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Re: seaint Digest for 31 Jul 1999
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- Subject: Re: seaint Digest for 31 Jul 1999
- From: Colin Blaney <colin(--nospam--at)crosbygroup.com>
- Date: Fri, 06 Aug 1999 13:21:19 -0700
Mark & Dennis, I just came across the following message and have an additional question/ comment to add. I have also always been under the impression that the effect of torsional shears (even regardless of the many ways they appear to be calculated) could not reduce the overall shear force for a particular wall. Mark, this is something you stated at the end of you attached example. In reviewing Volume 1 page 78 of the Seismic Design Manual recently published by SEAOC, it is stated that torsional shears can be subtracted if they are due to the reduced eccentricity (e-e accidental). Does anyone have any insight to this issue as it does appear to go against the intent of section 1603.3.3 of the 1994 UBC and what I also presume to be the implied intent of section 1605.2.1? Colin Blaney P.E. Redwood City, Ca > -------------------------------------------------------------------------- > 2 Message:0002 2 > -------------------------------------------------------------------------- > From: "Mark T. Swingle" <mswingle(--nospam--at)earthlink.net> > To: seaint(--nospam--at)seaint.org > Subject: Re: Displaced Center of Mass Question > > Dennis: > > 1997 UBC Section 1630.6 says in part "the > mass....shall be assumed to be displaced....in > each direction....perpendicular to the direction > of force under consideration." > > My understanding and interpretation is as > follows: If one is considering accidental > eccentricity, one must move the CM <both> > directions for each direction of force, yielding > two (2) separate analyses for each direction, or > four (4) total analyses for a (typical) building > with two (2) orthogonal lines of shear walls. The > four analyses can be accomplished by assuming the > CM to be in two different locations. > > I would agree that "displacing....can produce > added torsional shears in all walls" as you > quoted. The other method you mentioned appears to > be wrong the way you described it, IMHO. However, > displacing the mass does NOT ALWAYS increase all > wall shears, as I will describe two paragraphs > down. > > Assuming north = up = y positive axis and east = > right = x positive axis etc, and that the shear > walls line up with these major axes, let me > describe an example. Assume also that the lower > left hand corner of the building is at (0,0). > Suppose the building is 40' wide (EW) and 100' > long (NS), the CM is at the geometric center > (20,50), and the CR is at (19,49). To add the 5% > eccentricity, ONE WAY would be to assume the CM to > be at either of the following two points: point 1 > = (18,45) or point 2 = (22,55). The analysis > assuming point 1 as the CM would yield the highest > shear for all NS walls to the left of the CR and > all EW walls below the CR. The analysis assuming > point 2 as the CM would yield the highest shear > for all NS walls to the right of the CR and all EW > walls above the CR. > > In the particular case I described, the 5% > eccentricity "goes past" the CR on both sides, so > that the x- and y-values for the CR are bracketed > by the two CM values. However, this is not always > the case. Suppose in my example the CR was at > (16,43) and all other items remain the same. Now > the CR is not "between" the two displaced CM > points. In this case, using point 2 is still > valid and appropriate, since it is 5% from the CM > for both x and y. However, using point 1 results > in a LOWER shear for the walls to the left and > below the CR than would be obtained by using > DIRECT SHEAR ONLY. This is a no-no since 1994 UBC > Section 1603.3.3 says that "Forces shall not be > decreased due to torsional effects." [I THINK > it's in 1997 UBC Section 1605.2.1 also, but my 2nd > printing is missing that sentence. I'm pretty > sure it was put back in a later printing.] > Anyway, 97 UBC 1605.2.1 also says "Provision shall > be made for increased forces....resulting from > torsion....", implying that forces shall not be > reduced. > > Applying this to a building with wood shear walls > is another whole can of worms, however. And the > question of whether this results in a significant > difference depends upon the distribution of the > shear walls in plan. All buildings are > different. The more the walls are located at the > perimeter, the lower the effect of torsion will > be, as a percentage of the direct shear. > > I hope I have explained clearly. > > Mark Swingle, SE > Oakland, CA > > Dennis Wish wrote on 7/29/99: > > Displaced Center of Mass Question > > The design examples from the Feb 1998 SEAOC Wood > Seminar for the 97 UBC (and the draft of the > ICBO Design Manual Volume II) note "by displacing > the center of mass by 5% can result in the C.M. > being on either side of the C.R. and can produce > added torsional shears in all walls." > > However, the Reinforced Masonry Engineering > Handbook > by James Amrhein (5th edition 1994 UBC compliant) > simply adds the 5% of the diaphragm depth > perpendicular to the direction of loading and adds > it to the difference between the C.M. and C.R. > - leaving the displaced C.M. in only one location. > > Which is considered the standard of practice in > rigid diaprhagm analysis? If the more involved > method is applied in wood construction, has anyone > been able to calculate a significant difference > in added shear from torsion? If so, how many of > these buildings were residential (single and > mulitple residential) etc. > > Dennis Wish PE > > -
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