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Nail slip in plywood LRFD vs UBC

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Nail slip in plywood LRFD vs UBC

I would like to use the LRFD equation in lieu of the UBC tabulated nail slip
(en) values. Is the LRFD fastener slip equation acceptable as a "well
established principle of mechanics"? (1997 UBC, 1605.2 Rationality).

Ref 1996 ed. "LRFD Manual for Engineered Wood Construction" American Forest
& Paper Association, American Wood Council.

>From the LRFD: en=(Vn/A)^B   en= Nail Slip inch, Vn= pound/nail, A and B are
constants specific per fastener and framing moisture content. Green/Dry=
Fabricated Green and tested Dry This is similar to the UBC table 23-2-k.

Comparison of UBC vs LRFD slip values
Example: 8d nail: A,= 857, B=1.869
Green/Dry, 15/32" plywood, Structural I
UBC 8d@4", given: Vn=143.3 lb/nail, slope=0.0005"/lb en=0.0326" interpolated
from table
 LRFD given A= 857, B=1.869 At 143 lb/nail
          en=.0354 (8% more flexible than UBC table. Solution is between
steps of table)

Differences in nail slip references affect little in shear wall force
distribution because values are relatively correct. Drift is directly
influenced, but rarely controls. The UBC has no (en) change for changes in
spacing that affect the nail strength. The LRFD does include a (en) change
for nail spacing and may better represent relative wall stiffness. The LRFD
equation en=(Vn/A)^B also offers an accuracy by avoiding table steps and
interpolation. The LRFD gives values for stapled plywood.

A period calculation is needed for a two-stage static analysis. A 10% error
in drift will be less than a 5 % error in the value of the period. This is a
function of the square root of the stiffness and the stiffness is only
partly due to nail slip. A wood shear wall is rarely too stiff to be
considered for upper levels of a two stage analysis. Period can also be used
to consider reducing the base shear. This is allowed by code, rarely used,
and parametric studies should be made of a range of slip values.

Is there any compelling reason to not use the LRFD slip equation?

David Merrick

More information...
Constants A and B are easily found with a known slope in/lb for a determined
connection strength (lb/nail). To derive A and B Let slope= (delta
en)/(delta Vn), at a shear per nail value. B=(Vn*slope)/en,
A=(en/Vn^B)^(-1/B),  to back check: Slope=B*(A^-B)*(Vn^(B-1)).

Possibility...Hold-Down deflections and other wood connections may have
nonlinear-elastic curves similar to the nail slip curve. With a given a
deflection at a given maximum capacity, a slope, if not given, could be
estimated. Increase the linear slope (en/Vn) by about a factor of 2.3. If a
deflection is found for a given max working stress design load, then, less
accurately, increase the linear slope (en/Vn) by a factor of 1.7.  What do
you think?



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