Page 253 - Forensic Structural Engineering Handbook
P. 253

LOADS AND HAZARDS: THEIR NATURE, MAGNITUDE, AND CONSEQUENCES  7.37

               As a fire continues after catenary action has been initiated, beam temperatures and
             deformations continue to increase. Eventually, the strength of the heated steel may be
             insufficient to support catenary forces (steel yield stress is only approximately 15 percent
             of room-temperature yield strength when it is heated to approximately 1400°F (760°C). 32
               In the final phases of a fire, structural elements cool and contract as the fire is extin-
             guished. When beams have sagged or columns have been displaced during the heating
             phase, the subsequent contraction can develop high forces in directions reverse to those
             caused by heating. In beams, the cooling-phase forces can be tensile and additive with cate-
             nary forces. As such, it is possible to fail components such as connections after the heat of
             the fire has diminished.


             Stability of Vertical Members
             Maintenance of stiffness is critical to the survival of vertical members during fires. When
             stiffness is reduced or floors that provide lateral support for columns fail, structural stabil-
                                                    32
             ity is placed at risk. At approximately 1000°F (538°C) , steel members lose 50 percent of
             their stiffness. Depending upon the composition of and the load on a concrete member dur-
             ing heating, concrete can lose 50 percent of its stiffness at approximately 1100°F (593°C).
             At these temperatures, slender columns at their design load may be at risk of failure due to
             buckling.
               For columns and walls that are heated on one side only during a fire, the rates of tem-
             perature increase in the members are lower than when they are heated from all sides.
             However, nonsymmetric heating results in deformations that can affect the overall stabil-
             ity of the member. Due to expansion of the heated side, vertical members tend to bow
             toward the fire. This creates force eccentricities, distorts connections, and increases the
             compressive stress near the midheight of the column or wall. Fortunately, the stress
             increase occurs on the “cool” side where the material strength is relatively unaffected by
             heat. However, with sufficient deformation, yielding of steel, crushing of concrete, or
             member buckling is possible.


             Increased Gravity Loads
             Failure of a single floor can effectively double the weight on the floor below when debris
             falls from one level to the next. This added weight alone can be sufficient to exceed the fac-
             tor of safety designed for that floor. Successive failures can cause progressive collapse.
               With impact, such as by the dropping of one floor onto the other, short-term loads will
             be higher than twice the added static load. Properly designed and detailed bending mem-
             bers can be resilient enough to absorb the shock of a load failing from a short distance (if
             the members are not themselves damaged by high temperatures). However, if the falling
             weight lands near to the support of a beam, shear failure and progressive failure of succes-
             sive floor elements are possible.
               Firefighting water also adds weight to burning structures. Some firefighting equipment
             is capable of delivering huge volumes of water to buildings. If this water is retained in the
             structure, it adds to the weight already on floors and columns.


             Determination of Fire Loads
             Fire loads can be estimated by computing the amount of combustible material in a structure
             at the time of the fire. These data, together with an assessment of the extent of ventilation
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