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The flexural strength, flexural stiffness, and shear strength of
RC beams reduce considerably after exposure to fire. The
strength of concrete decreases after fire imposition and so is
the balance steel ratio.This will decrease the ductility of a
beam and the shear strength contribution of concrete.As a
result, the failure mode of the beam will possibly be changed
from flexural failure to shear failure. Twelve 30 cm×50 cm×340
cm RC beams, with several different stirrup spacings, have been
made and conducted to fire simulation tests and loading tests.
The results show that the flexural stiffness of RC beams reduce
pronouncedly after fireexposure, and the reduction of shear
strength of damaged beams is more evident than that of the
flexural strength. Both the modified conventional RC beam model
and the finite element model, account for the change of stress-
strain curves of the reinforcement and the concrete, have been
used to analyze the fire-damaged beams, Good agreements between
the test results and the analytical ones are obtained. As a
modified ACI shear strength equation for fire-damaged concrete
is adopted to calculate the shear strength of the damaged beam,
a reasonable however conservative result is achieved. An under-
reinforcement design and an adequately increase of shear
reinforcement, considering the weaker shear strength of damaged
concrete, will avoid the beam to have the brittle failure either
in compression or in shear, and enable the beam to retain a
reasonable ductility after fire damage.
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