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Re: 10/lw term in calculation of rho
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: Re: 10/lw term in calculation of rho
- From: "Swingle, Mark" <Mark.Swingle(--nospam--at)dgs.ca.gov>
- Date: Fri, 29 Oct 1999 11:30:30 -0700
Re: 1997 UBC 1630.1.1: Chuck- Rho is independent of the base shear for all structural systems. Rho is a property of and is dependent ONLY upon the geometry (location) and stiffness of the vertical elements (frames or walls). For shear walls, stiffness is dependent upon length as well as other factors. Obviously this only applies if the elements are assumed to be in a force range where deformations are linear-elastic. If one has a shear wall building with a calculated rho of 1.1 (for example), rho will remain the same even if the force level increases. This is because the denominator will increase by the same percentage as the numerator for all r(i) calculations. I fail to see how rho will increase if the loads are increased due to discovering a rho greater than 1. The point of the provision is to nudge the designer to ADD walls upon calculating a rho greater than 1. In this case, if the configuration is changed through the addition of walls, rho will decrease, as you pointed out, assuming walls are added or lengthened in the vicinity of the most-highly-stressed wall. Mark Swingle, SE Oakland, CA P.S. Please do not assume from the above response that I am in favor of this provision in its current form. There are several ways in which this provision is rife with potential pitfalls and in which it is weak technically. ---------------------------------------------------- Chuck Utzman wrote on 28 Oct 1999: Dan- The two guys that ran the S.F seminar didn't know wood very well at all. As Bill Allen correctly pointed out in previous comments on this subject, rho doesn't "penalize" short walls at all. What this provision really does is set a limit on the the maximum shear in any wall at each level. Set rho = 1 you'll find that Vmax = 2 x story load/square root of the base area. It actually works very well for design purposes. Once you know the story loads and base area, find Vmax, add up the total length of shearwalls for each floor and find Vave. at each level, if it exceeds Vmax. you're going to add some more wall length on that floor to keep rho under 1. As long as you don't mix long and short walls (under 6') and keep the eccentricity down, you'll find this works very accurately. If you let rho exceed 1, then the wall loads go up and rho goes up again--it usually takes several iterations to converge (or hit 1.5). You almost have to set up spreadsheets to do lateral analysis now. Chuck Utzman, P.E.
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