The graphs in Fig. 10 were prepared
to assist designers in estimating fire endurance of simply supported prestressed concrete slabs. These graphs
show the inter-relations among load intensity, cover thickness, fire endurance,
and concrete type. Note that fire endurance is improved by decreasing load
intensity or increasing cover. Also, for
the same cover and load intensity, fire
endurance is longer for lightweight concrete than for normal weight concrete.
One may ask, "Why be concerned
with load intensity if ASTM E-119 requires that a specimen support its design load during a fire test?" By using
rational design methods, the effect of
load intensity can be evaluated. In certain cases, such as precast prestressed
slabs, it may be more economical to
provide additional load capacity by using more strands than to provide additional cover. By evaluating the effects
of cover and load, an economical and
rational solution can be obtained.
Fig. 10 shows how results of fire tests
combined with rational design procedures can be combined to form relatively simple design aids. Modified design aids can be used for simply supported beams.