Resources

Fire Resistance Ratings of Concrete Masonry Assemblies

September 2020

INTRODUCTION

Concrete masonry is widely specified for fire walls and fire barriers because concrete masonry is noncombustible, provides durable fire resistance, and is economical to construct. Chapter 7 of the International Building Code (IBC) (ref. 2) governs materials and assemblies used for structural fire resistance and fire-rated separation of adjacent spaces. This TEK is based on the provisions of Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies, ACI 216.1/TMS 216 (ref. 1) , which outlines a procedure to calculate the fire resistance ratings of concrete masonry assemblies. The 2014 edition of the ACI 216.1/TMS 216 is referenced in the 2015 IBC for concrete and masonry materials. This TEK is based on both prescriptive details and tables as well as the calculated fire resistance procedure, which is practical, versatile and economical. The calculation procedure allows the designer virtually unlimited flexibility to incorporate the excellent fire-resistive properties of concrete masonry into a design. Included are methods for determining the fire resistance rating of concrete masonry walls, columns, lintels, beams, and concrete masonry fire protection for steel columns. Also included are assemblies composed of concrete masonry and other components, including plaster and gypsum wallboard finishes, and multi-wythe masonry components including clay or shale masonry units.

METHODS OF DETERMINING FIRE RESISTANCE RATINGS

Because full-scale fire testing of representative test specimens is not practical in daily practice due to time and financial constraints, the IBC outlines multiple options for fire rating determination:

standardized calculation procedures, such as those in the ACI 216.1/TMS 216 and in Section 722 of the IBC;
prescriptive designs such as those in Section 721 of the IBC;
engineering analysis based on a comparison with tested assemblies;
third party listing services, such as Underwriters Laboratory; and
alternative means approved by the building official per Section 104.11 of the IBC.

Of these, the calculation method is an economical and commonly used method of determining concrete masonry fire resistance ratings. The calculations are based on extensive research, which established relationships between the physical properties of materials and the fire resistance rating. The calculation method is fully described in ACI 216.1/TMS 216 and IBC Section 722, and determines fire resistance ratings based on the equivalent thickness of concrete masonry units and the aggregate types used to manufacture the units. Private commercial listing services allow the designer to select a fire rated assembly that has been previously tested, classified and listed in a published directory of fire rated assemblies. The listing service also monitors materials and production to verify that the concrete masonry units are and remain in compliance with appropriate standards, which usually necessitates a premium for units of this type. The system also is somewhat inflexible in that little variation from the original tested wall assembly is allowed, including unit size, shape, mix design, constituent materials, and even the plant of manufacture. More information on listing services for fire ratings is provided in CMU-FAQ 015-23 (ref. 16).

For prescriptive designs, the IBC provides a series of tables that describes requirements of various assemblies to meet the fire resistance ratings specified. The last two options listed above require justification to the building official that the proposed design is at least the equivalent of what is prescribed in the code.

CALCULATED FIRE RESISTANCE RATINGS

Background

The calculated fire resistance method is based on extensive research and testing of concrete masonry walls. Fire testing of wall assemblies is conducted in accordance with Standard Test Methods for Fire Tests of Building Construction and Materials, ASTM E119 (ref. 3), which measures four performance criteria, as follows:

resistance to the transmission of heat through the wall assembly;
resistance to the passage of hot gases through the wall, sufficient to ignite cotton waste;
load-carrying capacity of loadbearing walls; and
resistance to the impact, erosion and cooling effects of a hose stream on the assembly after exposure to the standard fire.

The fire resistance rating of concrete masonry is typically governed by the heat transmission criteria. From the standpoint of life safety (particularly for fire fighters) and reuse, this failure mode is certainly preferable to a structural collapse endpoint, characteristic of many other building materials.

The calculated fire resistance rating information presented here is based on the IBC and ACI 216.1/TMS 216 (refs. 1, 2).

Equivalent Thickness

Extensive testing has established a relationship between fire resistance and the equivalent solid thickness of concrete masonry walls, as shown in Table 1. Equivalent thickness is essentially the solid thickness that would be obtained if the volume of concrete contained in a hollow unit were recast without core holes (see Figure 1). The equivalent thickness is determined in accordance with Standard Methods of Sampling and Testing Concrete Masonry Units, ASTM C140 (ref. 4), and is reported on the C140 test report. If the equivalent thickness is unknown, but the percent solid of the unit is, the equivalent thickness of a hollow unit can be determined by multiplying the percent solid by the unit’s actual thickness.

The equivalent thickness of a 100% solid unit or a solid grouted unit is equal to the actual thickness. For partially grouted walls where the unfilled cells are left empty, the equivalent thickness for fire resistance rating purposes is equal to that of an ungrouted unit. For partially grouted walls with filled cells, see the following section. Loadbearing units conforming to ASTM C90 (ref. 5) that are commonly available include 100% solid units, 75% solid units, and hollow units meeting minimum required face shell and web dimensions. Typical equivalent thickness values for these units are listed in Table 2.

Filling Cells with Loose Fill Material

If all cells of hollow unit masonry are filled with an approved material, the equivalent thickness of the assembly is the actual thickness. This also applies to partially grouted concrete masonry walls where all ungrouted cells are filled with an approved material.

Applicable fill materials are: grout, sand, pea gravel, crushed stone, or slag that comply with ASTM C33 (ref. 6); pumice, scoria, expanded shale, expanded clay, expanded slate, expanded slag, expanded fly ash, or cinders that comply with ASTM C331 (ref. 7); perlite meeting the requirements of ASTM C549 (ref. 8); or vermiculite complying with C516 (ref. 9).

Wall Assembly Fire Ratings

The fire resistance rating is determined in accordance with Table 1 utilizing the appropriate aggregate type used in the masonry unit and the equivalent thickness.

Units manufactured with a combination of aggregate types are addressed by footnote C, which may be expressed by the following equation (see also the blended aggregate example, below):

Blended aggregate example:

The required equivalent thickness of an assembly constructed of units made with expanded shale (80% by volume), and calcareous sand (20% by volume), to meet a 3-hour fire resistance rating is determined as follows. From Table 1:

Multi-Wythe Wall Assemblies

The fire resistance rating of multi-wythe walls (Figure 2) is based on the fire resistance of each wythe and the air space between each wythe using the following equation:

For multi-wythe walls of clay and concrete masonry, use the values in Table 3 for the brick wythe in the above equation.

Reinforced Concrete Masonry Columns

Concrete masonry column fire testing evaluates the ability of the column to carry design loads under standard fire test conditions. Based on a compendium of fire tests, the fire resistance rating of reinforced concrete masonry columns is based on the least plan dimension of the column as indicated in Table 4. The minimum required cover over the vertical reinforcement is 2 in. (51 mm).

Concrete Masonry Lintels

Fire testing of concrete masonry beams and lintels evaluates the ability of the member to sustain design loads under standard fire test conditions. This is accomplished by ensuring that the temperature of the tensile reinforcement does not exceed 1,100°F (593°C) during the rating period. The calculated fire resistance rating of concrete masonry lintels is based on the nominal thickness of the lintel and the minimum cover of longitudinal reinforcement (see Table 5). The cover requirements protect the reinforcement from strength degradation due to excessive temperature during the fire exposure period. Cover requirements may be provided by masonry units, grout, or mortar. Note that for 3 and 4 hour requirements, not enough cover is available for 6-in. (152 mm) masonry; however, if a special analysis indicates that the reinforcement is not necessary or not needed, such as when conditions for arching action are present, the cover requirements may be waived. See TEK 17-01D (ref. 11) for lintel design and conditions for arching action.

Control Joints

Figure 3 shows control joint details in fire-rated wall assemblies in which openings are not permitted or where openings are required to be protected. Maximum joint width is 1/2 in. (13 mm). Although these details are not directly in the IBC, they are included by reference in ACI 216.1/TMS 216.

In addition to these prescriptive fire resistance rated control joints, other control joints may be permitted in fire rated masonry walls. For example, the IBC and ACI 216.1/1/TMS 216 include provisions for ceramic fiber joint protection for precast panels, which are similar to concrete masonry walls in that both rely on concrete for fire protection, and both are governed by the ASTM E119 heat transmission criteria (see Figure 4). The first two categories of aggregate types in Table 1 would correspond to the carbonate or siliceous aggregate concrete curve and the last two aggregate categories of Table 1 would correspond to the semi-lightweight or lightweight concrete curve. For example, for an 8-in. (203-mm) limestone aggregate concrete masonry wall with a maximum control joint width of 1/2 in. (13 mm), a 1 in. (25 mm) thickness (measured perpendicular to the face of the wall) of ceramic fiber in the joint can be used in walls with fire resistance ratings up to 3 hours, while a 2 in. (51 mm) thickness can be used in the joints of a 4-hour wall.

Steel Columns Protected by Concrete Masonry

Fire testing of a steel column protected by concrete masonry evaluates the structural integrity of the steel column under fire test conditions, by measuring the temperature rise of the steel. The calculated fire resistance rating of steel columns protected by concrete masonry, as illustrated in Figure 5, is determined by:

Effects of Finish Materials on Fire Resistance Ratings

In many cases, drywall, plaster or stucco finishes are used on concrete masonry walls. While finishes are normally applied for architectural reasons, they can also provide additional fire resistance. The IBC and ACI 216.1/TMS 216 include provisions for calculating the additional fire resistance provided by these finishes.

Note that when finishes are used to achieve the required fire rating, the masonry alone must provide at least one- half of the total required rating and the contribution of the finish on the non-fire-exposed side cannot be more than one-half of the contribution of the masonry alone. This is to assure structural integrity during a fire. The finish material must also be continuous over the entire wall.

Certain finishes deteriorate more rapidly when exposed to fire than when they are on the non-fire side of the wall. Therefore, two separate tables are required. Table 7 applies to finishes on the non fire-exposed side of the wall, and Table 8 applies to finishes on the fire-exposed side. For finishes on the non-fire exposed side of the wall, the finish is converted to an equivalent thickness of concrete masonry by multiplying the finish thickness by the factor given in Table 7. The result, Tef, is then added to the concrete masonry wall equivalent thickness, Te, and used in Table 1 to determine the wall’s fire resistance rating (i.e., the equivalent thickness of concrete masonry assemblies, Tea = Te Tef).

For finishes on the fire-exposed side of the wall, a time (from Table 8) is assigned to the finish. This time is added to the fire resistance rating determined for the base wall and nonfire-exposed side finish, if any. The times listed in Table 8 are essentially the length of time the various finishes will remain intact when exposed to fire (i.e., on the fire-exposed side of the wall).

When calculating the fire resistance rating of a wall with finishes, two calculations are performed, assuming each side of the wall is the fire exposed side. The fire rating of the wall assembly is the lower of the two. Typically, for an exterior wall with a fire separation distance greater than 5 ft (1,524 mm), fire needs be considered on the interior side only

Installation of Finishes

Finishes that contribute to the total fire resistance rating of a wall must meet certain minimum installation requirements. Plaster and stucco are applied in accordance with the provisions of the building code without further modification. Gypsum wallboard and gypsum lath are to be attached to wood or metal furring strips spaced a maximum of 16 in. (406 mm) o.c., and must be installed with the long dimension parallel to the furring members. All horizontal and vertical joints must be supported and finished.

UNCONVENTIONAL AGGREGATES

In recent years, manufacturers of concrete masonry products have been exploring the use of alternative materials in the production of concrete masonry units. Some of these materials have not been evaluated using standardized fire resistance test methods or have been evaluated only to a limited degree. Such unconventional materials, which are typically used as a replacement for conventional aggregates, may not be covered within existing codes and standards due to their novelty or proprietary nature.

While test methods such as ASTM E119 define procedures for evaluating the fire resistance properties of concrete masonry assemblies, including those constructed using unconventional constituent materials, there has historically been no defined procedure for applying the results of ASTM E119 testing to standardized calculation procedures available through ACI 216.1/TMS 216. To provide consistency in applying the results of full scale ASTM E119 testing to established calculation procedures, CMHA has developed CMU-FAQ-013-23 (Ref. 15).

This guideline stipulates that when applying the fire resistance calculation procedure of ACI 216.1/TMS 216 to products manufactured using aggregate types that are not listed in ACI 216.1/TMS 216, at least two full scale ASTM E119 tests must be conducted on assemblies containing the unconventional material. Based on the results of this testing, an expression can be developed in accordance with this industry practice that permits the fire resistance of units produced with such aggregates to be calculated for interpolated values of equivalent thickness and proportion of non listed aggregate.

REFERENCES

Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies, ACI 216.1- 14/TMS216-14. American Concrete Institute and The Masonry Society, 2014.
International Building Code 2015. International Code Council, 2015.
Standard Test Methods for Fire Tests of Building Construction and Materials, ASTM E119-16a. ASTM International, Inc., 2016.
Standard Methods for Sampling and Testing Concrete Masonry Units, ASTM C140-16. ASTM International, Inc., 2016.
Standard Specification for Loadbearing Concrete Masonry Units, ASTM C90-16. ASTM International, Inc., 2016.
Standard Specification for Concrete Aggregates, ASTM C33-16e1. ASTM International, Inc., 2016.
Standard Specification for Lightweight Aggregates for Concrete Masonry Units, ASTM C331-14. ASTM International, Inc., 2014.
Standard Specification for Perlite Loose Fill Insulation, ASTM C549-06(2012). ASTM International, Inc., 2012.
Standard Specification for Vermiculite Loose Fill Thermal Insulation, ASTM C516-08(2013)e1. ASTM International, Inc., 2013.
Steel Column Fire Protection, TEK 07-06A. Concrete Masonry & Hardscapes Association, 2009.
ASD of Concrete Masonry Lintels Based on the 2012 IBC/2011 MSJC, TEK 17-01D. Concrete Masonry & Hardscapes Association, 2011.
Standard Specification for Concrete Building Brick, ASTM C55 14a. ASTM International, Inc., 2014.
Standard Specification for Calcium Silicate Brick (SandLime Brick), ASTM C73-14. ASTM International, Inc., 2014.
Standard Specification for Prefaced Concrete and Calcium Silicate Masonry Units, ASTM C744-16. ASTM International, Inc., 2016.
How is the fire resistance of a concrete masonry assembly calculated when using unconventional aggregates?, CMU-FAQ-013-23. Concrete Masonry & Hardscapes Association, 2023.
What is the difference between fire resistance ratings for masonry assemblies obtained through IBC versus a listing service such as UL or FM?, CMU-FAQ-015-23. Concrete Masonry & Hardscapes Association, 2023.

Evaluating Fire-Exposed Concrete Masonry Walls After a Fire

August 2018

INTRODUCTION

Fire safety requires that a wall not only halt the spread of fire from one area to another, but also retain its structural integrity throughout the fire and fire-fighting operations. If occupants, firefighters and building contents are to be fully protected, the structure must not collapse, add fuel to the fire nor emit toxic gases during the fire.

Concrete masonry fire walls provide maximum safety during and after severe fire exposure. Because concrete masonry is a noncombustible structural material which neither adds fuel to a fire nor emits toxic gases, it is widely used to provide compartmentation—containing a fire until it can be brought under control by fire fighters. In addition, even after severe fires, concrete masonry walls can typically be repaired by simply patching cracks and tuckpointing mortar joints, rather than requiring demolition and replacement. Experience with building fires has shown that the most damage to concrete masonry walls during a fire often occurs due to lost support rather than as a direct result of fire on the masonry.

This TEK provides general information on assessment methods and repair techniques and discusses what can be expected after concrete masonry walls have been subjected to fire.

EVALUATING FIRE-EXPOSED WALLS

Preliminary Inspection

After a fire occurs, a preliminary inspection should be conducted as soon as possible to assess: the condition of the structure, the type and severity of problems observed in the affected area(s), the feasibility of rehabilitation and the need for conducting a detailed investigation. After collecting data on the building structure and the fire event, the preliminary investigation should take place as soon as safe entry into the building can be arranged.

The first step in the preliminary investigation is a visual inspection of structural members in the fire-affected areas. Indications of cracking, spalling, deflections, distortions, misalignment of elements and/or exposure of steel reinforcement should be documented. Measurements of deflections, deformations and geometry can be taken of any suspect members for comparison to unexposed members in the same structure. These observations should be recorded, documenting the type of damage and its severity for each affected member. This summary helps identify damaged members in need of more detailed investigation, as well as the extent and nature of any necessary repairs.

As an adjunct to visually assessing the structural members in fire affected areas, the building contents in these areas should be observed. The melting points of various materials (see Table 1) indicate the temperature ranges that have occurred in localized areas, providing an estimate of the maximum temperatures achieved during the fire. These estimated maximum temperatures help establish the severity of the fire relative to the Standard Test Methods for Fire Tests of Building Construction and Materials, ASTM E 119 (ref. 2) fire test, or to another recognized baseline. If the maximum temperatures during the fire are similar to those of the E 119 test, the potential damage to the concrete masonry is somewhat predictable, based on the history of E 119 testing on concrete masonry.

The ASTM E 119 fire test time/temperature protocol is shown in Figure 1.

There is a large body of data on concrete masonry walls tested according to the ASTM E 119 protocol. This test method evaluates walls subjected to the standard test fire. Performance criteria include: resistance to temperature rise on the unexposed side of the wall; resistance to the passage of hot gases or flames through the wall; structural stability during the test; and resistance of the masonry to deterioration under direct exposure to a fire hose stream immediately following the fire test. Research has shown that the fire resistance ratings of concrete masonry walls are invariably determined by the temperature rise on the cold (unexposed) side of the wall.

Field Testing Procedures

Part of the preliminary inspection is determining the need for further testing and evaluation. Nondestructive field tests, such as the use of an impact rebound hammer, are typically not used with concrete masonry, as the hollow cells interfere with obtaining meaningful results in many cases. In addition, extensive field testing is not always prudent, as removal and replacement of the fire-damaged element can sometimes be more economical than extensive testing. A solid understanding of both structural engineering and the effects of fire on building materials is invaluable to this decision-making process. When necessary, destructive test methods can be used to assess the strength of the in-situ concrete masonry (see reference 3). However, nonuniform fire damage on opposite sides of the wall and corresponding strength differences can lead to unreliable results. In most cases, strength testing is unnecessary.

ASSESSING THE CONCRETE MASONRY

In general, structural concrete masonry without excessive deformations, deflections, misalignments or large extensive cracks can typically be repaired rather than replaced. When these types of conditions are present, it indicates that the member’s load-carrying capacity may be impaired, which would require removal and replacement of the suspect members.

Fire distress such as soot and smoke deposits, pitting of aggregates, hairline cracks, shallow spalling and other surface damage generally require only cosmetic repairs. The following sections provide some more detailed guidance on assessing various concrete masonry characteristics after a fire.

Cracks

Cracks wider than about 1/16 in. (1.6 mm) should be further investigated to determine the potential structural impact. When the reinforcing steel in conventionally-reinforced masonry has not been exposed, the load-carrying capacity of the wall can typically be assumed to be relatively unaffected (see also Exposed Reinforcement, below).

Mortar Joint Damage

Mortar joints often appear to be more affected by fire exposure than the adjacent surface of the masonry units. When concrete masonry walls are subjected to a fire hose immediately after fire exposure in ASTM E 119 testing, mortar in the dehydrated state is sometimes flushed out, typically to a depth of about 1/4 in. (6.7 mm). In actual fires, mortar joints subjected to the most severe fire exposures can become softened or chalky, although this damage is typically not deeper than about 3/4 in. (19 mm). However, this loss of mortar does not affect the load-carrying ability of the concrete masonry wall (ref. 4), so can most often be adequately repaired by tuckpointing.

Exposed Reinforcement

Reinforcement exposed during or after a fire must be evaluated for quenching, buckling and/or loss of prestress. The investigator must consider that any exposed steel may have been quenched during fire fighting operations. This rapid cooling causes a loss of ductility in the steel that can reduce the load-carrying capacity of the member. A visual inspection of any exposed structural reinforcement can help asses the potential structural damage. This assessment must be tied to the element under consideration: either a conventionally-reinforced wall or prestressed wall, as follows. In a conventionally-reinforced wall, local buckling of exposed reinforcing bars usually indicates that the steel has been directly exposed to fire. When steel is exposed to temperatures of 1,100 o F (593 o C) or higher, the bars lose about half of their yield strength and buckling occurs. If the bars are exposed but not buckled or otherwise deformed, spalling may have occurred after the fire exposure. In general, flexural reinforcement that is not visibly deformed is unlikely to have suffered significant permanent damage. Similarly, if the spalling does not expose the reinforcement, i.e. the cover protection remains intact, the wall strength is unlikely to be compromised.

In prestressed concrete masonry walls, on the other hand, significant loss of prestress can occur without any visible distress to exposed tendons. Therefore, for prestressed masonry, any exposed prestressing tendons should indicate the need for a more in-depth structural evaluation. Tendon buckling is rarely observed, as the tendon typically remains in tension, even with significant loss of prestress.

EFFECT OF FIRE EXPOSURE ON WALL STRENGTH—EXPERIMENTAL RESULTS

One effect of fire exposure, as determined by testing (ref. 4), was reduced wall compressive strength due to the gradual dehydration of the cement and, depending on the aggregate type, to the expansion and changes in the physical properties of the aggregate used in the concrete masonry units. Reductions in compressive strength for 8-in. (203-mm) units exposed to 3 to 3 1/2 hours of fire varied widely, resulting in maximum reductions of 50 percent for some types of concrete masonry units. Lightweight aggregates, manufactured by expanding certain minerals in a kiln, are stable under fire exposure, so minimize loss of strength. During testing, limestone aggregate concrete masonry units also showed substantial stability and minimized loss of strength after fire exposure (ref. 4). For the wide range of masonry units tested, the wall strength after fire exposure remained directly proportional to the concrete masonry unit compressive strength before fire exposure.

A number of 8-in. (203-mm) walls underwent 2 1/2 to 3 1/2 hours of fire exposure, were cooled, then subjected to another 2 1/2 hour fire before being tested for compressive strength. These results showed that these walls were able to carry the same, or slightly higher, loads as similar walls exposed once for three to four hours, as well as serving as an effective fire barrier during the second fire.

PREPAIRING FIRE-EXPOSED CONCRETE MASONRY

For fire-exposed concrete masonry free from large cracks or deflections, repairs should be minimal. Crack repair and mortar joint tuckpointing procedures and recommendations are covered in detail in Maintenance of Concrete Masonry Walls, TEK 08-01A (ref. 5). Recommended cleaning procedures are covered in Cleaning Concrete Masonry, TEK 08-04A (ref. 6).

SUMMARY

In conventionally-reinforced concrete masonry, if reinforcing steel is not exposed, there is little likelihood of structural damage.
Lintels and beams free from excessive deflections are unlikely to be structurally impaired.
Softening of the top surface of mortar results in little loss of load carrying capacity and can be easily repaired by tuckpointing.
Walls subjected to fire one time without structural damage can be expected to perform just as well in a second fire.
Field tests are typically not conducted to assess fire damaged concrete masonry walls. Post-fire investigation typically consists only of visual inspection.
If no severe distortion, cracking or displacement of concrete masonry walls is present, complete reinstatement of the wall can usually be accomplished by patching cracks and tuckpointing mortar joints.

REFERENCES

Assessing the Condition and Repair Alternatives of Fire-Exposed Concrete and Masonry Members. National Codes and Standards Council of the Concrete and Masonry Industries, August, 1994.
Standard Test Methods for Fire Tests of Building Construction and Materials, ASTM E 119-05. ASTM International, 2005.
Evaluating Existing Concrete Masonry Construction, TEK 18-09A. Concrete Masonry & Hardscapes Association, 2003.
Menzel, Carl A. Tests of the Fire Resistance and Strength of Walls of Concrete Masonry Units. Portland Cement Association, January, 1934.
Maintenance of Concrete Masonry Walls, TEK 08-01A. Concrete Masonry & Hardscapes Association, 2004.
Cleaning Concrete Masonry, TEK 08-04A. Concrete Masonry & Hardscapes Association, 2005.

Balanced Design Fire Protection for Multifamily Housing

August 2018

INTRODUCTION

The application of balanced design principles is particularly important in multifamily residential buildings. The risk of fires in apartments and condominiums is high because of the numerous kitchens, furnaces, hot water heaters, and other causes of residential fires. In addition, there is always a potential for sleeping occupants, children, and others who may be at a disadvantage for evacuating quickly. Fire safety in multifamily housing requires an understanding of the hazards involved, so that both the potential for fire and the threat to life-safety during a fire are minimized. The purpose of balanced design is to protect life and property during a fire. Death and injury from fire is caused by asphyxiation from toxic smoke and fumes; burns from direct exposure to the fire; heart attacks caused by stress and exertion; and impact due to structural collapse, explosion and falls. Life safety in multifamily housing is influenced by the design of the building, its fire protection features, and by the materials, building contents, quality of construction, and maintenance.

BALANCED DESIGN

Balanced design relies on three complementary life safety systems to reduce the risk of death due to fire:

a detection system to warn occupants of a fire
a containment system to limit the extent of fire and smoke
an automatic suppression system to control the fire until it can be extinguished.

Each of these essential systems contributes to lowering the risk of death and injury from fire in multifamily housing. The three balanced design components complement each other by providing fire protection features that are not provided by the other components. Some features of each balanced design component are intended to be redundant, so that in the event that one system is breached, or fails to perform, the other components continue to provide safety.

Although not a tangible element in fire protection, a strong education program should be an integral part of any good fire protection plan in addition to the physical components of a balanced design system.

DETECTION

Accurate early warning is the first line of defense against the slow smoldering fire, typical in dwelling units with a low heat release rate that doesn’t activate a sprinkler head. Detectors that respond to light smoke are important from a life-safety standpoint to alert occupants so they can evacuate.

Other detection or alarm systems may be used to notify the fire department, thus decreasing response time, expediting rescue operations, and limiting the resulting fire spread and property damage. Detectors wired to a central alarm and installed in corridors and common areas notify all building occupants, permit timely and orderly evacuation, and decrease the potential for injury and death.

The National Fire Protection Association (NFPA) National Fire Alarm Code, NFPA 72 (ref. 3), covers minimum performance, location, installation, testing and maintenance of detectors. The Standard includes heat, smoke, flame and gas sensing fire detectors.

The most common detector used in multifamily housing is the smoke sensing fire detector. Detectors should be wired into a continuous power supply. Their location in multifamily housing is determined in accordance with the building code.

Each dwelling unit should be equipped with detectors in all sleeping rooms and in areas adjacent to all sleeping rooms, and on each level of the building, including the basement. The amount of air movement and obstructions within the space, such as partitions, ceiling height, and other factors, will guide the fire protection engineer in the proper selection of the detector location.

The performance of detectors is vulnerable to unpredictable malfunctions, among which are lack of maintenance due to human error and neglect, faulty power supply, and even acts of sabotage by arsonists. Young children, the disabled, elderly, and the deaf may not be able to respond to auditory alarms.

Tests on smoke detectors (ref. 4) indicate the need for a regularly scheduled maintenance and testing program, as well as the periodic replacement of some components.

SUPPRESSION

The function of automatic sprinkler systems is to control a fire at the point of origin. While not designed to extinguish a fire, residential sprinklers have been shown to be very reliable and effective in controlling a fire in the room of origin until it can be extinguished. Sprinklers reduce the likelihood of flashover. Flashover, the rapid ignition of volatile gasses, is particularly hazardous in exit corridors. Suppression of a fire allows access to the building to permit rescue and fire fighting efforts to proceed. Sprinklers are credited with preventing multiple deaths in fires.

The NFPA maintains minimum standards for the design and installation of sprinkler systems. Sprinkler systems for multifamily housing more than four stories in height are covered by NFPA Standard 13 (ref. 1). NFPA Standard 13R (ref. 2) covers the installation of sprinkler systems in multifamily housing up to and including four stories in height. When the interior construction or building contents contain a large amount of combustibles, sprinkler systems should meet the NFPA 13 Standard, regardless of height, to ensure protection in attics, closets, and other concealed spaces built with combustible materials, and to provide additional suppression in all areas due to the higher fuel loadings.

The NFPA Standards cover the design, installation, acceptance testing, and maintenance of sprinkler systems. In addition, there are specific spaces which are not required by NFPA 13R to be sprinklered. The Standards require an adequate water supply and a piping system designed to deliver sufficient water to the sprinkler head. Sprinkler head requirements ensure proper water coverage based on the room dimensions, area to be covered, and fuel loading. The Standards also list exceptions for specific spaces which are not required to be sprinklered. Once installation is complete, the Standards require inspection and acceptance of the system’s piping, valves, pumps and tanks. Testing also includes verification of adequate water flow to the sprinkler heads. After the sprinkler system is in use, it must be maintained; however, specific maintenance requirements and frequency of maintenance are not specified by the Standards.

Performance of automatic sprinklers can be vulnerable to system failures due to inadequate maintenance and inspection, or inadequate water supply. Sprinklers are not intended to control electrical and mechanical equipment fires or fires of external origin, such as fires from adjacent buildings, trash fires, and brush fires. Fires in concealed spaces, including some attics, closets, flues, shafts, ducts, and other spaces where sprinkler heads are not required to be installed, can compromise life safety due to the spread of toxic fumes and smoke. Inadequate water supply can occur due to low pressure in the municipal water system, broken pipes due to earthquakes or excavation equipment, explosions, freezing temperatures, closed valves due to human error, arson or vandalism, corrosion of valves, pump failure due to electrical outage, and lack of system maintenance.

COMPARTMENTATION

Compartmentation contains a fire until it can be brought under control by fire fighters as illustrated in Figure 1. Compartmentation limits the extent of fire by dividing a building into fire compartments enclosed by fire walls or fire separation wall assemblies, and by fire rated floors and ceilings. Compartments also minimize the spread of toxic fumes and smoke to adjacent areas of a building. Conflagrations beyond the fire compartment are prevented by limiting the total fuel load contributing to the fire. Compartmentation provides safe areas of refuge for handicapped, young, elderly, incapacitated and other occupants who may not be capable of unassisted evacuation. Compartmentation also provides safe areas of refuge for extended periods, where evacuation is precluded due to smoke filled exit ways or blocked exits. Compartmented construction provides protection for fire and rescue operations. Highly hazardous areas, such as mechanical, electrical or storage rooms, can be isolated by fire walls from other occupied areas of a building. Fire separation walls and floor/ceiling assemblies between dwelling units in multifamily housing afford protection from fires caused by the carelessness of other occupants.

Each compartment is enclosed by fire resistive components. Floor and wall elements forming the boundaries of each compartment should have a fire resistance rating of at least two hours, and should be constructed of noncombustible materials that are capable of preserving the structural integrity of the building throughout the duration of the fire.

Openings through compartment boundaries should be protected openings. Doors should be self closing when fire or smoke is detected. In multifamily housing, each dwelling unit should form a separate compartment. In addition, interior exit ways, as well as storage, electrical, and mechanical rooms, should be separate compartments. Exterior walls should be fire rated to form a barrier to the penetration of exterior fires and to contain interior fires.

The value of compartmentation could be reduced when joints between floors and walls, typically exterior curtain walls, or between walls and ceilings, are not properly fire-stopped. Damage caused by equipment, abuse, or the installation of utilities which are not properly sealed, can allow the passage of smoke and gas. Unsealed openings around penetrations can also allow the convective spread of smoke. Self-closing mechanisms on doors in compartment walls may fail if not maintained, or if blocked open.

PROPERTY PROTECTION

The initial cost of providing fire safety can be significant; however, balanced design offers advantages which offset initial costs. The higher level of protection for both the structure and its contents limits the potential loss due to fire. Immediate and long term savings will be reflected in lower insurance rates for both the building and its furnishings. Balanced design limits both fire and smoke damage of the building’s contents to the compartment of fire origin. Noncombustible compartment boundaries limit damage to the structure itself and reduce repair time following a fire. Repair is generally nonstructural, but may include the replacement of doors and windows; electrical outlets, switches and wiring; heating ducts and registers; and floor, wall and ceiling coverings.

SUMMARY

Fire protection engineering is as much an art as it is a science. The number of unknowns and potential fire propagation scenarios are numerous. Fire protection is, therefore, generally based more on risk assessment than precise calculation. Currently, building code prescriptive criteria, (ref. 5, 6) along with an understanding of the science of fire protection, guides the designer in addressing fire safety.

Some of the more significant fire safety issues requiring consideration are listed in Table 1, along with the effectiveness of each component of balanced design. As shown by the table, there may be more than one component which is considered effective in mitigating a particular hazard. Since none of the components are fail-safe, overlapping functions are needed to provide the required level of safety. In addition, there are some functions listed in the table which are addressed by only one component of balanced design. The appendix of NFPA Standard 13R (ref. 2) also recognizes the need for compartmentation and detection, along with sprinklers, to ensure a reasonable degree of life-safety protection.

There is a general agreement among the engineering and scientific communities that the desired approach to improving fire safety in buildings is through computer modeling. Limited success to date has been achieved in developing a system (ref. 7) that will assess risk and design protection levels for each of the three components of balanced design. Future efforts will be directed through the government and the private sector in a cooperative effort to develop this tool.

REFERENCES

Installation of Sprinkler Systems, NFPA 13, National Fire Protection Association, 2007.
Standard for the Installation of Sprinkler Systems in Residential Occupancies up to and Including Four Stories in Height, NFPA 13R, National Fire Protection Association, 2007.
National Fire Alarm Code, NFPA 72 National Fire Protection Association, 2007.
Why We Need to Test Smoke Detectors, Leon Cooper, Fire Journal, National Fire Protection Association, November 1986.
Life Safety Code, NFPA 101, National Fire Protection Association, 2006.
Standard on Types of Building Construction, NFPA 220, National Fire Protection Association, 2006.
Fire Hazard Assessment Method, Hazard I, NIST Handbook No. 146, National Institute of Standards and Technology, U.S. Department of Commerce, 1989.

Detailing Concrete Masonry Fire Walls

August 2018

INTRODUCTION

Concrete masonry, due to its inherent durability, reliability and superior fire resistance characteristics, is well suited to a range of fire protection applications.

The International Building Code (IBC) (ref. 1) defines three wall types for fire protection— fire wall, fire barrier and fire partition—depending on the level of protection provided for the type of occupancy and intended use. Of the three defined fire-rated assemblies, a fire wall is generally considered to provide the highest level of robustness and fire safety. As such, it is intended to provide complete separation and must be structurally stable under fire conditions.

Generally, fire barriers and fire partitions are required to provide the minimum protection necessary to assure that building occupants can evacuate a structure without suffering personal injury or loss of life. In addition to these requirements, fire walls reduce the likelihood of fire spread into the adjoining space, thus minimizing major property loss. Potentially significant architectural and economic advantages can be gained from using fire walls since each portion of a building separated by fire walls is considered a separate building for code compliance purposes.

Designing and detailing fire walls is a complex task with many facets, including structural criteria, fire resistance, vertical and horizontal continuity, and criteria for protecting openings and joints. It is beyond the scope of this TEK to include every code provision and exception for fire wall design for all project conditions. While much of the information in this TEK is applicable to both the IBC and the NFPA 5000 (ref. 2) building codes, the provisions are based on the 2003 IBC, so certain provisions may be different from NFPA 5000 requirements. Hence, the information may or may not conform to local building code requirements and should be carefully reviewed to ensure compliance. In addition, the details shown here are not the only ones that will comply, but are included as examples. Project specific needs will dictate the final detailing decisions.

FIRE WALLS

By Code (ref. 1), fire walls are required to have the minimum fire-resistance rating acceptable for the particular occupancy or use group which they separate and must also have protected openings and penetrations. A fire wall must have both vertical and horizontal continuity to ensure that the fire does not travel over, under or around the fire wall. In addition, the wall must have sufficient structural stability under fire conditions to remain standing for the duration of time indicated by the fire-resistance rating even with the collapse of construction on either side of the fire wall.

Fire-Resistance Rating

Because fire walls provide a complete separation between adjoining spaces, each portion of the structure separated by fire walls is considered to be a separate building. Fire walls in all but Type V construction must be constructed of approved noncombustible materials. Table 1 shows minimum required fire-resistance ratings. Information on determining the fire-resistance ratings of concrete masonry assemblies is contained in Fire Resistance Rating of Concrete Masonry Assemblies, TEK 07-01D and Standard Method for Determining Fire Resistance of Concrete and Masonry Construction Assemblies (refs. 3, 4).

Protected Openings and Penetrations

The IBC states that fire walls must have closures such as fire doors or shutters which automatically activate to secure the opening in the event of a fire. Further, openings in fire walls are restricted to a maximum size of 120 ft2 (11.2 m2). An exception permits larger openings provided both buildings separated by the fire wall are equipped throughout with automatic sprinkler systems. In all cases, the aggregate width of all openings at any floor level is limited to 25 percent of the wall length.

Through-penetrations in fire walls must utilize either fire-resistance rated assemblies or a firestop system which is tested in accordance with either ASTM E 814 (ref. 5) or UL 1479 (ref. 6). The annular space between steel, iron or copper pipes or steel conduits and surrounding concrete masonry fire walls may be filled with concrete, grout or mortar for the thickness required to provide a fire-resistance rating equivalent to the fire-resistance rating of the wall penetrated. In addition, the penetrating item is limited to a 6-in. (152-mm) nominal diameter and the opening is limited to 144 in.2 (92,900 mm2). Openings for steel electrical outlet boxes are permitted provided they meet the Code specified requirements.

Combustible members, such as wood, are permitted to be framed into concrete masonry fire walls provided that, when framed on both sides of the wall, there is at least 4 in. (102 mm) between the embedded ends of the wood framing. The full thickness of the fire wall 4 in. (102mm) above and below, as well as in between, the combustible member must be filled with noncombustible materials approved for fireblocking.

Voids created at the junction of walls and floor/ceiling/ roof assemblies must be protected from fire passage by using fireresistant joint systems tested in accordance with ASTM E 1966 or UL 2079 (refs. 7, 8). Control joints in fire walls must have fire-resistance ratings equal to or exceeding the required rating of the wall. Recommendations for locating and spacing control joints in concrete masonry walls also apply to concrete masonry fire walls. Crack Control Strategies for Concrete Masonry Construction, CMU-TEC 009-23 (ref. 9) includes control joint spacing criteria and illustrates control joint details for various fire-resistance ratings.

Vertical and Horizontal Continuity

The IBC mandates vertical continuity of a fire wall by requiring that the wall extend continuously from the foundation to a termination point at least 30 in. (762 mm) above both adjacent roofs. Exceptions permitting the fire wall termination at the underside of the roof deck or slab are listed in the Code. These exceptions require the use of Class B roof coverings (minimum), no openings within 4 ft (1.22 m) of the fire wall and other criteria for roof assembly protection. Buildings located over parking garages and stepped buildings are subject to additional requirements and permitted exceptions.

Horizontal continuity limits the spread of fire around the ends of a fire wall. The IBC requires that fire walls be continuous from exterior wall to exterior wall and that they extend at least 18 in. (457 mm) beyond the exterior surface of exterior walls. As with the vertical continuity requirements, there are criteria for terminating the fire wall at the interior surface of an exterior wall based on the types and fire resistance ratings of the intersecting wall constructions and on the presence of an automatic sprinkler system installed per Code requirements.

Structural Stability Under Fire Conditions

While concrete masonry remains structurally stable during the extreme temperatures experienced under fire conditions, steel framing undergoes a reduction in strength as the surrounding temperature and duration of exposure are increased. This decreased structural capacity is evidenced by a dramatic increase in the deflection and twisting of steel members. Wood framing may burn, collapse, shrink and/or deform under fire exposure and it too loses its load-carrying capability. For these reasons, concrete masonry firewalls should be designed and detailed to withstand any loading imposed by fire-compromised framing systems or detailed so that those loads are not imparted to the fire wall during a fire. This is perhaps the most difficult detailing provision in fire wall design.

DETAILING CONSIDERATIONS FOR STRUCTURAL STABILITY

Because most fire wall criteria relating to fire-resistance rating, protected openings and penetrations, and vertical and horizontal continuity are prescriptive, the designer’s primary challenge when engineering and detailing a concrete masonry fire wall relates to maintaining the structural stability of the wall under fire conditions.

There are various methods of designing, detailing and constructing fire walls for structural stability during a fire. Among the systems recommended for use as fire walls are: (a) cantilevered or freestanding walls, (b) laterally supported and tied walls, and (c) double wall construction.

Cantilevered or Freestanding Walls

Cantilevered walls (Figure 1) do not depend on the roof framing for structural support. The wall is cantilevered from the foundation by grouting and reinforcing, or by prestressing. Freestanding walls may also be designed to span horizontally between pilasters or masonry columns integral to the wall.

It can be difficult to design a cantilevered single wythe loadbearing fire wall. Thermal stresses may cause deformation in steel or wood joists or framing systems which can eccentrically load the top of the fire wall. Designing the wall to remain stable under that loading condition may be difficult especially for tall or slender walls. For this reason, cantilevered single wythe fire walls are often designed as nonbearing walls with the primary roof framing system running parallel to the fire wall. Column lines on either side of the wall support the roof framing. Details for cantilevered/freestanding fire walls are similar to those for laterally supported walls (shown in Figures 2, 3 and 4) with the exception that cantilevered walls do not include through-wall ties or break-away connectors.

Laterally Supported or Tied Walls

Laterally supported or tied walls rely on the building frame for lateral stability. The fire wall is laterally supported on each side by the framing system. As such, forces due to the collapse of the structure on one side of the fire wall are resisted by the structural framework on the other side of the wall. Adequate clearance, as listed in Table 2, between the framing and the concrete masonry fire wall is necessary to allow framing expansion or deformation without exerting undue pressure on the wall.

Laterally supported fire walls may utilize break-away connectors
manufactured with metals having melting points lower than structural steel (generally about 800° F (427° C)), so that, in the event of fire, the connectors on the fire side of the wall will give way before those on the non-fire side. In Figures 2 and 3, the structural diaphragm on the side of the wall opposite the fire provides the stability. The connections between the roof and wall must be designed to resist these forces. If the diaphragms occur at different elevations, the wall must be designed to withstand the anticipated flexural forces that will be generated as well. Figure 4 shows a laterally supported fire wall with combustible framing supported by metal joist hangers. Joist hanger manufacturers may have alternate details as well. Note that there may be code limitations on the use of combustible framing.

Figure 5 shows design and detailing options for tied fire walls. Tied fire walls are a type of laterally supported fire wall where the roof structure is not supported by the fire wall, but rather by the roof structure on the other side of the fire wall, thus the two roof structures are tied together across the fire wall. Figure 5a illustrates one choice for a “double column” detail which uses a through-wall tie to connect the primary steel on both sides of the fire wall. In this detail, the primary roof framing steel is parallel to the fire wall and supported on fireproofed columns. One column is used on each side of the fire wall to support the roof system for that building. Both steel columns and primary support beams/trusses should be aligned vertically and horizontally with the columns and beams/trusses on the opposite side of the wall and should be fireproofed. If the primary steel is not parallel to the fire wall Figure 5b shows a through-wall tie which can be used.

As an alternative to using two steel columns, Figure 5c shows one steel support column encased entirely within the concrete masonry fire wall. Fire protection requirements for steel columns are included in Steel Column Fire Protection, TEK 07-06A (ref. 11). This system creates a single column line tied at the top of the wall to horizontal roof framing. Detailing the connection of the steel beams to the concrete masonry fire wall varies based on the framing layout, but the wall must be supported at the top and the connection must be fire protected.

Double Wall Fire Wall

Double wall construction (Figure 6) is generally easy to design and detail for loadbearing conditions, especially for taller walls. It utilizes two independent concrete masonry walls side by side, each meeting the required fire-resistance rating. In the event one wall is pulled down due to fire, the other wall remains intact, preventing fire spread. Floor and roof connections to each fire wall are the same as for conventional concrete masonry construction. These walls are often cantilevered or freestanding but may be tied or laterally supported as well if so detailed and designed. This system is also easy to use when a building addition requires a fire wall between the existing structure and the new construction.

REFERENCES

International Building Code 2003. International Code Council, 2003.
Building Construction and Safety Code – 2003 Edition, NFPA 5000. National Fire Protection Association, 2003.
Fire Resistance Rating of Concrete Masonry Assemblies, CMHA TEK 07-01D. Concrete Masonry & Hardscapes Association, 2018.
Standard Method for Determining Fire Resistance of Concrete and Masonry Construction Assemblies, ACI 216.1-97/ TMS 0216-97.
American Concrete Institute and The Masonry Society, 1997.
Standard Test Method for Fire Tests of Through Penetration Fire Stops, ASTM E 814-02. ASTM International, 2002.
Fire Tests of Through-Penetration Firestops, UL
Underwriters Laboratory, 2003.
Standard Test Method for Fire-Resistive Joint Systems, ASTM E 1966-01. ASTM International, 2001.
Tests for Fire Resistance of Building Joint Systems, UL
Underwriters Laboratory, 2004
Crack Control Strategies for Concrete Masonry Construction, CMU-TEC-009-23. Concrete Masonry & Hardscapes Association, 2023.
Criteria for Maximum Foreseeable Loss Fire Walls and Space Separation, Property Loss Prevention Data Sheets 1-22. Factory Mutual Insurance Company, 2000.
Steel Column Fire Protection, CMHA TEK 07-06A. Concrete Masonry & Hardscapes Association, 2003.