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Re: UBC 97 Diaphargm Nailing

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Jim, here are answers to your questions.

1.  Another respondent has correctly noted that the derivation of allowable shear values by principles of mechanics for diaphragms and shear walls are described in APA Reports 138 and 154 respectively. Both are based on allowable lateral loads for fasteners x no. of fasteners per foot = allowable shear load
per foot. But there are other factors that are considered, such as splitting of framing when fasteners are closely spaced, bearing of panels on adjacent panels due to shear rotation (especially in the case of horizontal diaphragms with staggered joints), uniform shear along the length (height) of shear
walls vs. linearly reducing shear for diaphragms.  If you base the design on principles of mechanics and use current fastener values which are based on European Yield Model (EYM), often the allowable shear load is nearly the same, or sometimes less than the code value. When allowable shear values were
considered, more attention was given to maximum shear strength (strength limit state) reduced by a load factor, than to allowable shear strength as determined by fastener loads which were based on low slip applications. The values were "fit" into a matrix to present a reasonable progression considering
these factors, while providing a reasonable margin of "safety" (e.g., load factor).  Also, deflection of the diaphragm or shear wall was a consideration.  These factors were considered when establishing design values in the codes, which have been in place for over 40 years (diaphragms) or 30 years (shear
walls).
2.  1997 UBC Table 23-II-H does not give allowable shear loads for 7/16" Structural I wood structural panels since this grade and thickness is not commonly used for roof diaphragms in non-residential buildings. One reason for this is that manufacturers of built-up roofing and other types used for
nonresidential buildings have specifications for min. 15/32" wood structural panel roof decks as a substrate for their products. (Asphalt shingles are commonly used over 7/16" wood structural panels, however.) It is permitted to use 7/16" wood structural panels oriented parallel to supports, as for
panelized roofs (see Table 23-II-E-2), but the allowable vertical roof load is low (20psf LL plus 10psf DL).  However, this construction is rarely used because of the roofing specification. If someone wanted to do it, the shear values for 8d common nails and 3/8" wood structural panel sheathing would apply;
note that panel thickness is *minimum* nom. panel thickness.  However, 7/16" thick wood structural panels are shown in the lower half of Table 23-II-H for diaphragms, and for shear walls (Table 23-II-1-1) as you noted.
3. Table 23-II-H for diaphragms lists common nails only because that was the basis for most of early diaphragm construction (still true). For shear walls, which also include plywood siding where galvanized box nails are specified for installation, box nails were added to Table 23-II-1-1 based on results of
APA shear wall tests conducted in the mid-60s. These showed that common and box nails gave practically the same maximum shear capacity (strength limit state) although shear wall deflection at low shear loads was slightly but not significantly greater for box nails. More recent cyclic load testing of shear
walls also appears to confirm these earlier results, but the whole issue will be revisited in one of the subtasks of the CUREe Woodframe Project that is getting underway now.
4. For siding applications, galvanized nails produced by hot-dipping or hot-tumbled galvanized nails seem provide adequate corrosion resistance to prevent staining and rusting in exposed-to-weather applications.  Some electro-galvanized nails that are protected with a yellow chromate coating also appear to
have adequate corrosion resistance.

John Rose/APA - The Engineered Wood Association
Tacoma, WA


James Allen wrote:

> I have a few questions to take you mind off the worrying about the calendar and other horrible things.
>
> 1. Tables 23-ll-H and 23-ll-I-1 list shear values for nailing horizontal and vertical diaphragms. The shear values for blocked horizontal diaphragms are not the same as for vertical diaphragms. Why? The NDS (the basis for these tables) does not differentiate between horizontal and vertical diaphragms.
> 2. Table 23-ll-H does not list values for 8d nails in sheathing Struct I 7/16 nnd 15/32 thick. It does list values for 8d nails in these thickness for other grades of sheathing. Why? Table 23-ll-I-1 lists shear values for 8d nails in these thickness for all grades. Why the difference between the tables?
> 3. Table 23-ll-H lists only common nails while Table 23-ll-I-1 lists common or galvanized box. Why the difference?
> 4. See Table 23-ll-I-1, footnote 5, what are tumbled nailed?
>