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RE: 97 UBC
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: 97 UBC
- From: "Yousefi, Ben" <Ben.Yousefi(--nospam--at)ci.sj.ca.us>
- Date: Fri, 2 Jul 1999 17:16:51 -0700
As it was pointed out earlier the delta s is based on strength design level forces. Now I'll try to take a crack at what I believe the source of confusion is. When checking drift, you only need to verify that lateral displacement of your vertical lateral force resisting system (in your case the tilt-up walls) meets the drift limitations. This is not cumulative with the diaphragm deflection. However, the cumulative displacement should be used to check for deformation compatibility of the gravity members, such as the interior columns and walls perpendicular to the direction of load. Hope this helps Ben Yousefi, SE San Jose, CA -----Original Message----- From: Gerard Madden [SMTP:GerardM(--nospam--at)crjarch.com] Sent: Friday, July 02, 1999 11:40 AM To: 'seaint(--nospam--at)seaint.org' Subject: 97 UBC Question For all the Engineers interpreting the 97 UBC. I am designing Tilt-ups to the 1997 UBC and I find it extremely confusioning mainly due to the factored or strength level seismic forces obtained in the base shear equations and the usual guessing game of what the code is really asking me to check. At first glance, I decided to design the bldg using the Simplified Method. This would mean about a 20% increase in force compared to the non-simplified method. What this would allow me to do is ignore the messy calculation of determining the drift of the structure which must include foundation rotation, cracked sections & diaphragm deflection. However, being that I am 4.3km from a type A fault in Zone 4, My diaphragm design force is 0.3W (working stress) compared to the .183W of the 1994 UBC. My structure design force is .267W for everything else but the diaphragms. This makes for extremely heavily nailed diaphragms and my collector forces are ridiculus even when assuming the diaphragm yields at 2X the allowable capacity. Overturning of the interior shearwall on my building controls the collector force fortunately but with the force level about 45% higher (using the simplified method) versus the 94 UBC, many connections simply can't work in a slender wall. I began to re-examine the need to do the simplified method. The diaphragm deflection calc is not difficult. The wall deflection in-plane is insignificant. Leaving me to figure out the foundation rotation and its contribution to the drift during rocking. For the time being, I'm assuming that the foundation, as long as it meets the bearing pressure requirements under seismic loads, will have a mininal contribution to the drift. Now, I calcuate my diaphragm deflection which turned out to be about 3.5 inches. I have a 21ft tall building with the walls designed for the p-delta effect per 1914.8 alternate design method for slender walls. The walls are designed for a displacement greater than the diaphragm deflection and I could say that they are adequate for this deflection. But when I read the code, it says Delta S shall be calculated using the design seismic forces. I am confused, does this mean STRENGTH DESIGN SEISMIC FORCES only or either the WORKING STRESS LEVEL FORCES or Strength Design Forces. To get the DELTA M drift the 0.7RDelta S is compared with the 0.025H limit which I would think is based on the Strength design force. This is all destroyed if I am forced to compare the wall deflection capacity to a factored force diaphragm deflection. The wall deflection is based on the Nominal Moment and Cracked Section properties of the wall. The reinforcing and adequacy of the wall is based on the 1.1x1.4xFp. Fp being the working stress seismic coefficient. If anyone has any insight on the simplified method and drift I would appreciate there opinion. By the way, when are the seismic design manuals II & III available? I'm tired of these examples that have given forces, displacements, etc... that all figure to a clean answer or some assumption that is pulled out of nowhere. Gerard Madden CRJ Associates Design Engineer, Structural Department 650-324-0691x129 650-324-0927-fax gerardm(--nospam--at)crjarch.com
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