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Code min. Design requirements
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- Subject: Code min. Design requirements
- From: Lynn <lhoward(--nospam--at)silcom.com>
- Date: Sun, 01 Aug 1999 20:30:49 -0700
The following is a very quick case study of the change in seismic design requirements that have occurred here in Santa Barbara between the 1976 UBC and the 1997 UBC Let's consider a two story wood framed house, totaling 3,000 square feet The ground floor is 2,000 square feet and the second floor is 1,000 square feet. Assume the weight of the roof plus the upper section of second floor walls is 35 Kips (concrete tile roof). Assume the weight of the floor plus the lower half of the second floor walls and the upper half of the ground floor walls is 40 Kips. Assume at the ground floor level you have 60 lineal feet of shear wall available in each direction to resist shear forces. Plate heights are 8 foot, and the minimum length of any shear wall is 5' 4", and the maximum length is 25'. For simplicity, the shear forces will be divided uniformly to all walls. 1976 UBC Chapter 2312, Equation 12-1 V=ZIKCSW Z= .75, I = 1.0, K = 1.33, CS = .14 V=.14 W Vbase = .14 x 75 Kips = 10.5 Kips Unit shear on ground floor shear walls = 10,500 lbs. / 60 ft. = 175 plf. Therefore, all shear walls can be 5/8" thick drywall, blocked edges and nailed at 4" o.c. FEMA 310 states that this house will provide a life safety level of performance for the occupants. 1997 UBC Seismic Zone 4 Soil Sd Na = 1.3 R = 4.5 V= .227 x Rho. Rho in this idealized case turned out to be 1.1, so V = .25W, 80% higher than the 1976 UBC Vbase = .25 x 75 = 18.75 Kips Unit shear in walls = 18,750 lbs. / 60 ft. = 312.5 plf Per Table 25 - I in the 1997 UBC, the allowable shear for 5/8" gypboard, blocked and nailed at 4" o.c. is 87.5 plf. These walls are over-stressed 350 percent by current Code standards, a very significant number. Obviously, the 1997 Code would imply this structure would NOT provide a basic life safety level of performance. So which is it? Lynn
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