April, 2007
Feature
Inspection News and Views from the American Society of Home Inspectors



The Effects of Fire on Structural Systems

U.S. DEPARTMENT OF HOUSING AND URBAN DEVELOPMENT

First published by the U.S. Department of Housing and Urban Development in 1984, the Rehabilitation Inspection Guide has been updated and expanded as part of the Partnership for Advancing Technology in Housing. Today it includes current techniques and standards, information about additional building materials, and a broader coverage of hazardous substances and the effects of earthquake, wind and floods. Home inspectors may find
articles of general interest, such as the one printed here.
To view the entire guide, visit www.huduser.org/publications/destech/inspection.

Building fires, which normally reach temperatures of about 1000 ºC, can affect the loadbearing capacity of structural bearing elements in a number of ways.

Apart from such obvious effects as charring and spalling, there can be a permanent loss of strength in the remaining material and thermal expansion may cause damage in parts of the building not directly affected by the fire.

In assessing fire’s effects, the main emphasis should be placed on estimating the residual load-carrying capacity of the structure and then determining the remedial measures, if any, needed to restore the building to its original design for fire resistance and other requirements. Obviously, if weaknesses in the original design are exposed, these should be corrected.

All building materials except timber are likely to show significant loss of strength when heated above 250 ºC, strength that may not recover after cooling. Thus, it is useful to estimate the maxi maximum temperature attained in a fire. Molded glass objects soften or flow at 700 or 800 ºC. Metals form drops or lose their sharp edges as follows: 300 to 350 ºC for lead, 400 ºC for zinc, 650 ºC for aluminum and alloys, 950 ºC for silver, 900 to 1000 ºC for brass, 1000 ºC for bronze, 1100 ºC for copper and 1100 to 1200 ºC for cast iron. There are also the well-known color changes in concrete or mortar.
The development of red or pink coloration in concrete or mortar containing natural sands or aggregates of appreciable iron oxide content occurs at 250 to 300 ºC and, nor normally, 300 ºC may be taken as the transition temperature. Table A-1 provides specifics.

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Making an analysis of the damage and assessment of the necessary repairs may be possible within a reasonable degree of accuracy, but final acceptance may depend on proof by a load test, where performance is generally judged in terms of the recovery of deflection after load removal.

1 – Timber


Timber browns at about 120 to 150 ºC, blackens around 200 to 250 ºC, and emits combustible vapors at about 300 ºC. Above a temperature of 400 to 450 ºC (or 300 ºC if a flame is present), the surface of the timber will ignite and char at a steady rate. Table A-2 shows the rate of charring.

Analysis and Repair
Generally, any wood that is not charred should be considered to have full strength. It may be possible to show by calculation that a timber section or structural element subjected to fire still has adequate strength once the char is removed. Where additional strength is required, it may be possible to add strengthening pieces. Joints that may have opened and metal connections that may have conducted heat to the interior are points of weakness that should be carefully examined.

tablea-2.gif


2 – Masonry


The physical properties and mechanisms of failure in masonry walls exposed to fire have never been analyzed in detail. Behavior is influenced by edge conditions and there is a loss of compressive strength as well as unequal thermal expansion of the two faces. For solid bricks, resistance to the effects of fire is directly proportional to thickness. Perforated bricks and hollow clay units are more sensitive to thermal shock. There can be cracking of the connecting webs and a tendency for the wythes to separate. In cavity walls, the inner wythe carries the major part of the load. Exterior walls can be subjected to more severe forces than internal walls by heated and expanding floor slabs. All types of brick give much better performance if plaster is applied, which improves insulation and reduces thermal shock.

Analysis and Repair
As with concrete, it is possible to determine the degree of heating of the wall from the color change of the mortar and bricks. For solid brick walls without undue distortion, the portion beyond the pink or red boundary may be considered serviceable and calculations should be made accordingly. Perforated and hollow brick walls should be inspected for the effects of cracks indicating thermal shock. Plastered bricks sometimes suffer little damage and may need repairs only to the plaster surfaces.

3 – Steel

The yield strength of steel is reduced to about half at 550 ºC. At 1000 ºC, the yield strength is 10 percent or less. Because of its high thermal conductivity, the temperature of unprotected internal steelwork normally will vary little from that of the fire. Structural steelwork is, therefore, usually insulated.

Apart from losing practically all of its load-bearing capacity, unprotected steelwork can undergo considerable expansion when sufficiently heated. The coefficient of expansion is 10-5 per degree Celsius. Young’s modulus does not decrease with temperature as rapidly as does yield strength.

Cold-worked reinforced bars, when heated, lose their strength more rapidly than do hot-rolled high-yield bars and mild-steel bars. The differences in properties are even more important after heating. The original yield stress is almost completely recovered on cooling from a temperature of 500 to 600 ºC for all bars but on cooling from 800 ºC, it is reduced by 30 percent for cold-worked bars and by 5 percent for hot-rolled bars.

The loss of strength for prestressing steels occurs at lower stressing temperatures than that for reinforcing bars. Cold-drawn and heat-treated steels lose a part of their strength permanently when heated to temperatures in excess of about 300 ºC and 400 ºC, respectively.

The creep rate of steel is sensitive to higher temperatures and becomes significant for mild steel above 450 ºC and for prestressing steel above 300 ºC. In fire resistance tests, the rate of temperature rise when the steel is reaching its critical temperature is fast enough to mask any effects of creep. When there is a long cooling period, however, as in prestressed concrete, subsequent creep may have some effect in an element that has not reached the critical condition.

Analysis and Repair
In general, a structural steel member remaining in place with negligible or minor distortions to the web, flanges,
or end connections should be considered satisfactory for further service. Exceptions are the relatively small number of structures built with cold-worked or tempered steel, where there may be permanent loss of strength.
This may be assessed using estimates of the maximum temperatures attained or by on-site testing. Where necessary, the steel should be replaced, although reinforcement with plates may be possible. Microscopy can be used to determine changes in microstructure. Since this is a specialized field, the services of a metallurgist are essential.

4 – Concrete

Concrete’s compressive strength varies not only with temperature but also with a number of other factors, including the rate of heating, the duration of heating, whether the specimen was loaded or not, the type and size of aggregate, the percentage of cement paste, and the water/cement ratio. In general, concrete heated by a building fire always loses some compressive strength and continues to lose it on cooling. However, where the temperature has not exceeded 300 ºC, most strength eventually is recovered.

Because of the comparatively low thermal diffusivity of concrete (of the order of 1 mm/s), the 300 ºC contour may be at only a small depth below the heated face. Concrete’s modulus of elasticity also decreases with temperature, although it is believed that it will recover substantially with time, provided that the coefficient of thermal expansion of the concrete is on the order of 10-5 per degree Celsius (but this varies with aggregate). Creep becomes significant at quite low temperatures, being of the orders of 10-4 to 10-3 per hour over the temperature range of 250 to 700 ºC, and can have a beneficial effect in relaxing stresses.

Analysis and Repair


• Effective cross section.
Removal of the surface material down to the red boundary (see Table A-1) will reveal the remaining cross section that can be deemed effective. Compression tests of cores can indicate the strength of the concrete, yielding a value for use in calculations.

• Cracks.
Most fine cracks are confined to the surface. Major cracks that could influence structural behavior are generally obvious. A wide crack or cracks near supports may mean there has been a loss of anchorage of the reinforcement.

• Reinforcing steel.
Provided that mild steel or hot-rolled high-yield steel is undistorted and has not reached a temperature above about 800 ºC, the steel may be assumed to have resumed its original properties except that cold-worked bars will have suffered some permanent loss.

• Prestressing steel.
It is likely that prestressing steel will have lost some strength, particularly if it has reached temperatures over 400 ºC. There will also be a loss of tensile stress. These effects can be assessed for the estimated maximum temperature attained.

In some situations, the replacement of a damaged concrete structural member may be the most practical and economic solution. Elsewhere, the repair of the member, even if extensive, will be justified to avoid inconvenience and damage to other structural members.

Where new members are connected to existing ones, monolithic action must be ensured. This calls for careful preparation of the concrete surfaces and the continuity of reinforcing steel. For repair, the removal of all loose friable concrete is essential to ensure adequate bonding. Extra reinforcement should be fastened only by experienced welders.

New concrete may be placed either by casting in forms or by the gunite method. With the latter, it may be possible to avoid increasing the original dimensions of the member. The choice of method will depend on the thickness of the new concrete, the surface finish required, the possibility of placing and compacting the concrete in the forms, and the degree of importance attached to an increase in the size of the section.

Large cracks can be sealed by injecting latex solutions, resins, or epoxies. Various washes or paints are available to restore the appearance of finely cracked or crazed surfaces.