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Concrete Masonry Curtain and Panel Wall Details

  • August 2018

INTRODUCTION

Steel and concrete structural frames often rely on nonloadbearing masonry curtain or panel walls to enclose the structure. Panel and curtain walls are distinguished by the fact that a panel wall is wholly supported at each story, while a curtain wall is supported only at its base, or at prescribed interims. Both are designed to resist lateral wind or seismic loads and transfer these lateral loads to the structural frame. They typically do not carry any vertical loads other than their own weight. Curtain and panel walls differ from anchored masonry veneer in that veneer is continuously supported by a backup material.

Curtain and panel walls must be isolated from the frame to prevent the unintentional transfer of structural loads and to allow differential movement between the frame and the masonry. Anchorage between the concrete masonry and structural frame must also account for different construction tolerances for each building material.

Concrete masonry curtain and panel walls should incorporate flashing and weep holes as for other concrete masonry construction. Design for Dry Single-Wythe Concrete Masonry Walls, Flashing Strategies for Concrete Masonry Walls and Flashing Details for Concrete Masonry Walls (refs. 3, 4 & 5) provide detailed information.

PANELWALLS

Concrete masonry panel walls are supported at each building story by means of concrete beams, concrete slabs or steel members.

Supports must take into account the strains and deformations in both the concrete masonry panel wall and the structural frame. Steel supports, often in the form of shelf angles, can be attached to the frame either by welding or bolting, although bolting is often preferred because slotted bolt holes permit adjustments to be made for proper alignment with the masonry. In addition, bolted connections are inherently more flexible than welded connections, allowing a limited amount of movement between the masonry and the frame. Care should be taken, however, to ensure proper bolt tension to avoid slipping once positioned.

For high-rise construction, allowance should be made for differential movement between the shelf angle and the panel wall below due to creep of the frame and/or masonry thermal expansion. This is accomplished by leaving an open (mortarless) space between the bottom of the shelf angle and the masonry below or by filling the space with compressible material. The joint is then sealed with caulking to prevent moisture intrusion. The horizontal movement joint below the shelf angle also helps prevent vertical loads from inadvertently being transferred to the concrete masonry panel wall below the shelf angle.

Flashing and weep holes should be installed immediately above all shelf angles to drain moisture. In multi-wythe panel walls, wall ties between the exterior and interior masonry wythes should be located as close to the shelf angle as possible. Figures 1 and 2 show steel shelf angle attachments to concrete and steel, respectively.

CURTAIN WALLS

Concrete masonry curtain walls can be designed to span either vertically or horizontally between supports. They can also incorporate reinforcement to increase lateral load resistance and the required distance between lateral supports.

Anchors used to provide lateral support must be sufficiently stiff in the out-of-plane direction to transfer lateral loads to the frame and be flexible enough in-plane to allow differential movement between the curtain wall and the frame. In addition, Building Code Requirements for Masonry Structures (ref. 1) includes specific corrosion-resistance requirements to ensure long-term integrity of the anchors, consisting of AISI Type 304 stainless steel or galvanized or epoxy coatings.

Anchors are required to be embedded at least 11/2 in. (38.1mm) into the mortar bed when solid masonry units are used (ref.1) to prevent failure due to mortar pullout or pushout. Because of the magnitude of anchor loads, it is also recommended that they be embedded in filled cores when using hollow units. As an alternative to completely filling the masonry core, this can be accomplished by placing a screen under the anchor and building up 1 to 2 in. (25 to 51 mm) of mortar into the core of the block above the anchor.

For both concrete and steel frames, the space between the column and the masonry should be kept clear of mortar to avoid rigidly bonding the two elements together.

Figures 3 through 5 show curtain wall attachments to concrete and steel frames.

CONSTRUCTION TOLERANCES

Tolerances are allowable variations, either in individual component dimensions or in building elements such as walls or roofs. Construction tolerances recognize that building elements cannot always be placed exactly as specified, but establish limits on how far they can vary to help ensure the finished building will function as designed.

When using masonry with another structural system, such as steel or concrete, construction tolerances for each material need to be accommodated, since construction tolerances vary for different building materials.

In general, masonry must be constructed to tighter tolerances than those applicable to steel or concrete frames (refs. 2, 7). Particularly in high-rise buildings, tolerances can potentially affect anchor embedment, flashing details and available support at the shelf angle. To help accommodate these variations in the field, the following recommendations should be considered.

  • Use bolted connections with slotted holes for steel shelf angles to allow the shelf angle location to be adjusted to meet field conditions. Steel shims can be used to make horizontal adjustments to the shelf angle location. Figure 6 shows an example of a shelf angle connection which is adjustable in all three directions. For connections like this, the bottom flange needs to be evaluated for adequate load carrying capability as does the beam for torsion.
  • When shimming shelf angles, use shims that are the full height of the vertical leg of the shelf angle for stability. Shimming is limited to a maximum of 1 in. (25 mm) (ref. 7).
  • Provide a variety of anchor lengths to allow proper embedment over the range of construction tolerances.
  • Use two-piece flashing to accommodate varying cavity widths.
  • Cut masonry units only with the permission of the architect or engineer (this may be proposed when the frame cants towards the masonry, making the cavity between the two materials too small).
  • Include instructions for handling building element misalignment in the construction documents.

REFERENCES

  1. Building Code Requirements for Masonry Structures, ACI
    530-99/ASCE 5-99/TMS 402-99. Reported by the Masonry
    Standards Joint Committee, 1999.
  2. Specification for Masonry Structures, ACI 530.1-99/ ASCE
    6-99/TMS 602-99. Reported by the Masonry Standards
    Joint Committee, 1999.
  3. Design for Dry Single-Wythe Concrete Masonry Walls,
    19-02B, Concrete Masonry and Hardscapes Association,
    2012.
  4. Flashing Strategies for Concrete Masonry Walls , TEK
    19-04A, Concrete Masonry and Hardscapes Association,
    2003.
  5. Flashing Details for Concrete Masonry Walls, TEK 19-05A,
    Concrete Masonry and Hardscapes Association, 2008.
  6. Laska, W. Masonry and Steel Detailing Handbook. The
    Aberdeen Group, 1993.
  7. Code of Standard Practice for Steel Buildings and Bridges,
    American Institute of Steel Construction, Inc., 2000.

Integrating Concrete Masonry Walls With Metal Building Systems

  • August 2018

INTRODUCTION

Metal buildings are used extensively for warehouses and other structures requiring large, open floor spaces. Part of their design flexibility comes from the ability to clad metal buildings with a variety of materials to provide different appearances or functions to the buildings. Concrete masonry walls are popular enclosure systems for metal buildings because of masonry’s aesthetic appeal, impact resistance, strength, and fire resistance. The durability of concrete masonry resists incidental impacts from hand carts and forklifts, provides maximum protection in disasters such as earthquakes and hurricanes, as well as superior security, fire resistance, and noise control.

Concrete masonry walls used for metal buildings can include: exterior full-height walls, either with or without a parapet; exterior partial-height or wainscot walls; and interior loadbearing walls or nonloadbearing walls or partitions. Architectural concrete masonry units, such as colored, split faced, burnished, or scored units, can be used to provide an almost limitless array of textures and patterns to the walls. These units can be used for the entire facade or for banding courses to achieve specific patterns or highlight certain design aspects of the building.

A more detailed discussion of the system, along with structural design and construction considerations, is included in Concrete Masonry Walls for Metal Building Systems (ref. 1). The manual is intended to
bridge the gap between the engineer who designs the metal building system and the engineer who designs the concrete masonry walls to unify their respective knowledge.

DETAILS

A typical metal building clad with masonry is shown in Figure 1. Figures 2 – 6 show some typical details used for exterior concrete masonry cladding on a metal building. These details may need to be modified to meet individual design conditions.

Because of the inherent material differences between steel and masonry, careful consideration must be given to accommodating differential movement between the two materials and their assemblies. In Serviceability Design Considerations for Low-Rise Buildings (ref. 2), a lateral drift limit of H/100 for a ten year recurrence wind loading based on main wind force resisting system loads is suggested for low rise buildings with exterior masonry walls reinforced vertically. See Table 12.12.1 of ASCE 7 (ref. 3) for the allowable story drift for seismic loading. Most reinforced masonry walls for metal buildings are designed to span vertically, supported by a steel spandrel at the top and by the foundation at the bottom.

WALL BASE

Because of stiffness and deformation incompatibilities between flexible steel and rigid masonry assemblies, and consequently to control the location of cracking in the masonry walls that may result from relatively larger steel frame deflections at the top of the structure, a “hinge” can be incorporated at the base of the masonry assembly to allow out-of-plane rotation.

Two such hinge connections are shown in Figures 2 and 3. The construction shown in Figure 2 uses through-wall flashing to break the bond at the base of the wall providing a simply supported condition allowing shear transfer but no moment for out-of-plane loading. In many cases the shear force can be adequately transferred by friction through the flashed bed joint. However, it is recommended that a positive shear connection be provided by extending foundation dowels across the joint. It is recommended that the number of bars extended across the horizontal joint be minimized, and that the extension be limited to 2 in. (51 mm), to ensure that the joint will behave as assumed. Therefore, every vertical bar otherwise required for strength at critical sections does not necessarily need to be extended through the joint.

Masonry shear walls are very strong and stiff and are often used to resist lateral loads. However, masonry wall sections used as shear wall segments must have vertical reinforcement continuous into the foundation as shown in Figure 3. Flashing is also incorporated at the floor level to allow the wall some out-of-plane rotation due to building drift. Design aids are included in Concrete Masonry Walls for Metal
Building Systems (ref. 1) for inplane and out-of-plane reinforced masonry walls as well as for lintels and anchor bolts. Appendix C also presents design examples. As shown in Figure 4, these walls normally span vertically and are laterally supported by a spandrel at the top of the masonry portion of the wall.

When the masonry is designed with a base hinge, it is important to properly detail the building corners to accommodate the movements.

A vertical isolation joint should be placed near the building corner and proper consideration should be given to the masonry and steel connections at corner columns. Flexible anchors and/or slotted connections should be used.

WAINSCOT WALLS

Although full height masonry walls provide the most benefit particularly when the masonry is used for shear walls, partial-height walls, or wainscots, are sometimes used. These walls are commonly 4 to 10 ft (1.2 to 3.0 m) high with metal panel walls extending from the top of the masonry to the roof. The masonry provides strength and
impact resistance for the portion of the wall most susceptible to damage.

COLUMN DETAIL

Figure 5 shows the connection of a rigid frame column to concrete masonry sidewalls with a coincident vertical control joint. The details show vertically adjustable column anchors connecting the wall to the column. For walls designed to span vertically, it is good practice to provide a nominal number of anchors connecting the wall to the column to add stiffness and strength to the edge of the wall. If rigid enough, these anchors can assist in laterally bracing the outside column flange. For larger lateral loads, more substantial connections may be required. Anchorage to end wall columns is very similar.

SPANDREL DETAIL

A typical spandrel detail is shown in Figure Spandrels should be placed as high as possible to reduce the masonry span above the spandrel, especially on walls with parapets. Depending on the rigid frame configuration used, rigid frame connection plates and diagonal stiffeners may restrict the spandrel location. The spandrel is designed by the metal building manufacturer. If the inner flange of the spandrel needs to be braced, the metal building manufacturer will show on the drawings where the braces are required along with the information needed for the masonry engineer to design them and their anchorage to the wall.

Shim plates should be used at spandrel/masonry connections to allow for camber in the spandrel and other construction tolerances (see Figure 6). The steel spandrel should never be pulled to the masonry wall by tightening the anchor bolts.

CONSTRUCTION SEQUENCE

Typically, construction of metal buildings with concrete masonry walls proceeds as follows: concrete footing and column placement; concrete masonry foundation wall construction to grade; concrete slab placement; steel erection; and concrete masonry wall construction. Note, however, that this sequence may need to be modified to meet the needs of a particular project. For example, this construction sequence changes when loadbearing end walls are used. In this case, the steel supported by the masonry is erected after the masonry wall is in place.

Coordination between the various trades is essential for efficient construction. Preconstruction conferences are an excellent way for contractors and subcontractors to coordinate construction scheduling and to avoid conflicts and delays.

REFERENCES

  1. Concrete Masonry Walls for Metal Building Systems, CMU-MAN-003-11. Concrete Masonry & Hardscapes Association, Metal Building Manufacturers Association, International Code Council, 2011.
  2. Serviceability Design Considerations for Steel Buildings, AISC Steel Design Guide #3. American Institute of Steel Construction, 2003.
  3. Minimum Design Loads for Buildings and Other Structures, ASCE 7-05. American Society for Civil Engineers, 2005.