2001 ASD Supplements - unprotected PDF - American Wood Council

SW-8DIAPHRAGMS3.1 GeneralPanel diaphragms have been used extensively forroofs, floors and walls, for new construction and rehabilitationof older buildings. A complete diaphragm analysisincludes analyzing chord forces, connections and, if applicable,cross ties and drag struts.A diaphragm acts in a manner analogous to a deepbeam or girder where the panels act as a “web” resistingshear while the diaphragm edge members perform thefunction of “flanges” resisting axial stresses. These edgemembers are commonly called chords in diaphragm design,and may be joists, ledgers, trusses, bond beams, studs,etc.Due to the great depth of most diaphragms in the directionsparallel to application of load, and to their meansof assembly, their behavior differs slightly from that ofthe usual, relatively shallow beam. Shear stresses havebeen shown to be essentially uniform across the depth ofthe diaphragm rather than showing significant parabolicdistribution as in the web of a shallow beam. Similarly,chords in a diaphragm carry all “flange” stresses acting insimple tension and compression rather than sharing thesestresses significantly with the web. As in any beam, considerationmust be given to bearing stiffeners, continuityof webs and chords, and to web buckling which is normallyresisted by framing members.Diaphragms vary considerably in load-carrying capacity,depending on whether they are “blocked” or“unblocked” (see Figure 3.1). Blocking consists of lightweightnailers, usually 2x4s, framed between the joists orother primary structural supports for the specific purposeof connecting the edges of the panels. Systems which providesupport framing at all panel edges, such as panelizedroofs, are also considered blocked. The reason for blockingin diaphragms is to allow connection of panels at alledges for better shear transfer. Design loads for unblockeddiaphragms may be limited by buckling of the unsupportedpanel edges. Additional nailing will have little effect onthe buckling performance of the unblocked diaphragm.For the same nail spacing, design load on a blocked diaphragmcan be designed to be as high as twice the designload of its unblocked counterpart.Figure 3.1 BlockingTable 3.1A and 3.1B present the tabulated values forboth blocked diaphragms and unblocked diaphragms forwind loading and seismic loading, respectively. Somemodel building codes have adopted a 40% increase forallowable diaphragm resistance when subjected to windloading. For the convenience of the user of this Supplement,Table 3.1A reflects the 40% increase that may bepermissible. The designer should confirm that the increaseis applicable under the local code; if notapplicable, use Table 3.1B. The basis for the increase isa change in design philosophy for estimating wind forces.In addition, the increase helps to account for better understandingof wind loads and the historical excellentperformance of diaphragms subjected to high wind events.3.2 Using Diaphragm TablesExample One:Given:• residential roof diaphragm• wind loading• trussed roof• unblocked diaphragm required• required diaphragm capacity is 250 lb./ft.• panel orientation is unknownFind:Panel thickness, nail size and nailing scheduleSolution:Using Table 3.1A, refer to the “Unblocked Diaphragm”area of the table. As panel orientation is unknown,use the “All other configurations…” column since thesevalues will be conservative. Check “SHEATHING...” rowsfirst since Structural I may not be readily available in allAPA – The Engineered Wood Association