AISC LRFD 1.pdf

256 PONDING [Comm. K2.would be determined by the intercept of a horizontal line representing the U s valueand the curve for C p = 0.The ponding deflection contributed by a metal deck is usually such a small part ofthe total ponding deflection of a roof panel that it is sufficient merely to limit itsmoment of inertia [in. 4 per foot (mm 4 per meter) of width normal to its span] to0.000025 (3 940) times the fourth power of its span length, as provided in the LRFDSpecification. However, the stability against ponding of a roof consisting of a metalroof deck of relatively slender depth-span ratio, spanning between beams supporteddirectly on columns, may need to be checked. This can be done using FigureA-K2.1 or A-K2.2 with the following computed values:U p = stress index for the supporting beamU s = stress index for the roof deckC p = flexibility constant for the supporting beamsC s = flexibility constant for one foot width of the roof deck (S = 1.0)Since the shear rigidity of the web system is less than that of a solid plate, themoment of inertia of steel joists and trusses should be taken as somewhat less thanthat of their chords.LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION