AISC LRFD 1.pdf

254 FLANGES AND WEBS WITH CONCENTRATED FORCES [Comm. K1.and recognizes the observed fact that when the panel-zone web has completelyyielded in shear, the axial column load is carried in the flanges.K2. PONDINGAs used in the LRFD Specification, ponding refers to the retention of water duesolely to the deflection of flat roof framing. The amount of this water is dependentupon the flexibility of the framing. Lacking sufficient framing stiffness, its accumulatedweight can result in collapse of the roof if a strength evaluation is not made.Representing the deflected shape of the primary and critical secondary member as ahalf-sine wave, the weight and distribution of the ponded water can be estimatedand, from this, the contribution that the deflection each of these members makes tothe total ponding deflection can be expressed (Marino, 1966):For the primary member:∆For the secondary member:wα p∆o[ 1+ 0. 25παs + 0. 25πρ (1+αs)]=1−025 . πα α2 2é p pùads o ê1 + ap+ (1 +ap) + 0.185aas p32 8rúdw=ëû1-0.25pa aIn these expressions o and o are, respectively, the primary and secondary beamdeflections due to loading present at the initiation of ponding,α p = C p /1 ( −Cp ), α s = C s /1 ( −Cs ), and ρ = δo / ∆o = Cs / C pUsing the above expressions for w and w , the ratios w / o and w / o can be computedfor any given combination of primary and secondary beam framing using,respectively, the computed value of parameters C p and C s defined in the LRFDSpecification.pspsFig. C-K1.5. Interaction of shear and axial force—inelastic.LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION