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Redundancy Factor
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: "Ron O. Hamburger" <ROH(--nospam--at)eqe.com>
- Subject: Redundancy Factor
- From: Charles Greenlaw <cgreenlaw(--nospam--at)speedlink.com>
- Date: Fri, 30 Jul 1999 17:39:55 -0700
- Cc: seismo-all(--nospam--at)seaint.org, seaint(--nospam--at)seaint.org
Thanks, Ron Hamburger, for the refreshingly candid and honorable revelation that the rho factor for shear walls went astray in Seismology Committee. Your info comfirms what had become readily apparent to me in the last two days since I finally began to get familiar with the rho factor on my own, reading the 97 UBC in its CBC-adopted edition I held out for, hoping it would bear corrections to glitches. I avoided the seminars and manuals in order to avoid being prejudiced by other people's notions until after forming my own from the code language itself. You confirm in your description that shear wall examples deliberated for the rho factor were concrete walls for tilt-ups, not wood panels like are ubiquitous in residential. This too I suspected. It is significant. And Bob Bossi confirms that it was Seismology Committee, not industry or others remote from SEAOC, that originated the rho factor, and that the committee has a combative stance toward dissenters. At this point I am going to copy in a message I sent privately to Dennis Wish very early this same day, Friday 7-30, before this thread took off. It will have to be in two installments. The first gives the results of my discoveries in the last two days about how haywire the rho factor is in wood frame work. It was readily discoverable just by taking an interest and testing among a few simple possibilities in a day's time starting cold. I thought the Committee did that sort of verification. The second part continues on the rho factor, and then gets into what I think the implications are for SEAOC, its damaged integrity, and a suggested reorganization to end the too-often recurrences of this type of debacle, which hits residential engineering practice and practitioners especially hard. This second part is brutal. But it is directed at problems that came from people, not at the individuals personally. Hurt feelings are likely, not improper, and not necessarily counterproductive. Many recent and present Committee members are respected friends from past years, and still will be. Charles O. Greenlaw SE Sacramento CA Past Seismology Comm. Member during 1982-88 Blue Book rewrite, specializing in Wood Section tasked to SEAOCC, Past SEAOC Director for two separate two-year terms. Presently on SEAOC Prof Pract Comm. -------------------------------------------------------------- [Copied-in previous message to Dennis Wish, part one:] Dennis, here's an off-list interim reply. I'm learning a bit on this new stuff, now. I have a job on a one story, flat-top home under major remodel since a dose of fire damage, and the 97 UBC overtook it, and I have to finish up the new shear design. So far it looks to me like the rho factor, as applied to residential shear walls, is totally bogus, and inconsistent with both common seismic principles and with the 96 Blue Book Appx C Commentary that only gives "redundancy" (one wall failing and leaving too few left) as the justification. Everyone promoting more complex codes always claims increased "rationality", but this rho factor utterly fails rationality, at least in its shear wall form, for houses. To get familiar with rho initially, I first built a table that tells me the lower and upper bound of r-max for floor square footages in residential ranges so I could sense when the rho hits 1.0, and when it hits 1.5 (it takes double the r-max to get to 1.5 from 1.0) And of course, the bigger the floor plan, the lower the r-max in order to not get hit with a rho above one. Then I postulated an example, simple 30ft by 30ft "house" and gave it initially "conventional construction"- scheduled wall panels, at exterior only (I'm in zone 3) namely, Three on each side at 4ft long each, and, without bothering with the accidental eccentricity at this stage of experimenting, I figured r-max and then rho, which came out much less than one. [I didn't use a numerical base shear, only an algebraic "V". Also I invented a generic term "V sub w" for the amount of V in any shear panel under consideration, and "Sigma L sub w" for the total panel length in the orthogonal direction of concern. Thus I only did algebra, not numerical loads and stresses, in this example.] After Three panels at 4ft each, I changed to TWO panels each side at 6ft long. Still 12 ft per ext wall. Same lbs/ft shear and same r-max and same rho, in spite of fewer panels. Then I checked using a Single panel ea wall at 12 ft long: same lbs/ft and still same rho. So much for protecting against a single wall failing! Bogus code provision! Then I started over with Three panels per ext wall, but with evenly reduced length of each panel without reducing the number of them. Lbs/ft went up, r-max went up, and rho went up. However the actual redundancy had not changed: 3 panels per wall. The same increase in rho resulted when the lbs/foot went up the same amount in all panels alike, regardless of the nunber of panels per exterior wall. Rho did not change even for one panel per wall (ie, no redundancy exists) as long as the feet of panel was the same as with multiple panels combined. I find that the rho formula penalizes you according to the total length of shear wall panels summed up, when panels are uniformly stressed per foot in the worst stressed wall line. It doesn't matter if some panels are long in plan and some short, in the same wall line, if you deem them to have the same shear per foot (by virtue of rigging their relative rigidity or whatever to get such a result.) In other words, if you have long panels everywhere the rho factor is lower than if you have the same number of panels, but not as long, and thus at higher stress per foot. So it can happen that if you had lots of long STUCCO-sheared walls you get off at rho less than one, but if you use say, narrow Simpson Strong Walls in equal numbers and locations, the maxed-out rho factor, due to the far fewer feet of wall in plan, can jack the design lateral force up by 50 percent, so that the whole house, including diaphragms and other elements, like upper story shear and hold-downs, has a seismic design load in excess of that for the stucco version, even though stucco started with a heavier R value (and worse ductility) than does Wood Structural Panels. And again it happened with no difference in the "real" redundancy concerns where a failed panel matters a lot. Figuring the accidental torsion eccentricity into my familiarization exercise is a big bother and would raise the r-max and rho results somewhat, but the generalities I found would still apply. [end of part one]
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