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Re: Effects of the New Code on Wood structures - good or bad????? -Part 1[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: Re: Effects of the New Code on Wood structures - good or bad????? -Part 1
- From: "Swingle, Mark" <Mark.Swingle(--nospam--at)dgs.ca.gov>
- Date: Thu, 22 Jul 1999 12:55:46 -0700
- Cc: "'mswingle(--nospam--at)earthlink.net'" <mswingle(--nospam--at)earthlink.net>
PART 1 OF 2 Dennis - I sincerely appreciate the efforts you are making to promote rationality with respect to seismic design of light-frame buildings with plywood shear walls. However, I would like you (or anyone else) to explain exactly what has changed in the 97 code that leads to your ranting and raving. I am unable to find the major changes of which you speak. You keep blaming the new code and speak with urgency about it due to the fact that it took effect in CA this July 1. The relevant portions of which you complain the most have not changed at all since the 1994 UBC, and have not changed significantly since 1967. Since at least the 1967 version (that's as far back as I have looked), consideration of the following three items has been required for earthquake design (and wind design) in the UBC: 1) The relative stiffnesses of the various shear walls with respect to one another, 2) The relative stiffness of the diaphragm vs the shear walls in each line, and 3) The eccentricity between the center of application of the forces and the center of rigidity of the resisting elements. These three items have not changed, and these are the main issues that you imply have changed so drastically in the 97 UBC. There is no provision in the code that I can find where it says you must balance deflection in each line, or balance the stiffness of each element in a line with respect to the others. Just because someone does a design example that way doesn't mean that the code requires it. If the stiffness don't match, then more or less force will be delivered to that wall, depending upon its relative stiffness, assuming that the diaphragm has sufficient stiffness to redistribute the forces. If the diaphragm is so stiff that its deflection is miniscule relative to the deflection of the walls, then the shear distribution will be based purely upon a "rigidity analysis", with no consideration of the deflection of the diaphragm. In that case accidental torsion must be added to the actual torsion. If the diaphragm is so flexible that the deflection is very large relative to the shear walls, then no significant redistribution can take place, and the forces are based upon tributary area, and no real or accidental torsion is considered. If the diaphragm is somewhere in between, then only partial redistribution takes place, and then the forces in the shear walls fall somewhere between the "rigid" and "flexible" cases. The problem is that most wood buildings fall somewhere in between, and there is no technically justified way of modeling the diaphragm or the shear walls to find out the actual forces in the walls. END OF PART 1 OF 2
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