Design Considerations for Fire Resistance - Simplified Design of Reinforced Concrete Buildings - page 269

(3) The test assembly fails to sustain the applied load.

(4) For certain restrained and all unrestrained floors, roofs and beams, the reinforcing steel temperature rises
to 1100°F.

Though the complete requirements of ASTM E 119 and the conditions of acceptance are much too detailed for
inclusion in this chapter, experience shows that concrete floor/roof assemblies and walls usually fail by heat
transmission (item 1); and columns and beams by failure to sustain the applied loads (item 3), or by beam
reinforcement failing to meet the temperature criterion (item 4).

Fire rating requirements for structural assemblies may differ from code to code; therefore, it is advisable that
the designer take into account the building regulations having jurisdiction over the construction rather than
relying on general perceptions of accepted practice.

10.4 DESIGN CONSIDERATIONS FOR FIRE RESISTANCE

10.4.1 Properties of Concrete

Concrete is the most highly fire-resistive structural material used in construction. Nonetheless, the properties of
concrete and reinforcing steel change significantly at high temperatures. Strength and the modulus of elasticity are
reduced, the coefficient of expansion increases, and creep and stress relaxations are considerably higher.

Concrete strength, the main concern in uncontrolled fires, remains comparatively stable at temperatures
ranging up to 900°F for some concretes and 1200°F for others. Siliceous aggregate concrete, for instance, will
generally maintain its original compressive strength at temperatures up to 900°F, but can lose nearly 50% of
its original strength when the concrete reaches a temperature of about 1200°F. On the other hand, carbonate
aggregate and sand-lightweight concretes behave more favorably in fire, their compressive strengths remaining
relatively high at temperatures up to 1400°F, and diminishing rapidly thereafter. These data reflect fire test
results of specimens loaded in compression to 40% of their original compressive strength.

The temperatures stated above are the internal temperatures of the concrete and are not to be confused with the
heat intensity of the exposing fire. As an example, in testing a solid carbonate aggregate slab, the ASTM
standard fire exposure after 1 hour will be 1700°F, while the temperatures within the test specimen will vary
throughout the section: about 1225°F at 1 / 4 in. from the exposed surface, 950°F at 3 / 4 in., 800°F at 1 in., and
600°F at 1- 1 / 2 in.; all within the limit of strength stability.

It is to be realized that the strength loss in concrete subjected to intense fire is not uniform throughout the
structural member because of the time lag required for heat penetration and the resulting temperature gradients
occurring across the concrete section. The total residual strength in the member will usually provide an
acceptable margin of safety.

This characteristic is even more evident in massive concrete building components such as columns and girders.
Beams of normal weight concrete exposed to an ASTM E 119 fire test will, at two hours when the exposing
fire is at 1850°F, have internal temperatures of about 1200°F at 1 in. inside the beam faces and less than 1000°F