EWPAA Structural Plywood and LVL Design Manual - Engineered ...

FIGURE 9.3 shows a plywood panel nailed to a pin-jointed timber frame.From statics:FIGURE 9.3: Shows panel subjected to shearΣF x = 0 : S A = S RΣF y = 0 : C s = C s (complimentary shears)ΣMo = 0 = S A .h-C s .S A / = C s / hThat is, the UNIT SHEAR along HORIZONTAL EDGES equals UNIT SHEAR along VERTICAL EDGESNOTE:The Unit Shear concept is of FUNDAMENTAL IMPORTANCE when re-distributingSHEARS around OPENINGS.9.3 Diaphragm Design – Diaphragm ActionDiaphragm capacity will vary considerably depending on nail frequency and capacity, and whether thediaphragm is “blocked” or “unblocked”. Blocking consists of lightweight framing, usually 90 x 45 timberframing, located between the joists or other primary structural supports, for the specific purpose of connectingthe edges of the plywood panels. The use of blocking to connect panels at all edges facilitates shear transferand increases diaphragm capacity. Unblocked diaphragm capacity is governed by buckling of unsupportedpanel edges, such that above a maximum load, increased nailing will not increase diaphragm capacity. Thecapacity of blocked diaphragms is 1.5 to 2 times the capacity of an equivalently nailed unblockeddiaphragm. Additionally, blocked diaphragms can be designed to carry lateral loads many timesgreater than those for unblocked diaphragms.Diaphragm action differs from simple beam action in that shear stresses have been shown to be essentiallyuniform across the depth rather than displaying the parabolic distribution associated with shallow beamwebs.Also, the chord members are responsible for transfer of bending moments, acting in uniaxial tension andcompression. Chord members must, however, be continuous over the length of the diaphragm. Theadvent of LVL, being available in long straight lengths has reduced the need for incorporating spliced jointsalong the chord lengths. Chord members of plywood sheathed, timber framed diaphragms are not restrictedto timber members. They could also be the face of a concrete or masonry wall, a reinforced or masonry beamor a steel beam.93