UBC Shear table mistake ?

[CTruax]

Structural components (trusses & pre-fab walls) shipped from Canada to the US are typically made of Spruce-Pine-Fir lumber. It becomes cost effective because there is no 20% tarriff on components, only bulk lumber.
Designing wood structural shear walls and diaphragms with nails penetrating into SPF requires a reduction in the allowable PLF table values which would typically be proportionate to the specific gravity ratio of SPFF

UBC'97 Table 23-II-I-1 has a footnote 1 which says
"multiply the Struc-1 values by a factor" .82 or .65

Example: T-1-11 siding applied 8d 6"oc edges
SG=.50 for DF framing: 160 plf (from the table)
SG=.42 for SPF framing: 230 x .82 = 188 plf (per footnote)
This would be an increase in allowable, not a reduction!

Shouldn't this footnote be re-written? (addendumized)

 

[JAE]

ctruax,

Perhaps I'm mistaken, but I think I would interpret it this way:

The capacity of a shearwall is governed by the smaller of the sheathing capacity or the nail capacity. The Table 23-II-I-1 gives the design values to be used. These design values may be either nail governed or sheathing governed (the table doesn't tell you).

If you are nailing into DF, then the table applies. If you are nailing into a WEAKER framing, there is the potential that the NAIL capacity will govern over the sheathing capacity. Thus, the footnote states that the "allowable shear values for NAILS" should be reduced based on the .82 or .65 calculated off the Structural I numbers.

So you would do the calculation you just did for us above, and compare the 188 plf vs. your original 160 plf and find that, in this case, the SHEATHING still controls and use the 160 plf.

With an even weaker lumber (small SG) the Structural I x the factor might actually control.

Does this make sense?

 

[sanjeevpillai]

UBC'97 Table 23-II-I-1 has a footnote 1 which says
"multiply the Struc-1 values by a factor" .82 or .65

Example: T-1-11 siding applied 8d 6"oc edges
SG=.50 for DF framing: 160 plf (from the table)
SG=.42 for SPF framing: 230 x .82 = 188 plf (per footnote)
This would be an increase in allowable, not a reduction!

Above is the exract from ur question. We have some questions on ur above values.
Pl. let us know so that it can be easy for us to interpret ur doubt.

1] PL clarify where is this T-1-11? Is this somewhere in UBC?
2] Is the value of 160plf for DF framing from "Panel grade" -: plywood panel siding in grades covered in UBC standard 23-2?

Our understanding is that as the table values are for DF & SP. So for any other species whatever values we get from table to be multiplied by the factors(as in footnote 1) depending upon sp. gravity.
For Ex:
For Structural I panel grade with 3/8 panel thickness & 8d nails @ 6"spacing the value as per table is 230plf.For SPF the SG is 0.42
So, the shear value would be 0.82 x 230 = 188plf

which is less that 230plf.
Hope this explanation might have cleared some of ur doubts.

thx