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[SEAOC] Re: [SEAOC] Seismic Design in Wood Frame Construction[Subject Prev][Subject Next][Thread Prev][Thread Next]
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- Subject: [SEAOC] Re: [SEAOC] Seismic Design in Wood Frame Construction
- From: dennismc(--nospam--at)mail.mcn.org
- Date: Tue, 09 Jul 1996 08:24:33 -0500
I have a few comments about plywood shear walls. 1. On sophisticated engineering projects with lots of scrutiny, I almost always provide a continuous tie across the top of the windows. This is accomplished with either CS straps or 1/8" steel straps running end to end of the building. I treat this as a collector and use it to redistribute loads to the walls below. The plywood panel above and below windows often have very low shear with plywood extending end to end of the building with few or no openings. Once at the window line, loads are distributed to the much smaller panels between openings. I use the top of the opening as the height of the panel to keep within h/d ratios. Sometimes, I put a similar strap at the bottom of the opening to redistribute the loads to the panels below the openings. In essence, each collector is treated as a floor line. By doing this, deflections can be computed for the entire wall instead of individual panels. The openings will of course reduce the capacity of the overall wall. If deflection is a real concern, I generally resolve the wall into 3 sections, one above the opening, one at the openings and one below the openings. Add them all up and come up with a figure. Since engineers are guessing at earthquake and wind loads anyway, how close can we really be in our calculations. This system, in essence, creates a plywood frame. Even though there is some give to wood and a true frame is not developed, it is extremely stiff, far more than just a shear wall by alone. I have used this method for a couple of decades to get jobs thru DSA with little resistance. 2. As far as wind load, I agree with what Ken indicates below. When the new wind loads came into the code in the late '80's, I resisted and still do. Buildings all over the state were failing because of earthquake forces. The earthquake forces were reduced and the wind loads increased. I have seen very little if any buildings failing as a result of wind loads but the loads went up. Does this seem logical? I I live on the north coast where we regularly get 100 mph winds each year. Exposure is almost always type "C". Old buildings and new buildings alike seem to hold up very well against them. Most are not engineered and do fine. The damage is usually to fences that were not engineered and minor things like roof covers attached to mobile homes. The real damage comes from the trees. Large trees fall over all the time, often times landing on houses. How do you design for that? At 09:52 PM 7/8/96 -0700, you wrote: >Does the Emergency Seismic Structural Requirements also apply to shear >walls where wind is the governing force. I design a lot of tract homes >where the local jurisdictions require that the wind speed of any where >between 70MPH and 90MPH be used along with the killer "EXPOSURE C", >which usually results in wind base shears double the seismic base shears. >To design using the new shear wall specs for conditions of wind factors >of 90MPH and EXPOSURE "C" would not be practical or economical >engineering design. Some one needs to wake up the local building >departments that earthquakes kills and destroys buildings not the >unwarranted and misuse of Sections 1614 and 1616 as applies to single >family dwellings and tract developments. > >Ken Wilkinson > >... > > > dennismc ...
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