Thinking Outside the Box: New Approaches to Very Large Flexible Diaphragms

This paper discusses the past design practices of flexible diaphragms, the current trend to very large diaphragms, and the new tools available to properly evaluate and engineer these large roof systems. Higher shear demands are accommodated by higher plywood/OSB and steel deck shear capacities accepted by the IBC. A new collective chord technique is presented to more accurately reflect large diaphragm behavior. In addition, these very large diaphragms have significant lateral deflection issues that can now be evaluated in the IBC. The new AF&PA deflection equations for wood diaphragms are presented as well as a technique to compute deflections of diaphragms with collective chords or multi-zoned nailing. Introduction California’s ports account for over 40% of the cargo flowing into the United States. With the ever-increasing demand for Asian imports, larger and larger distribution facilities are being built here in seismically active California to distribute these goods across the Country. Tilt-up buildings in excess of one million square feet are becoming commonplace, and the design of these very large flexible diaphragms is presenting new engineering challenges. Higher diaphragm shear capacities are now available in both wood and steel deck roof systems. The collective chord approach provides reduced chord forces while more accurately modeling diaphragm behavior. More accurate diaphragm deflection calculations are now possible, as engineers try to mitigate their impact. 1. Special High Diaphragm Shear Capacities: In California, the hybrid panelized roof is the most common large flat roof system being used today. This consists of wood structural-use panels such as plywood or oriented strand board (OSB) nailed to wood nailers factory installed to the top chord of open-web steel joists. Current trends are for larger buildings with more clear-space and taller clear-heights to facilitate state-of-the-art warehousing and distribution. Wood roof diaphragms are being required to span farther horizontally with higher shear stresses. Since the 1967 UBC, allowable horizontal diaphragm shears for wood sheathing have remained essentially the same; with a maximum tabular shear capacity topping out at 820 plf (1⁄2” Struct I plywood with 10d nailing at 2”, 3”, 12”). These shear capacities are an outgrowth of testing conducted beginning in 1952 and concluding in 1966 (Tissell, 2000). The traditional UBC table of allowable shears is currently incorporated in the 2006 IBC Table 2306.3.1 with no significant changes. Over the past couple decades though; engineers of very large wood diaphragms have had another option available for justifying very high allowable shear capacities. Sponsored by the American Plywood Association, the APA funded ICBO Evaluation Service document #1952 permiting allowable shear capacities up to 1800 plf. These very high shear capacities are achieved with 3⁄4” plywood, multiple lines of nails, on 3x and 4x framing, with special inspection. ER1952 is a direct result of full-scale diaphragm testing by APA, whose primary purpose was to provide higher diaphragm shear capacities in response to the higher demands of the added Seismic Zone 4 into the Building Code. Prior testing was always controlled by nail failure; these tests investigated diaphragm strengths governed by the shear strength of the plywood material. The results and analysis of the testing program are published in APA Research Report 138, (Tissell, 2000). The use of ER-1952 has been widely accepted in large tilt-up and masonry buildings (ICC ER1952, 2006). An important recent development has been the incorporation of APA’s ER-1952 directly into the IBC. The 2006 IBC contains Table 2306.3.2 that provides direct code approval of these special high shear capacities for all structural-use panels. Originally, only APA stamped 3⁄4” thick plywood products were specifically approved unless a separate justification was used with principles of mechanics. IBC Section 2306.3.1 still allows the calculation of high shears using principles of mechanics, but IBC Table 2306.3.2 has removed much of the need for this separate analysis.