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Nail slip in plywood LRFD vs UBC
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: "Dan Merrick" <danmerr(--nospam--at)ix.netcom.com>, "SEAINT" <seaint(--nospam--at)seaint.org>
- Subject: Nail slip in plywood LRFD vs UBC
- From: "David Merrick" <mrkgp(--nospam--at)winfirst.com>
- Date: Fri, 7 Mar 2003 15:54:12 -0800
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? ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********
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