Allowable Shear Table

It is important for designers to understand how to use IBC Table 2306.3.1 and UBC Table 23-II-H, "Allowable Shear for Horizontal Wood Structural Panel Diaphragms." This table can also be found in the Allowable Stress Design (ASD) Manual for Engineered Wood Construction and in the back of the Design of Wood Structures textbook (Breyer, Fridley and Cobeen, 1999).

The table relates sheathing thickness, nail size and spacing, sheathing grade and allowable unit shear to enable determination of an appropriate construction configuration. Depending on the given information, the table may be used in different ways to find the unknown design parameters. It is important to note that the footnotes of the table play a key role in the design process. The footnotes relate information on intermediate nail spacing (at panel edges), framing member thickness, and alternate allowable unit shear values. Often times the footnotes may dictate the design of the diaphragm. The following example demonstrates how the table may be used to find the necessary information for design. Keep in mind this is only one approach for using IBC Table 2306.3.1 and UBC Table 23-II-H.

Problem: A floor diaphragm is to be built using C-D grade plywood sheathing applied over nominal 2x10 floor joists that are 16 inches on center (o.c.). The sheathing will be attached using 8d common nails.  It has been determined that the diaphragm must resist a unit shear due to wind loading of 350 plf. Determine an appropriate sheathing thickness and the nail spacing to meet these requirements.

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