Design Considerations for Fire Resistance - Simplified Design of Reinforced Concrete Buildings - page 270

at 2 in. Obviously, the dimensionally larger concrete sections found in main framing systems will suffer far less
net loss of strength (measured as a percentage of total cross-sectional area) than will lighter assemblies.
Because of the variable complexities and the unknowns of dealing with the structural behavior of buildings
under fire as total multidimensional systems, building codes continue to specify minimum acceptable levels of
fire endurance on a component by component basis—roof/floor assemblies, walls, columns, etc. It is known,
for instance, that in a multi-bay building, an interior bay of a cast-in-place concrete floor system subjected to
fire will be restrained in its thermal expansion by the unheated surrounding construction. Such restraint
increases the structural fire endurance of the exposed assembly by placing the heated concrete in compression.
The restraining forces developed are large and, under elastic behavior, would cause the concrete to exceed its
original compressive strength were it not for stress relaxations that occur at high temperatures. According to
information provided in Appendix X3 of ASTM E 119, cast-in-place beams and slab systems are generally
considered restrained (see Table 10-5 in Section 10.4.3).
In addition to the minimum acceptable limits given in the building codes, the use of calculation methods for
determining fire endurance are also accepted, depending on the local code adoptions (see Reference 10.1 and 10.3).
10.4.2 Thickness Requirements
Test findings show that fire resistance in concrete structures will vary in relation to the type of aggregate used.
The differences are shown in Table 10-1 and 10-2.
Table 10-1 Minimum Thickness for Floor and Roof Slabs and Cast-in-Place Walls, in.
(Load-Bearing and Nonload-Bearing)—(Reference 10.4)
Table 10-2 Minimum Concrete Column Dimensions, in.—(Reference 10.4)
In studying the tables above it is readily apparent that there may be economic benefits to be gained from the
selection of the type of concrete to be used in construction. The designer is encouraged to evaluate the alternatives.
10.4.3 Cover Requirements
Another factor to be considered in complying with fire-resistive requirements is the minimum thickness of
concrete cover for the reinforcement. The concrete protection specified in ACI 318 for cast-in-place concrete