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.
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.