The standard APA panel diaphragm table contains a variety of diaphragm configurations that provide workable solutions for most designs. For some large commercial buildings, however, the maximum blocked diaphragm capacity of 820 plf can be a significant limitation. In the case when shear demand exceeds 820 plf, one can utilize the high load diaphragm table.

This table appears in a number of sources. It has been part of the International Building Code (IBC) since 2003. In the most recent IBC (2006), this information is found in Table 2306.3.2. Typically, this table is used in areas of the country that traditionally utilized the Uniform Building Code; hence, some may be more familiar with APA’s ICC ES Legacy Report ER-1952. The main concept behind the high load diaphragm table is that shear strength of wood structural panels is generally much higher than the shear forces transferred into the panels using nails or staples. Consequently, if more fasteners are added, the resistance of the diaphragm is increased accordingly. For the high load diaphragm tables, fasteners are driven into either two or three rows of fasteners. See schematic here. Obviously with multiple rows of fasteners, the wood members also have to be increased to nominal 3x, 4x or wider framing members. It also should be noted that high load diaphragms have all panel edges blocked, which is typically very easy to accommodate on large commercial roofs due to the typical framing configuration.

One important requirement of the high load diaphragm table is that special inspection is required. Please see previous blog article, Deciphering Classes of Inspection, for discussion on special inspection. Section 1704.6.1 of the 2006 IBC specifies that for high load diaphragms:

"… The special inspector shall inspect the wood structural panel sheathing to ascertain whether it is of the grade and thickness shown on the approved building plans. Additionally, the special inspector must verify the nominal size of framing members at adjoining panel edges, the nail or staple diameter and length, the number of fastener lines and that the spacing between fasteners in each line and at edge margins agree with the approved building plans."

In my opinion, this code section lends itself to Periodic Special Inspection. The items the inspector must verify can easily be noted after the panels have been "nailed off", with the exception that once a fastener has been driven, diameter and length might be difficult to inspect. There are several solutions to this, ranging from pulling a sample of nails for inspecting dimensions to periodically observing nails during the sheathing process. In fact, some manufacturers identify nails with color codes and/or numeric codes, which helps inspectors to identify nails after they have been driven. In summary, I see no strong reason why Periodic Special Inspection would not be adequate for special inspection of high load diaphragms. Of course, the special inspection should be completed before the wood structural panel decking has been roofed.

I encourage your questions and feedback. For more information, download the Diaphragms and Shear Walls Design/Construction Guide, Form L350.