJC 8.3.3.1)

MSJC Section 8.3.4.2.2 requires an additional check for fa alone. The design engineer is generally advised to perform this check. However, it rarely controls for diaphragm walls due to the stiff wall section. For this example, there is no applied axial load so the check is not required.

Therefore, this section checks using No. 8 bars at 24 in. on center (M#25 at 610 mm) in a fully grouted diaphragm wall. Note that this only applies to the end zone in suction. The design calculations should be repeated:

a. for pressure load on the end zone,

b. for pressure and suction over the interior zone,

c. over the height of the wall to reduce the amount of vertical reinforcement, and

d. the design should be checked adjacent to control joints and openings.

Using the walls to support of out-of-plane loads requires the foundations to be designed and detailed for the cantilever walls.

12. Check deflection at top of the wall for a cantilever

Using loads and section properties for b_effective.

Provide the control joints between the sidewalls and the front/ rear walls. Construct with sealant that has a shear capacity of 50% of the joint thickness, the joint thickness should exceed 2 x 0.56 in. = 1.12 in. (28 mm). See white arrow on Figure 5.

Figure 5—Maintenance Facility for Design Example 1

SUMMARY

Reinforced concrete masonry diaphragm walls provide opportunities for engineers to design a) very tall walls and b) brace walls using the diaphragm walls as cantilevers. For buildings, these are two unique options that are not normally available from traditional masonry walls.

NOTATIONS

A_n = net cross-sectional area of a member, in.² (mm²)
A_s = area of nonprestressed longitudinal tension reinforcement, in.²(mm²)
b = width of section, in. (mm)
b_effective = effective width of section, in. (mm)
C = resultant compressive force, lb (N)
c = distance from the fiber of maximum compressive
strain to the neutral axis, in. (mm)
d = distance from extreme compression fiber to centroid of tension reinforcement, in. (mm)
E_m = modulus of elasticity of masonry in compression, psi (MPa)
E_s = modulus of elasticity of steel, psi (MPa)
e_m = eccentricity of axial load, in. (mm)
F_m = allowable compressive stress, psi (MPa)
f_m = calculated compressive stress in masonry due to axial and flexure, psi (MPa)
F_v = allowable shear stress, psi (MPa)
F_s = allowable tensile or compressive stress in reinforcement, psi (MPa)
f_a = calculated compressive stress in masonry due to axial load only, psi (MPa)
f’_m = specified compressive strength of clay masonry or concrete masonry, psi (MPa)
f_r = modulus of rupture, psi (MPa)
f_s = calculated tensile or compressive stress in reinforcement, psi (MPa)
f_v = calculated shear stress in masonry, psi (MPa)
f_y = specified yield strength of steel for reinforcement and anchors, psi (MPa)
h = effective height of wall, in. (mm)
I_cr = moment of inertia of cracked cross-sectional area of a member, in ⁴ (mm⁴)
I_g = moment of inertia of gross cross-sectional area of a member,, in.⁴ (mm⁴)
j = ratio of distance between centroid of flexural compressive forces and centroid of tensile forces to
depth, d
k = ratio of the distance between the compression face of an element and the neutral axis to the effective depth d
M = maximum moment at the section under consideration, in.-lb (N-mm)
M_cr = nominal cracking moment strength, in.-lb (N-mm)
M_ser = service moment at midheight of a member, in.-lb (N-mm)
M_u = factored moment, magnified by second-order effects where required by the code, in.-lb (N-mm)
n = modular ratio, E_s/E_m
P = axial load, lb (N)
P_DL = axial load due to dead load, lb (N)
P_u = factored axial load, lb (N)
r = radius of gyration, in. (mm)
Sg = section modulus of the gross cross-sectional area
of a member, in.³(mm³)
T = resultant tensile force, lb (N)
t = nominal thickness of member, in. (mm)
t_face = specified thickness of masonry unit faceshell, in. (mm)
t_rib = specified thickness of diaphragm wall rib, in. (mm)
t_sp = specified thickness of member, in. (mm)
t_wall = specified thickness of wall, in. (mm)
t_wythe = specified thickness of the masonry wythe, in. (mm)
V = shear force, lb (N)
Vrib = shear capacity (resisting shear) of diaphragm wall rib, lb (N)
Vser = service level shear force, lb (N)
Vu = factored shear force, lb (N)
W = wind load, psf (kPa)
γ_g = grouted shear wall factor
δ = moment magnification factor
ε_m = compressive strain of masonry
ε_s = strain of steel
f = strength reduction factor
ρ = reinforcement ratio

References

Building Code Requirements for Masonry Structures, TMS 402-16, Reported by The Masonry Society 2016.
Construction of Reinforced Concrete Masonry Diaphragm Walls, TEK 03-15, Concrete Masonry & Hardscapes Association, 2017.
Aggregate Concrete Blocks: Unreinforced Masonry Diaphragm Walls, Data Sheet 10. Concrete Block Association of Great
Britain, March 2003.
Flashing Details for Concrete Masonry Walls, TEK 19-05A, Concrete Masonry & Hardscapes Association, 2008.
Design for Dry Single-Wythe Concrete Masonry Walls, TEK 19-02B, Concrete Masonry & Hardscapes Association, 2012.
International Building Code. International Code Council, 2015/2018.
Crack Control Strategies for Concrete Masonry Construction, CMU-TEC-009-23, Concrete Masonry & Hardscapes Association, 2023.
TEK 14-07C, ASD of Concrete Masonry (2012 IBC & 2011 MSJC). Concrete Masonry & Hardscapes Association, 2013.
TEK 14-11B, Strength Design of CM Walls for Axial Load & Flexure. Concrete Masonry & Hardscapes Association, 2003.
Minimum Design Loads for Buildings and Other Structures, ASCE/SEI 7-10. American Society of Civil Engineers, 2010.
Masonry Designers’ Guide, Seventh Edition, MDG-7. The Masonry Society, 2013.
Standard Specification for Loadbearing Concrete Masonry Units, ASTM C90-14. ASTM International, Inc., 2014.
Standard Specification for Deformed and Plain CarbonSteel Bars for Concrete Reinforcement, ASTM A615/ A615M-14. ASTM International, Inc., 2014.
Standard Specification for Grout for Masonry, ASTM C476-10. ASTM International, Inc., 2010.
Standard Specification for Mortar for Unit Masonry ASTM C270-14. ASTM International, Inc., 2014.
Weights and Section Properties of Concrete Masonry Assemblies, CMU-TEC-002-23.Concrete Masonry & Hardscapes Association, 2023.

Design and Construction of Dry-Stack Masonry Walls

August 2018

INTRODUCTION

Construction of masonry wall systems is possible without the use of mortar. The use of standard CMU units laid dry and subsequently surface bonded with fiber reinforced surfaced bonding cement has been well documented in the past. (ref. 16) With the use of specially fabricated concrete masonry units known as “dry-stack units,” construction of these mortarless systems is simple, easy and cost effective. This TEK describes the construction and engineering design of such mortarless wall systems.

The provisions of this TEK apply to both specialty units manufactured specifically for dry-stack construction and conventional concrete masonry units with the following system types:

Grouted, partially grouted or surface bonded
Unreinforced, reinforced, or prestressed

Note that dry-stacked prestressed systems are available that do not contain grout or surface bonding. The provisions of this TEK do not apply to such systems due to a difference in design section properties (ref 8).

Specially designed units for dry-stack construction are available in many different configurations as shown in Figure 1. The latest and most sophisticated designs incorporate face shell alignment features that make units easier and faster to stack plumb and level. Other units are fabricated with a combination of keys, tabs or slots along both horizontal and vertical faces as shown in Figure 1 so that they may interlock easily when placed. Physical tolerances of dry-stack concrete units are limited to ±¹/₁₆ in. (1.58 mm.) which precludes the need for mortaring, grinding of face shell surfaces or shimming to even out courses during construction. Interlocking units placed in running bond resist flexural and shear stresses resulting from out-of-plane loads as a result of the keying action: (a) at the top of a web with the recess in the web of the unit above, (b) at two levels of bearing surface along each face shell at the bed joint, and (c) between adjacent blocks along the head joint. The first of these two interlocking mechanisms also ensures vertical alignment of blocks.

The interlocking features of dry-stack units improve alignment and leveling, reduce the need for skilled labor and reduce construction time. Floor and roof systems can be supported by mortarless walls with a bond beam at the top of the wall which expedites the construction process.

Wall strength and stability are greatly enhanced with grouting which provides the necessary integrity to resist forces applied parallel, and transverse to, the wall plane. Vertical alignment of webs ensures a continuous grout column even when the adjacent cell is left ungrouted. Grouting is necessary to develop flexural tensile stress normal to the bed joints, which is resisted through unit-mortar bond for traditional masonry construction. Strength of grouted dry-stack walls may also be enhanced by traditional reinforcement, prestressing, post-tensioning or with external fiber-reinforced surface coatings (surface bonding) as described in the next section.

Typical applications for mortarless concrete masonry include basement walls, foundation walls, retaining walls, exterior above-grade walls, internal bearing walls and partitions. Dry-stack masonry construction can prove to be a cost-effective solution for residential and low-rise commercial applications because of it’s speed and ease of construction, strength and stability even in zones of moderate and high seismicity. More information on design and construction of dry-stack masonry can be found in Reference 5.

CONSTRUCTION

Dry-stack concrete masonry units can be used to construct walls that are grouted or partially grouted; unreinforced, reinforced or prestressed; or surface bonded. With each construction type, walls are built by first stacking concrete masonry units.

For unreinforced construction as shown in Figure 2a, grouting provides flexural and shear strength to a wall system. Flexural tensile stresses due to out-of-plane bending are resisted by the grout cores. Grout cores also interlace units placed in running bond and thus provide resistance to in-plane shear forces beyond that provided by friction developed along horizontal joints. Grout cores can also be reinforced to increase flexural strength.

Reinforcement can be placed vertically, in which case only those cells containing reinforcement may be grouted as shown in Figure 2b, as well as horizontally, in which case the masonry must be fully grouted. Another version is to place vertical prestressing tendons in place of reinforcement. Vertical axial compressive stress, applied via the tendons, increases flexural and shear capacity. Tendons may be bonded to grout, or unbonded, based upon the design. Placement of grout may be optional. Horizontally reinforced bond beam lintels can be created using a grout stop beneath the unit to contain grout.

As an alternative to reinforcing or prestressing, wall surfaces may be parged (coated) with a fiber-reinforced surface bonding cement/stucco per ASTM C887(ref. 14) as illustrated in Figure 2c. This surface treatment, applied to both faces of a wall, bonds concrete units together without the need for grout or internal reinforcement. The parging material bridges the units and fills the joints between units to provide additional bonding of the coating to the units through keying action. The compressive strength of the parging material should be equal to or greater than that of the masonry units.

Figure 2–– Basic Dry-Stack Masonry Wall Types

Laying of Units

The first course of dry-stack block should be placed on a smooth, level bearing surface of proper size and strength to ensure a plumb and stable wall. Minor roughness and variations in level can be corrected by setting the first course in mortar. Blocks should be laid in running bond such that cells will be aligned vertically.

Grout and Reinforcement

Grout and grouting procedures should be the same as used in conventional masonry construction (ref. 1, 10) except that the grout must have a compressive strength of at least 2600 psi (190 MPa) at 28 days when tested in accordance with ASTM C 1019 (ref.12). Placement of grout can be accomplished in one lift for single-story height walls less than 8 ft (2.43 m). Grout lifts must be consolidated with an internal vibrator with a head size less than 1 in. (25 mm).

Vertical Reinforcing

As for conventional reinforced masonry construction, good construction practice should include placement of reinforcing bars around door and window openings, at the ends, top and bottom of a wall, and between intersecting walls. Well detailed reinforcement such as this can help enhance nonlinear deformation capacity, or ductility, of masonry walls in building systems subjected to earthquake loadings – even for walls designed as unreinforced elements. Additional information on conventional grouting and reinforced masonry wall can be found in TEK 09-04A and TEK 03-03B (refs. 9 & 6).

Pre-stressed Walls

Mortarless walls can also be prestressed by placing vertical tendons through the cores. Tendons can be anchored within the concrete foundation at the base of a wall or in a bottom bond beam and are tensioned from the top of a wall.

Surface Bonded Walls

For walls strengthened with a surface bonding, a thin layer of portland cement surface bonding material should be troweled or sprayed on to a wall surface. The thickness of the surface coating should be at least ⅛ in. (3.2 mm.) or as required by the material supplier.

ENGINEERING PROPERTIES

Walls constructed with mortarless masonry can be engineered using conventional engineering principles. Existing building code recommendations such as that produced by the building code (ref. 1) can serve as reference documents, but at the time of this printing it does not address mortarless masonry directly. It is thus considered an alternate engineered construction type. The International Building Code (ref. 7) does list allowable stresses based on gross-cross-sectional area for dry-stacked, surface-bonded concrete masonry walls. These values are the same as presented in TEK 03-05A (ref. 16). Suggested limits on wall or building height are given in Table 1.

Test data (refs. 2, 3 and 4) have shown that the strength of drystack walls exceeds the strength requirements of conventional masonry, and thus the recommended allowable stress design practices of the code can be used in most cases. When designing unreinforced, grouted masonry wall sections, it is important to deduct the thickness of the tension side face shell when determining the section properties for flexural resistance.

Table 1 –– Summary of Wall Heights for 8” (203 mm) Dry-stacked Units (ref. 5)

* Laterally supported at each floor

Unit and Masonry Compressive Strength

Units used for mortarless masonry construction are made of the same concrete mixes as used for conventional masonry units. Thus, compressive strength of typical units could vary between 2000 psi (13.79MPa) and 4000 psi. (27.58 MPa) Standard Methods of Sampling and Testing Concrete Masonry Units (ref. 11) can be referred to for determining strength of dry-stack units.

Masonry compressive strength f’_m can conservatively be based on the unit-strength method of the building code (ref . 15), or be determined by testing prisms in accordance with ASTM C1314 (ref. 4). Test prisms can be either grouted or ungrouted depending on the type of wall construction specified.

Solid Grouted, Unreinforced Construction

Out-of-Plane & In-Plane Allowable Flexural Strength

Because no mortar is used to resist flexural tension as for conventional masonry construction, flexural strength of mortarless masonry is developed through the grout, reinforcement or surface coating. For out-of-plane bending of solid grouted walls allowable flexural strength can be estimated based on flexural tensile strength of the grout per Equation 1.

Consideration should be given to the reduction in wall thickness at the bed joints when estimating geometrical properties of the net effective section.

Correspondingly, flexural strength based on masonry compressive stress should be checked, particularly for walls resisting significant gravity loads, using the unity equation as given below.

Buckling should also be checked. (Ref. 8)

In-Plane Shear Strength

Shear strength for out-of-plane bending is usually not a concern since flexural strength governs design for this case. For resistance to horizontal forces applied parallel to the plane of a wall, Equation 3 may be used to estimate allowable shear strength.

F_v is the allowable shear strength by the lesser of the three values given in Equation 4.

Grouted, Reinforced Construction

Mortarless masonry that is grouted and reinforced behaves much the same as for conventional reinforced and mortared construction. Because masonry tensile strength is neglected for mortared, reinforced construction, flexural mechanisms are essentially the same with or without the bed joints being mortared provided that the units subjected to compressive stress are in good contact. Thus, allowable stress design values can be determined using the same assumptions and requirements of the MSJC code. (ref.1)

Out-of-Plane & In-Plane Allowable Flexural Strength

Axial and flexural tensile stresses are assumed to be resisted entirely by the reinforcement. Strains in reinforcement and masonry compressive strains are assumed to vary linearly with their distance from the neutral axis. Stresses in reinforcement and masonry compressive stresses are assumed to vary linearly with strains. For purposes of estimating allowable flexural strengths, full bonding of reinforcement to grout are assumed such that strains in reinforcement are identical to those in the adjacent grout.

For out-of-plane loading where a single layer of vertical reinforcement is placed, allowable flexural strength can be estimated using the equations for conventional reinforcement with the lower value given by Equations 5 or 6.

In-Plane Shear Strength

Though the MSJC code recognizes reinforced masonry shear walls with no shear, or horizontal reinforcement, it is recommended that mortarless walls be rein- forced with both vertical and horizontal bars. In such case, allowable shear strength can be determined based on shear reinforcement provisions (ref. 1) with Equations 7, 8 and 9.

Where F_v is the masonry allowable shear stress per Equations 8 or 9.

Solid Grouted, Prestressed Construction

Mortarless masonry walls that are grouted and pre- stressed can be designed as unreinforced walls with the prestressing force acting to increase the vertical compres- sive stress. Grout can be used to increase the effective area of the wall. Flexural strength will be increased because of the increase in the f_a term in Equation 1. Shear strength will be increased by the N_v term in Equation 4.

Because the prestressing force is a sustained force, creep effects must be considered in the masonry. Research on the long-term behavior of dry-stacked masonry by Marzahn and Konig (ref. 8) has shown that creep effects may be accentuated for mortarless masonry as a result of stress concentrations at the contact points of adjacent courses. Due to the roughness of the unit surfaces, high stress concentrations can result which can lead to higher non-proportional creep deformations. Thus, the creep coefficient was found to be dependent on the degree of roughness along bed-joint surfaces and the level of applied stress. As a result, larger losses in prestressing force is probable for dry-stack masonry.

Surface-Bonded Construction

Dry-stack walls with surface bonding develop their strength through the tensile strength of small fiberglass fibers in the 1/8” (3.8mm) thick troweled or surface bonded cement-plaster coating ASTM C-887(Ref. 14). Because no grouting is necessary, flexural tension and shear strength are developed through tensile resistance of fiberglass fibers applied to both surfaces of a wall. Test data has shown that surface bonding can result in a net flexural tension strength on the order of 300 psi.(2.07 MPa) Flexural capacity, based on this value, exceeds that for conventional, unreinforced mortared masonry construction, therefore it is considered conservative to apply the desired values of the code (ref. 1) for allowable flexural capacity for portland cement / lime type M for the full thickness of the face shell.

Out-of-Plane and In-Plane Flexural Strength

Surface-bonded walls can be considered as unreinforced and ungrouted walls with a net allowable flexural tensile strength based on the strength of the fiber-reinforcement. Flexural strength is developed by the face shells bonded by the mesh. Allowable flexural strength can be determined using Equation 1 with an F_t value determined on the basis of tests provided by the surface bonding cement supplier. Axial and flexural compressive stresses must also be checked per Equation 2 considering again only the face shells to resist stress.

Surface Bonded In-Plane Shear Strength

In-plane shear strength of surface-bonded walls is attributable to friction developed along the bed joints resulting from vertical compressive stress in addition to the diagonal tension strength of the fiber coating. If the enhancement in shear strength given by the fiber reinforced surface parging is equal to or greater than that provided by the mortar-unit bond in conventional masonry construction, then allowable shear strength values per the MSJC code (ref. 1) may be used. In such case, section properties used in Equation 3 should be based on the cross-section of the face shells.

Figure 3 – A Mortarless Garden Wall Application

Figure 4 – A Residential, Mortarless, Single-Family Basement – Part of a 520 Home Development

REFERENCES

Building Code Requirements for Masonry Structures), ACI 530-02/ ASCE 5-02/TMS 402-02. Reported by the Masonry Standards Joint Committee (MSJC), 2002.
Drysdale, R.G., Properties of Dry-Stack Block, Windsor, Ontario, July 1999.
Drysdale, R.G., Properties of Surface-Bonded Dry-Stack Block Construction, Windsor, Ontario, January 2000.
Drysdale, R.G., Racking Tests of Dry-Stack Block, Windsor, Ontario, October 2000.
Drysdale, R.G., Design and Construction Guide for Azar Dry-Stack Block Construction, JNE Consulting, Ltd., February 2001.
Grout for Concrete Masonry, TEK 09-04A, Concrete Masonry & Hardscapes Association, 2002.
2000 International Building Code, Falls Church, VA. International Code Council, 2000.
Marzahn, G. and G. Konig, Experimental Investigation of Long-Term Behavior of Dry-Stacked Masonry, Journal of The Masonry Society, December 2002, pp. 9-21.
Hybrid Concrete Masonry Construction Details, TEK 0303B. Concrete Masonry & Hardscapes Association, 2009.
Specification for Masonry Structures, ACI 530.1-02/ASCE 6-02/ TMS 602-02. Reported by the Masonry Standards Joint Committee (MSJC), 2002.
Standard Methods of Sampling and Testing Concrete Masonry Units, ASTM C140-02a, ASTM International, Inc. , Philadelphia, 2002.
Standard Method of Sampling and Testing Grout, ASTM C1019-02, ASTM International, Inc., Philadelphia, 2002.
Standard Specification for Grout for Masonry, ASTM C 476-02. ASTM International, Inc., 2002
Standard Specification for Packaged, Dry, Combined Materials for Surface Bonding Mortar, ASTM C 887-79a (2001). ASTM International, Inc., 2001.
Standard Test Method for Compressive Strength of Masonry Assem blages, ASTM C1314-02a, ASTM International, Inc., Philadelphia, 2002.
Surface Bonded Concrete Masonry Construction, TEK 03-05A. Concrete Masonry & Hardscapes Association, 1998.

NOTATION

A_n net cross-sectional area of masonry, in² (mm²)
A_s effective cross-sectional area of reinforcement, in2 (mm2)
b width of section, in. (mm)
d distance from extreme compression fiber centroid of tension reinforcement, in. (mm)
F_a allowable compressive stress due to axial load only, psi (MPa)
F_b allowable compressive stress due to ß exure only, psi (MPa)
F_s allowable tensile or compressive stress in reinforcement, psi (MPa)
F_t flexural tensile strength of the grout, psi(MPa)
F_v allowable shear stress in masonry psi (MPa)
f_a calculated vertical compressive stress due to axial load, psi (MPa)
f_b calculated compressive stress in masonry due to ß exure only, psi (MPa)
f’ specified compressive strength of masonry, psi (MPa)
I moment of inertia in.⁴ (mm⁴)
j ratio of distance between centroid of flexural compressive forces and centroid of tensile forces to depth, d
k ratio of the distance between compression face of the wall and neu tral axis to the effective depth d
M maximum moment at the section under consideration, in.-lb (N-mm)
N_v compressive force acting normal to the shear surface, lb (N)
Q first moment about the neutral axis of a section of that portion of the cross section lying between the neutral axis and extreme fiber in.³ (mm³)
S_g section modulus of uncracked net section in.³ (mm³)
V shear force, lb (N)

Concrete Masonry Fence Design

August 2018

INTRODUCTION

Concrete masonry fences and garden walls are used to fulﬁll a host of functions, including privacy and screening, security and protection, ornamentation, sound insulation, shade and wind protection.

In addition, concrete masonry provides superior durability, design ﬂexibility and economy. The wide range of masonry colors and textures can be used to complement adjacent architectural styles or blend with the natural landscape.

Because fences are subjected to outdoor exposure on both sides, selection of appropriate materials, proper structural design and quality workmanship are critical to maximize their durability and performance.

STRUCTURAL DESIGN

Masonry fences are generally designed using one of ﬁve methods:

as cantilevered walls supported by continuous footings;
as walls spanning between pilasters, that are, in turn, supported by a footing pad or caisson;
as walls spanning between wall returns that are sufﬁcient to support the wall;
as curved walls with an arc-to-chord relationship that provides stability; or
as a combination of the above methods.

This TEK covers cases (a) and (d) above, based on the provisions of the 2003 and 2006 editions of the International Building Code (refs. 1, 2). Although fences up to 6 ft (1,829 mm) high do not require a permit (refs. 1 and 2, Ch.1), this TEK provides guidance on design and construction recommen- dations. Fences designed as walls spanning between pilasters (case b) are covered in TEK 14-15B, Allowable Stress Design of Pier and Panel Highway Sound Barrier Walls (ref. 3). In addition, fences can be constructed by dry-stacking and surface bonding conventional concrete masonry units (see ref. 4), or by utilizing proprietary dry-stack fence systems.

Figure 1—Typical Construction Requirements for a Cantilevered Fence

CANTILEVERED FENCE STRUCTURAL DESIGN

Tables 1, 2 and 3 provide wall thickness and vertical reinforcement requirements for cantilevered walls for three lateral load cases: lateral load, w ≤ 15 psf (0.71 kPa), 15 < w ≤ 20 psf (0.95 kPa), and 20 < w ≤ 25 psf (1.19 kPa), respectively. For each table, footnote A describes the corresponding wind and seismic conditions corresponding to the lateral load, based on Minimum Design Loads for Buildings and Other Structures, ASCE 7 (ref. 5).

Assumptions used to develop Tables 1, 2 and 3 are:

strength design method
except as noted, designs comply with both the 2003 and 2006 International Building Code,
running bond masonry,
ASTM C 90 (ref. 6) concrete masonry units,
speciﬁed compressive strength of masonry, f’_m = 1,500 psi (10.3 MPa)
ASTM C 270 (ref. 7) mortar as follows: Type N, S or M portland cement /lime mortar or Type S or M masonry cement mortar (note that neither Type N nor masonry cement mortar is permitted to be used in SDC D),
ASTM C 476 (ref. 8) grout,
Grade 60 reinforcing steel, reinforcement is centered in the masonry cell,
depth from grade to top of footing is 18 in. for 4- and 6-ft (457 mm for 1.2- and 1.8-m) high fences; 24 in. for 8-ft (610 mm for 2.4-m) high fences, and
reinforcement requirements assume a return corner at each fence end with a length at least equal to the exposed height. Where fence ends do not include a return, increase the design lateral load on the end of the fence (for a length equal to the exposed height) by 5 psf (34.5 kPa).

Table 1—Cantilevered Fences Subject to Lateral Loads up to 15 psf (0.71 kPa)

Table 2—Cantilevered Fences Subject to Lateral Loads up to 20 psf (0.95 kPa)

Table 3—Cantilevered Fences Subject to Lateral Loads up to 25 psf (1.19 kPa)

FOOTINGS

For cantilevered walls, the footing holds the wall in position and resists overturning and sliding due to lateral loads. Dowels typically extend up from the footing into the wall to transfer stresses and anchor the wall in place. Dowels should be at least equal in size and spacing to the vertical fence reinforcement. The required length of lap is determined according to the design procedure used and type of detail employed. For the design conditions listed here, the No. 4 (M#13) reinforcing bars require a minimum lap length of 15 in. (381 mm), and the No. 5 (M#16) bars require a minimum lap length of 21 in. (533 mm). Refer to TEK 12-06a, Splices, Development and Standard Hooks for Concrete Masonry (ref. 9) for detailed information on lap splice requirements.

Footings over 24 in. (610 mm) wide require transverse reinforcement (see footnotes to Table 4). For all footings, the hook should be at the bottom of the footing (3 in. (76 mm) clearance to the subgrade) in order to develop the strength of the bar at the top of the footing.

The footing designs listed in Table 4 conform with Building Code Requirements for Reinforced Concrete, ACI 318 (ref. 10). Note that concrete for footings placed in soils containing high sulfates are subject to additional requirements (refs. 1, 2).

Table 4—Footing Sizes for Cantilevered Fences

SERPENTINE WALLS

Serpentine or “folded plate” wall designs add interesting and pleasing shapes to enhance the landscape. The returns or bends in these walls also provide additional lateral stability, allowing the walls to be built higher than if they were straight.

Serpentine and folded plate walls are designed using empirical design guidelines that historically have proven successful over many years of experience. The guidelines presented here are based on unreinforced concrete masonry for lateral loads up to 20 psf (0.95 kPa). See Table 2, footnote A for corresponding wind speeds and seismic design parameters.

Design guidelines are shown in Figure 2, and include:

wall radius should not exceed twice the height,
wall height should not exceed twice the width (or the depth of curvature, see Figure 2),
wall height should not exceed ﬁfteen times the wall thickness, and
the free end(s) of the serpentine wall should have additional support such as a pilaster or a short-radius return.

A wooden template, cut to the speciﬁed radius, is helpful for periodically checking the curves for smoothness and uniformity. Refer to TEK 5-10A, Concrete Masonry Radial Wall Details (ref. 11) for detailed information on constructing curved walls using concrete masonry units.

CONSTRUCTION

All materials (units, mortar, grout and reinforcement) should comply with applicable ASTM standards. Additional material requirements are listed under the section Cantilevered Fence Structural Design, above.

To control shrinkage cracking, it is recommended that horizontal reinforcement be utilized and that control joints be placed in accordance with local practice. In some cases, when sufficient horizontal reinforcement is incorporated, control joints may not be necessary. Horizontal reinforcement may be either joint reinforcement or bond beams. See CMU-TEC-009-23, Crack Control Strategies for Concrete Masonry Construction (ref. 12) for detailed guidance.

In addition, horizontal reinforcement in the top course (or courses if joint reinforcement is used) is recommended to help tie the wall together. For fences, it is not structurally necessary to provide load transfer across control joints, although this can be accomplished by using methods described in CMU-TEC-009-23 if deemed necessary to help maintain the fence alignment.

Copings provide protection from water penetration and can also enhance the fence’s appearance. Various materials such as concrete brick, cast stone, brick and natural stone are suitable copings for concrete masonry fences. Copings should project at least ½ in. (13 mm) beyond the wall face on both sides to provide a drip edge, which will help keep dripping water off the face of the fence. In cases where aesthetics are a primary concern, the use of integral water repellents in the masonry units and mortar can also help minimize the potential formation of efﬂorescence.

REFERENCES

2003 International Building Code. International Code Council, 2003.
2006 International Building Code. International Code Council, 2006.
Allowable Stress Design of Pier and Panel Highway Sound Barrier Walls, NCMA TEK 14-15B. Concrete Masonry & Hardscapes Association, 2004.
Design and Construction of Dry-Stack Masonry Walls, TEK 14-22. National Concrete Masonry Association, 2003.
Minimum Design Loads for Buildings and Other Structures, ASCE 7-02 and ASCE 7-05. American Society of Civil Engineers, 2002 and 2005.
Standard Speciﬁcation for Loadbearing Concrete Masonry Units, ASTM C 90-01a and C 90-03. ASTM International, Inc., 2001 and 2003.
Standard Speciﬁcation for Mortar for Unit Masonry, ASTM C 270-01a and C 270-04. ASTM International, Inc., 2001 and 2004.
Standard Speciﬁcation for Grout for Masonry, ASTM C 476-01 and C 476-02. ASTM International, Inc., 2001 and 2002.
Splices, Development and Standard Hooks for Concrete Masonry, TEK 12-06A. Concrete Masonry & Hardscapes Association, 2007.
Building Code Requirements for Structural Concrete, ACI 318-02 and ACI 318-05. Detroit, MI: American Concrete Institute, 2002 and 2005.
Concrete Masonry Radial Wall Details, TEK 5-10A. Concrete Masonry & Hardscapes Association, 2006.
Crack Control in Concrete Masonry Walls, TEK 10-1A. Concrete Masonry & Hardscapes Association, 2005.

Concrete Masonry Hurricane and Tornado Shelters

August 2018

INTRODUCTION

Extreme windstorms, such as hurricanes and tornadoes, can pose a serious threat to buildings and their occupants in many parts of the country. Hurricanes and tornadoes produce wind pressures and generate flying debris at much higher levels than those used to design most commercial and residential buildings. Hence, these storms require residents to either evacuate the area or seek protection in dedicated shelters. Storm shelters are buildings, or parts of buildings, that are designed and built specifically to provide a highly protected space where community members or occupants can seek refuge during these events.

The newly-developed standard ICC-500, Standard on the Design and Construction of Storm Shelters (ref. 1), provides design and construction requirements for hurricane and tornado shelters. The standard covers structural design requirements for these shelters, as well as requirements for ventilation, lighting, sanitation, egress and fire safety.

ICC-500 covers both hurricane and tornado shelters, and includes requirements for two types of shelters: community shelters, buildings specifically dedicated to provide shelter during a storm; and residential shelters, which are typically reinforced rooms within a home, where the occupants can safely seek refuge during a hurricane or tornado.

Prior to the publication of ICC-500, builders and homeowners seeking storm shelter guidance have used the FEMA 320 publication Taking Shelter From the Storm: Building a Safe Room Inside Your House, and the FEMA 361 publication Design and Construction Guidance for Community Shelters, (refs. 2, 3). Research performed at the Texas Tech University Wind Science and Engineering Research Center (ref. 4), however, found that the FEMA recommendations were overly conservative for concrete masonry for impact resistance. Concrete masonry walls have been tested to withstand the ICC500 criteria, resulting in more economical wall designs than those previously recommended by FEMA.

TEK 05-11, Residential Details for High-Wind Areas (ref. 5), provides prescriptive requirements for reinforced concrete masonry homes in hurricane-prone areas, based primarily on providing a continuous load path from roof to foundation. These are general residential details, and do not address storm shelters. In contrast, the requirements described in this TEK apply only to dedicated shelters, or to shelter areas within a home, meant to provide temporary protection during a storm. Concrete masonry walls capable of meeting the ICC-500 requirements are presented, as well as the results of impact testing on concrete masonry walls. Note that this TEK does not address all requirements of ICC-500.

ICC-500 WIND DESIGN CRITERIA FOR SAFE ROOM WALLS AND FLOORS

General design considerations for storm shelters include:

adequate wall and roof anchorage to resist overturning and uplift,
walls and ceiling, as well as openings such as doors and windows, must withstand design wind pressures and resist penetration by windborne objects and falling debris, and
connections between building elements must be strong enough to resist the design wind loads. Figure 2 shows a typical detail for connecting a concrete roof slab to concrete masonry shelter walls, using reinforcing bars to provide adequate load transfer. ICC-500 defines design tornado wind speeds across the United States, and hurricane design wind speeds for applicable coastal areas. When the shelter is to provide shelter from both hurricanes and tornadoes, the most restrictive of the two design criteria should be used for design. The reader is referred to the standard (ref. 1) for maps defining these speeds. Note that wind speeds in ICC-500 are much higher than wind speeds in ASCE7 (ref. 6) or the International Building Code (refs. 7, 8), and are considered to provide the maximum or ultimate tornado or hurricane design wind speed at a site. Therefore, the wind load contribution in the load combinations is adjusted accordingly.

For example, 1.0W rather than 1.6 W is used as the factored wind load in strength design combinations. In allowable stress design, 0.6W is used instead of W. Wind pressures are to be based on exposure C, although exposure B is permitted if it exists for all wind directions.

In addition to being designed for these design wind speeds, shelter walls and ceilings must be able to withstand impact from flying debris, whose projectile speed varies with the design wind speed. The ICC 500 design criteria vary with location. The concrete masonry walls tested at Texas Tech were tested at the most stringent of the ICC-500 wind speeds and impact requirements, as follows. For tornado shelters, the highest design wind speed prescribed by ICC-500 is 250 mph (402 km/h). Corresponding walls and ceilings must withstand impact from a 15 lb (6.8 kg) wooden 2 x 4, propelled at 100 mph (161 km/h) and 67 mph (108 km/h), respectively.

These conditions will more than satisfy the less stringent requirements for hurricane shelters. For hurricane shelters, the highest design wind speed in ICC-500 is 237 mph (381 km/h) (with the exception of Guam, which has a design hurricane wind speed of 256 mph (412 km/h)). In addition, walls subject to this 237 mph (381 km/h) design wind speed must be capable of withstanding impact from a 9 lb (4.1 kg) wooden 2 x 4 propelled at 100 mph (161 km/h). Ceilings and other horizontal surfaces must withstand impact from the same projectile propelled a 25 mph (40 km/h).

In addition to these requirements, ICC-500 defines requirements for tie-down to the foundation and adequate foundation sizing to resist the design overturning and uplift forces.

CONCRETE MASONRY ASSEMBLIES FOR STORM SHELTERS

A typical concrete masonry storm shelter design is shown in Figure 1. Several concrete masonry systems have been successfully tested to withstand the 15 lb (6.8 kg) 2 x 4 propelled at 100 mph (161 km/h) (ref. 4). Solidly grouted 8-in. (203-mm) concrete masonry walls with No. 5 (M #16) reinforcement at 48 in. (1,219 mm) o.c., with one horizontal No. 5 (M#16) min. at the top of the wall and in the footing or bottom of the wall, can withstand these conditions. All weight classes of concrete masonry meet the strength and impact-resistance requirements. The engineer will use the masonry weight in the shelter design to resist overturning. Regardless of the concrete masonry density, the weight of the grouted masonry assembly provides increased overturning resistance compared to low-mass systems.

Although solidly grouted 6-in. (152-mm) concrete masonry walls with No. 4 (M #13) bars at 32 in. (813 mm) o.c. successfully passed the impact test, they may not have enough weight to resist overturning for the most severe tornado loading, based on a 250 mph (402 km/h) wind speed. Hence, the details included in this TEK show 8-in. (203 mm) storm shelter walls. Solidly grouted 6-in. (152-mm) walls may be adequate for lower wind requirements, however.

A ceiling system using 7-in. (178-mm) deep bottom chord bearing steel joists infilled with concrete masonry units and grout to a nominal 8-in. (203-mm) depth was also tested and found to withstand the 15 lb (6.8 kg) 2 x 4 at 67 mph (108 km/h) protocol (ref. 4). No. 4 (M #13) reinforcing bars were placed perpendicular to the joists, at 8 in. (203 mm) o.c. Note that all assemblies were successfully tested using standard masonry grout per ASTM C 476 (ref. 9). Some previous references recommend the use of concrete to fill the masonry cores, rather than grout, but this is contrary to the building code and is highly discouraged.

RESIDENTIAL SHELTERS

The purpose of an in-home shelter is to provide an area where the occupants can safely shelter during a high wind event. In flood prone areas, the shelter must not be built where it can be flooded. The shelter should be accessible from all areas of the house and should be free of clutter to provide immediate shelter. If not within the residence, the shelter needs to be within 150 ft (45.72 m) of the residence (ref. 1). FEMA (ref. 2) suggests a basement, an interior room on the first floor on a foundation extending to the ground or on top of a concrete slab-on-grade foundation or garage floor as good locations for an in-home shelter.

Below-ground safe rooms provide the greatest protection, as long as they are designed to remain dry during the heavy rains that often accompany severe windstorms. When shelters are located below grade, the soil surrounding the walls can be considered as protection from flying debris during a high wind event, as long as the wall is completely below grade and soil extends at least 3 ft (914 mm) away from the wall, with a slope no greater than two inches per foot (167 mm/m) for that 3 ft (914 mm) distance. When these conditions are met, the walls do not need to meet the missile impact requirements described above. Below-grade ceilings must have a minimum of 12 in. (305-mm) of soil cover to be exempt from the impact testing requirements.

Sections of either interior or exterior residence walls that are used as walls of the safe room must be separated from the structure of the residence so that failure of the residence, which is designed for a much lower loading, will not result in a failure of the safe room.

RESIDENTIAL RETROFIT

Special consideration must be given when retrofitting a shelter into an existing home. Figures 3 through 5 illustrate typical details for connecting shelter elements to an existing basement wall.

The results of recent testing (ref. 4) has improved the economy of constructing retrofits. Previously, a concrete masonry storm shelter would have required a large dedicated foundation. Research confirms, however, that considering the weight of fully grouted concrete masonry, a large foundation is not required to adequately resist the uplift and overturning forces.

Accordingly, ICC-500 allows concrete masonry storm shelters to be constructed within one and two family dwellings on existing slabs on grade without a dedicated foundation, under the following conditions:

the calculated soil pressure under the slab supporting the storm shelter walls does not exceed 2,000 psf (95.8 kPa) for design loads other than the design storm events and 3,000 psf (143.6 kPa) for design storm shelter events,
at a minimum, the storm shelter is anchored to the slab at each corner of the structure and on each side of the doorway opening (see Figure 4), and
the ICC-500 slab reinforcement requirements are waived if the slab dead load is not required to resist overturning.

COMMUNITY SHELTERS

Requirements for community shelters are similar to those for residential, but require a larger area and additional features in anticipation of sheltering more people. For example, community storm shelters require: signage to direct occupants to storm shelter areas; wall, floor and ceiling assemblies with a minimum 2-hour fire resistance rating; as well as additional ventilation and sanitation facilities.

REFERENCES

Standard on the Design and Construction of Storm Shelters, ICC-500. International Code Council and National Storm Shelter Association, 2008.
Taking Shelter From the Storm: Building a Safe Room Inside Your House, FEMA 320. Federal Emergency Management Agency, 2004.
Design and Construction Guidance for Community Shelters, FEMA 361. Federal Emergency Management Agency, 2000.
Investigation of Wind Projectile Resistance of Concrete Masonry Walls and Ceiling Panels with Wide Spaced reinforcement for Above Ground Shelters, CMHA Publication MR 21. Texas Tech University Wind Science and Engineering Research Center, 2003.
Residential Details for High-Wind Areas, TEK 05-11, Concrete Masonry & Hardscapes Association, 2003.
Minimum Design Loads for Buildings and Other Structures, ASCE 7-02 and ASCE 7-05. American Society of Civil Engineers, 2002 and 2005.
2003 International Building Code. International Code Council, 2003.
2006 International Building Code. International Code Council, 2006.
Standard Specification for Grout for Masonry, ASTM C 476-07. ASTM International, Inc., 2007.

Construction of Reinforced Concrete Masonry Diaphragm Walls

August 2018

INTRODUCTION

Diaphragm walls are composed of two wythes of masonry with a large cavity or void. The wythes are bonded together with masonry ribs or crosswalls in such a way that, structurally, the wythes function compositely—as though the entire thickness is effectively solid.

Figure 1 shows a stone-clad university building with reinforced concrete masonry diaphragm walls, used to recreate the campus’ Gothic architecture. The use of reinforced diaphragm walls allowed support of the tall sidewalls and gable ends.

Figure 2 shows a cross-section of a typical diaphragm wall. The reinforced wythes can be fully or partially grouted. The exterior face can be constructed with a weathering face, like a conventional single wythe wall, or finished with a veneer. The voids can be used for placement of utilities and/or insulation.

This TEK discusses construction considerations for diaphragm walls: TEK 14-24, Design of Reinforced Concrete Masonry Diaphragm Walls, (ref. 1) covers the structural design.

CONSTRUCTION ADVANTAGES

Reinforced diaphragm walls present several construction benefits. These include:

As shown in Figure 1, thick walls can be created efficiently using standard units bonded together. Thicker walls can be used to create taller walls.
The wall can have exposed finished surfaces both inside and out. In addition, those finishes can be different because they are created by two different masonry wythes and can, therefore, feature different unit types/sizes/colors.
The wall construction proceeds very much as conventional single wythe or cavity wall construction.
The exterior wythe can be constructed with a veneer.
The large interior voids allow for easy placement of utilities and/or insulation.

KEY CONSTRUCTION FEATURES

Construction Sequence

The construction sequence for diaphragm walls can vary based upon how the ribs are interconnected with the two wythes. Building Code Requirements for Masonry Structures (ref. 2), referred to as TMS 402, Section 5.1.1.2.5 provides three methods for connecting intersecting walls to allow shear transfer:

At least fifty percent of the masonry units at the interface must interlock. This means the ribs could be constructed in running bond with every other course interlocking with the wythes. Thus, the wythes and the ribs would be constructed concurrently.
Walls must be anchored by steel connectors grouted into the wall and meeting the following requirements: (a) Minimum size: 1/4 in. x 1-1/2 in. x 28 in. (6.4 x 38.1 x 711 mm) including 2-in. (50.8-mm) long, 90-degree bend at each end to form a U or Z-shape. (b) Maximum spacing: 48 in. (1,219 mm). Thus, it is possible to build the ribs separately from the wythes, which provides significant flexibility in construction.
Intersecting reinforced bond beams must be provided at a maximum spacing of 48 in. (1,219 mm) on center. The area of reinforcement in each bond beam must be not less than 0.1 in.2 per ft (211 mm2/m) multiplied by the vertical spacing of the bond beams in feet (meters). Reinforcement must be developed on each side of the intersection.

Again, this provides flexibility in sequencing the wall construction. However, the grouting must be done simultaneously with the wythe construction.

Masonry Bond

TMS 402 Section 5.1.1.2.1 requires that the masonry at intersecting walls be laid in running bond for composite action between wythes to be effective. This requirement controls the entire construction of a diaphragm wall and mandates running bond for both the wythes and the ribs.

Reinforcement

Vertical reinforcement is typically placed in the cells of the wythes as is done in single-wythe construction. Posttensioning can be placed either in the cells of the wythes or within the void itself. If placed within the void and laterally restrained tendons are specified, tendon restraints must be fabricated. TEK 03-14, Post-Tensioned Concrete Masonry Wall Construction (ref. 3) provides a more detailed overview. Depending on the project’s seismic and/or loading requirements, horizontal reinforcement can be placed in either grouted bond beams or in the bed joints of the wythes and ribs. Horizontal bond beams are beneficial in that they can also serve as the interlock between the ribs and wythes, as well as shear reinforcement for the ribs.

Ribs (Crosswalls)

The structural design will determine whether or not the ribs require vertical reinforcement. The interlock with the wythes transfers shear forces across the intersections, and the vertical reinforcement in the wythes acts as the total wall reinforcement.

Wall Grouting

The requirement for full or partial wall grouting is a design decision. Any cells or bond beams with reinforcement must be grouted. The need for additional grouting is determined based on the design requirements. Both low-lift and high-lift grouting techniques are suitable to diaphragm walls. See TEK 03-02A, Grouting Concrete Masonry Walls, (ref. 4) for more detailed information.

Water Management

Strategies for water penetration resistance of conventional masonry walls depend on whether the wall is singlewythe or a cavity wall. Water penetration resistance for the exterior wythe of a diaphragm wall follows the strategies employed for single wythe construction. If the exterior wythe has a veneer and cavity, it is flashed and weeped the same way as a single wythe masonry cavity wall. With no veneer and cavity, the exterior wythe of a diaphragm wall is flashed and weeped the same way as a similarly constructed partially grouted single wythe wall. Flashing and weeps are not necessary if the exterior wythe is solid grouted.

Figure 3 shows a typical wall base detail for a diaphragm wall with an exterior veneer and cavity. The cavity between the exterior diaphragm wythe may contain insulation and an air/moisture barrier, as required. The veneer is anchored to the exterior wythe of the diaphragm wall and is weeped and flashed. TEK 19-05A, Flashing Details for Concrete Masonry Walls, (ref. 6) provides additional details applicable to this construction.

Figure 4 shows a wall base detail applicable to an exterior diaphragm wythe without a cavity and veneer. TEK 19-02B, Design for Dry Single Wythe Concrete Masonry Walls, (ref. 7) provides additional details for single wythe construction.

Openings through diaphragm walls, roof/floor intersections, etc. are also flashed and weeped similar to conventional concrete masonry walls.

Top of the Wall

Diaphragm walls require closure at the top to transfer vertical loads and close off the void. Figure 5 shows one common detail for capping the walls. The cast-in-place capping slab at the top takes the place of what would normally be bond beams in single-wythe walls. For post tensioned walls, the top slab provides a convenient anchorage point for the tendons.

Utilities and Insulation

The voids offer several opportunities not common in masonry walls. They provide chases for duct work and utilities with minimal cutting of the units and allow for additional insulation if desired. Diaphragm walls can be insulated on the exterior, by using a veneer and insulated cavity, or by using an exterior insulation system. They can also be insulated on the interior, using furring, insulation and gypsum wallboard. When insulation is placed in the voids, however, the ribs produce a large thermal bridge, reducing the effectiveness of the insulation. 06-11A, Insulating Concrete Masonry Walls, (ref. 5) provides more detailed information.

Openings

Constructing openings in diaphragm walls is also very similar to single-wythe walls (see Figure 6). The entire void should be spanned/filled at the opening and the exterior wythe flashed above (as appropriate), as shown in Figure 4. Figure 6 Option 1 shows a reinforced concrete slab that has been designed as a header for the opening. Figure 6 Option 2 has lintels to support the wythes over the opening. The void at the headers and sills is infilled with a nonmasonry material, such as exterior gypsum sheathing. The jambs should be infilled with masonry wherever they don’t already align with the ribs. Note that Figure 6 does not show flashing that may be necessary.

Control Joints

Control joints are provided in concrete masonry walls to control cracking primarily from movement due to shrinkage and thermal effects. In diaphragm walls, the ribs will tend to restrict some of that movement, however, because there is currently no research to quantify these effects, current practice is to place control joints at intervals based upon CMU-TEC-009-23, Crack Control Strategies for Concrete Masonry Construction, (ref. 8). TEK 14-24 discusses these criteria and provides an example for determining control joint spacing for a diaphragm wall.

Although the inner wythe will generally be exposed principally to shrinkage with only minor thermal effects, it is common to place control joints in the same locations and to provide similar shrinkage reinforcement in both wythes.

Figure 7 shows two methods of creating control joints in a diaphragm wall. Option 1, with ribs on both sides of the control joint, does a better job keeping water out of the void than Option 2 because a failure of the sealant would allow water to penetrate between the ribs, rather than into the void itself. The control joints in both wythes should be sealed for water protection.

CMU-TEC-009-23 contains additional control joint constructions/details that can also be used on diaphragm walls, including fire-rated joints and control joints that allow shear transfer.

SUMMARY

Diaphragm walls provide several beneficial features and are applicable to a wide variety of projects. Constructing reinforced concrete masonry diaphragm walls uses methods and techniques commonly known to most masons. The added thickness of the wall provides some variations in the overall reinforcement and layout concepts but the techniques are typical for masonry.

REFERENCES

Design of Reinforced Concrete Masonry Diaphragm Walls, TEK 14-24. Concrete Masonry & Hardscapes Association, 2014.
Building Code Requirements for Masonry Structures, TMS 402-16, Reported by The Masonry Society 2016.
Post-Tensioned Concrete Masonry Wall Construction, TEK 03-14. Concrete Masonry & Hardscapes Association, 2002.
Grouting Concrete Masonry Walls, TEK 03-2A. Concrete Masonry & Hardscapes Association, 2005.
Insulating Concrete Masonry Walls, TEK 06-11A. Concrete Masonry & Hardscapes Association, 2010.
Flashing Details for Concrete Masonry Walls, TEK 19-05A.
Concrete Masonry & Hardscapes Association, 2008.
Design for Dry Single Wythe Concrete Masonry Walls, TEK 19-02B. Concrete Masonry & Hardscapes Association, 2012.
Crack Control Strategies for Concrete Masonry Construction, CMU-TEC-009-23, Concrete Masonry & Hardscapes Association, 2023.

Hybrid Concrete Masonry Construction Details

August 2018

INTRODUCTION

Hybrid masonry is a structural system that utilizes reinforced masonry walls with a framed structure. While the frame can be constructed of reinforced concrete or structural steel, the discussion here includes steel frames with reinforced concrete masonry walls. The reinforced masonry infill participates structurally with the frame and provides strength and stiffness to the system. It can be used in single wythe or cavity wall construction provided the connections and joints are protected against water penetration and corrosion. The hybrid walls are constructed within the plane of the framing. Depending on the type of hybrid wall used, the framing supports some or all of the masonry wall weight.

Hybrid masonry/frame structures were first proposed in 2006 (ref. 1). There are several reasons for its development but one primary reason is to simplify the construction of framed buildings with masonry infill. While many designers prefer masonry infill walls as the backup for veneers in framed buildings, there is often a conflict created when structural engineers design steel bracing for the frame which interferes with the masonry infill. This leads to detailing and construction interferences trying to fit masonry around braces. One solution is to eliminate the steel bracing and use reinforced masonry infill as the shear wall bracing to create a hybrid structural system.

The concept of using masonry infill to resist lateral forces is not new; having been used successfully throughout the world in different forms. While common worldwide, U.S. based codes and standards have lagged behind in the establishment of standardized means of designing masonry infill.

The hybrid masonry system outlined in this TEK is a unique method of utilizing masonry infill to resist lateral forces. The novelty of the hybrid masonry design approach relative to other more established infill design procedures is in the connection detailing between the masonry and steel frame, which offers multiple alternative means of transferring loads into the masonry—or isolating the masonry infill from the frame.

Prior to implementing the design procedures outlined in this TEK, users are strongly urged to become familiar with the hybrid masonry concept, its modeling assumptions, and its limitations particularly in the way in which inelastic loads are distributed during earthquakes throughout the masonry and frame system. This system, or design methods, should not be used in Seismic Design Category D and above until further studies and tests have been performed; and additional design guidance is outlined in adopted codes and standards.

CLASSIFICATION OF WALLS

There are three hybrid wall types, Type I, Type II and Type III. The masonry walls are constructed within the plane of the framing. The classification is dependent upon the degree of confinement of the masonry within the frame.

Type I walls have soft joints (gaps that allow lateral drift at the columns or vertical deflection at the top) at the columns and the top of the wall. The framing supports the full weight of the masonry walls and other gravity loads.

Type II walls have soft joints at the columns and are built tight at the top of the wall.

Type III walls are built tight at the columns and the top of the wall.

For Type II and III walls, the masonry walls share the support of the vertical loads, including the wall weight, with the framing.

CONSTRUCTION

Type I Hybrid Walls

Practically speaking, the concept of Type I walls is that the masonry wall is a nonloadbearing shear wall built within the frame which also supports out-of-plane loads (see Figure 1). The details closely match those for current cavity wall construction where the infill masonry is within the plane of the frame, except that the vertical reinforcement must be welded to the perimeter framing at supported floors.

Since the walls are generally designed to span vertically, the walls may not have to be anchored to the columns. The engineer’s design should reflect whether anchors are required but only for out-of-plane loads. The masonry does have to be isolated from the columns so the columns do not transmit loads to the walls when the frame drifts.

In multi-story buildings, each wall is built independently. Walls can be constructed on multiple floors simultaneously. Because the steel framing is supporting the entire wall weight, Type 1 walls are more economical for lower rise buildings. It is possible with Type 1 walls to position the walls outside the framing so they are foundation supported as in caged construction (ref. 1), providing a more economical design for the framing.

Type II Hybrid Walls

With Type ll walls, the masonry wall is essentially a loadbearing shear wall built within the frame: it supports both gravity and out-of-plane loads (see Fig. 1).

There are two options: Type IIa and Type IIb. The engineer must indicate which will be used. For Type IIa walls, the vertical reinforcement (dowels) must be welded to the perimeter framing to transfer tension tie-down forces into the frame. The vertical dowels also transfer shear. For Type IIb walls, vertical reinforcement only needs to be doweled to the concrete slab to transfer shear forces because tie-down is not required. This simplifies the construction of multi-story buildings.

The top of the masonry wall must bear tight to the framing. Options include grouting the top course, using solid units, or casting the top of the wall. The top connectors must extend down from the framing to overlap with the vertical wall reinforcement.

Since the walls generally span vertically, the engineer must decide whether column anchors are needed similar to Type I walls. These anchors only need to transmit out-of-plane loads.

The design must take into account the construction phasing. In multi-story buildings, each wall may be structurally dependent on a wall from the floor below which is very similar to a loadbearing masonry building.

Type III Hybrid Walls

This wall type is fully confined within the framing—at beams and columns. Currently, there are no standards in the United States that govern Type III design. Standards are under development and research is underway to help determine structural and construction requirements. Therefore, no details are provided at this time.

DETAILS

Sample construction details were developed in conjunction with the National Concrete Masonry Association, International Masonry Institute (IMI), and David Biggs. They are hosted on the CMHA web site at www.masonryandhardscapes.org and the IMI web site at www.imiweb.org. Alternate details for hybrid construction are continually under development and will be posted on the web sites. There are several key details that must be considered, including: the wall base, the top of the wall, at columns, and parapets.

Base of Wall

As previously noted for Type I and Type IIa walls, vertical reinforcement must be anchored to either foundation or frame to provide tension-tie downs for the structure. Figure 2 shows the reinforcement anchored to the foundation with a tension lap splice, and also shows the reinforcement anchored at a floor level and tension lap spliced.

For Type IIb walls, the vertical reinforcement does not have to be anchored for tension forces because it only transfers shear forces. Figure 3 shows the reinforcement anchored to the foundation. Figure 4 shows the reinforcement anchored at a floor level. The designer must determine if the dowel can be effectively anchored to the slab for shear or if it must be welded to the framing as shown for Type I and Type IIa walls.

Figure 2—Type I and IIa Foundation and Floor Detail

Top of Wall

For all wall types, the top of the wall must be anchored to transfer in-plane shear loads from the framing to the wall. It also accommodates out-of-plane forces. This is accomplished by a connector. Figures 5 and 5A show an example with bent plates and slotted holes. For Type I walls, the gap at the top of the wall must allow for the framing to deflect without bearing on the wall or loading the bolts. For Type II walls, the gap is filled tight so the framing bears on the wall.

The vertical reinforcement must overlap with the connectors at the top of the wall. Since the top course could be a solid unit, the connector should extend down to a solid grouted bond beam.

Top of wall construction raises the most concern by designers. Constructability testing by masons has been successfully performed. The design concept for the connectors is:

Determine the out-of-plane loads to the wall top.
Design the top bond beam to span horizontally between connectors. Connector spacing is a designer’s choice but is generally between 2 and 4 ft (6.09 and 1.22 m) o. c.
Using the in-plane loading, analyze the connector and design the bolts.
If the design does not work, repeat using a smaller connector spacing.

The steel framing is affected by out-of-plane load transfer to the beam’s bottom flange. Beam analysis and flange bracing concerns for the steel are identical to those for any infill wall.

Figure 5—Top of Wall Details (continued)

Column

For Type I and IIa walls, the wall must be kept separated from the columns so that when the frame drifts it does not bear on the wall. Lightweight anchors can be used to support out- of-plane loads if desired. Figure 6 shows a possible anchor.

Parapet

Parapets can be constructed by cantilevering off the roof framing. Details vary depending on the framing used but are similar to Figure 2. Figure 7 shows three variations for: concrete slab, wide flange framing, and bar joist framing. There is a plate on the beam’s top flange for the bar joist and wide flange framing options.

QUALITY ASSURANCE

Special inspections should be an essential aspect of the quality assurance plan. Besides verifying the vertical reinforcement is properly installed as required by Building Code Requirements for Masonry Structures (ref. 2), the connector must be checked as well. If Type I walls are used, the bolts from the connector to the wall must allow for vertical deflection of the framing without loading the wall.

CONCLUSIONS

Hybrid masonry offers many benefits and complements framed construction. By using the masonry as a structural shear wall, the constructability of the masonry with the frames is improved, lateral stiffness is increased, redundancy is improved, and opportunities for improved construction cost are created.

For now, Type I and Type II hybrid systems can be designed and constructed in the United States using existing codes and standards. Criteria for Type III hybrid systems are under development.

Design issues for hybrid walls are discussed in TEK 14-09A and IMI Tech Brief 02.13.01 (refs. 3, 4).

REFERENCES

Biggs, D.T., Hybrid Masonry Structures, Proceedings of the Tenth North American Masonry Conference. The Masonry Society, June 2007.
Building Code Requirements for Masonry Structures, ACI 530-08/ASCE 5-08/TMS 402-08. The Masonry Society, 2008.
Hybrid Concrete Masonry Design, TEK 14-09A. Concrete Masonry & Hardscapes Association, 2009.
Hybrid Masonry Design, IMI Technology Brief 02.13.01. International Masonry Institute, 2009.

INTRODUCTION

ADVANTAGES

HISTORICAL PERSPECTIVE

REINFORCED DIAPHRAGM WALLS

Design Detailing

a) Spacing of Ribs

b) Flange Thickness

c) Grouting

d) Masonry Bond

e) Connecting the Ribs to the Wythes

f) Control (Movement) Joints

Allowable Stress Design of Reinforced Diaphragm Walls

Strength Design of Reinforced Diaphragm Walls

Reinforced Concrete Masonry Diaphragm Walls Using Post-tensioned Masonry Design

Seismic Design

DESIGN EXAMPLE: WINGWALL DESIGN FOR A REINFORCED CONCRETE MASONRY MAINTENANCE STORAGE FACILITY

1. Proposed wall section

2. Select control joint spacing

3. Determine wind loads

4. Determine base of wall loads

5. Determine beffective

6. Determine minimum twall to satisfy shear capacity

7.Determine minimum twall due to moment capacity

8. Determine wall dead load at base of wall

9. Load combination

10. Determine n

11. Design for PDL and M

12. Check deflection at top of the wall for a cantilever

SUMMARY

NOTATIONS

References

INTRODUCTION

CONSTRUCTION

Laying of Units

Grout and Reinforcement

Vertical Reinforcing

Pre-stressed Walls

Surface Bonded Walls

ENGINEERING PROPERTIES

Unit and Masonry Compressive Strength

Solid Grouted, Unreinforced Construction

Out-of-Plane & In-Plane Allowable Flexural Strength

In-Plane Shear Strength

Grouted, Reinforced Construction

Out-of-Plane & In-Plane Allowable Flexural Strength

In-Plane Shear Strength

Solid Grouted, Prestressed Construction

Surface-Bonded Construction

Out-of-Plane and In-Plane Flexural Strength

Surface Bonded In-Plane Shear Strength

REFERENCES

NOTATION

INTRODUCTION

STRUCTURAL DESIGN

CANTILEVERED FENCE STRUCTURAL DESIGN

FOOTINGS

SERPENTINE WALLS

CONSTRUCTION

REFERENCES

INTRODUCTION

ICC-500 WIND DESIGN CRITERIA FOR SAFE ROOM WALLS AND FLOORS

CONCRETE MASONRY ASSEMBLIES FOR STORM SHELTERS

RESIDENTIAL SHELTERS

RESIDENTIAL RETROFIT

COMMUNITY SHELTERS

REFERENCES

INTRODUCTION

CONSTRUCTION ADVANTAGES

KEY CONSTRUCTION FEATURES

Construction Sequence

Masonry Bond

Reinforcement

Ribs (Crosswalls)

Wall Grouting

Water Management

Top of the Wall

Utilities and Insulation

Openings

Control Joints

SUMMARY

5. Determine b_effective

6. Determine minimum t_wall to satisfy shear capacity

7.Determine minimum t_wall due to moment capacity

11. Design for P_DL and M