Page 252 - Forensic Structural Engineering Handbook

7.36 CAUSES OF FAILURES

cause it to yield. Although in building structures members are not fully restrained, the
effects of expansion and restraint of expansion are significant.
The effects of thermal expansion are not all detrimental. The benefits of thermal expan-
sion to the survival of restrained structural beams during fires has been studied and docu-
38
mented. When beams are heated from below, the compressive stresses that develop in the
expanding lower flange reduce tension due to external loads. This reduction of stress par-
tially offsets the concurrent reduction in tensile strength due to heating.
Columns, which may be restrained from free vertical growth in multistory buildings,
can experience increased compressive loads at the same time that the strength of the mem-
ber is reduced by heat if the expansion occurs at a rate that exceeds the rate of reduction of
the modulus of elasticity of the material. Columns also can fail in shear due to expansion
of the floor system, particularly in multibay fires and with relatively short columns.

Explosive Overpressures

When buildings that are on fire are not vented, fumes composed of partially consumed
combustion products can build in interior spaces. Under proper combinations of heat and
oxygen content, these explosive gases and material surfaces can ignite suddenly during the
fire, causing an explosion that can damage windows and walls, and spread the fire rapidly.
This phenomenon sometimes is referred to as “flash-over.” Flash-over can damage rooms
and building components outside the areas of active combustion.

Catenary Beam Action
Eventually, when the temperature of a beam during a fire increases enough to generate sig-
nificant expansion and to affect the material’s stiffness, the beam will begin to sag. This
deformation results from the reduction of modulus at elevated temperature, creep (particu-
larly in concrete beams), buckling if the restraint against expansion is rigid, and reduction
in material strength.
Steel members are likely to sag due to yielding of the flanges at approximately 900°F
(482°C). At similar temperatures, concrete beams also will sag if the cover over the bottom
steel has spalled, exposing the steel to fire heat.
For simple span beams without axial end restraint, yielding of the beam at midspan
usually results in collapse. As the deflection of the beam increases to approximately 10 to
20 percent of the span length, the ends of the beam can overstress their connections or lose
bearing.
For beams with restraint for axial loads, failure can be delayed as the beams sag. (Of
course, continuous beams with negative moment capacity also have reserve strength after a
hinge forms at midspan.) First, as beams undergo this inelastic deflection, they elongate. This
reduces fire-induced compression. Eventually, member tension develops if suitable end-
restraint details exist and the beam sags enough. This tension allows catenary action to
develop in deflected beams. This alternate load-carrying mechanism can support weights on
beams heated to temperatures far in excess of those which causes the initial hinge to form.
For steel beams proportioned for deflections on the order of L/360, where L is the span
length, the maximum tensile stress in beams supporting loads by catenary action is on the
order of 10 to 15 percent of the room-temperature yield strength of the steel. The horizon-
tal component of this force, pulling inward on supports, is much less than the heat-induced
outward force that usually precedes it in the fire, but often is significantly more than axial
forces assumed during the design of connections. If the connections can support the force,
the collapse of the beam will be delayed.