Shear wall dead weight & overturning
Shear wall dead weight & overturning
(OP)
When designing shear walls, I have typically used only the dead weight of the wall itself (studs & sheathing) to calc the required holddowns. Of course, the wall could never lift independently of the rest of the bldg. What is a more realistic approach to obtaining the actual deadweight that resists overturning? For example, if an interior shear wall intersects with an exterior wall, how should one consider the dead weight of the exterior wall?
Chris
Chris






RE: Shear wall dead weight & overturning
For example, if you have a shearwall on the end of a 60 foot long building, say its 20 feet long and 10 feet high, then we would include a 10 foot length of the intersecting wall (10 ft. out of the 60 ft) in our dead weight calculations. We also include floor or roof dead load in this as well.
Just make sure that the CONNECTION of this intersecting wall is adequate for its 10 ft. length of weight.
RE: Shear wall dead weight & overturning
That is an interesting approach, and I can see an intuitive justification for the height=length approach. Do you know of an experimental or rational justification for your approach? How do you handle an interior shearwall that intersects an exterior wall? Do you take lengths of the exterior wall in both directions equal to the height of the shear wall? For example, would a 10' high shear wall be held down by the weight of 20' feet of exterior wall (10' in both directions)? Are you also saying that you would count the dead weight of the floor bearing on that length of exterior wall? Also, what nature of attachment to that exterior wall would be required?
Thank you for your response.
RE: Shear wall dead weight & overturning
Yes, for an interior shearwall intersecting an exterior wall, we'd used the weight of the exterior wall in both directions.
The floor/roof weights would also be used, but only if they are bearing ON the exterior wall, not hanging from them below.
The connection, then, between the exterior "weight" and the shearwall, would have to be able to support the dead weight you are counting on. So if you have an exterior wall that is 10 feet long, and 10 feet high (10 x 10 x 9 psf weight = 900 lbs.) The 900 lbs would have to be transferred to the shearwall across the 10 foot height = 90 plf. This is usually accomplished through the blocking at the intersection and the sheathing in both walls. Thus, we make sure that the exterior wall is properly sheathed with gyp-board or plywood that can handle 90 plf.
RE: Shear wall dead weight & overturning
RE: Shear wall dead weight & overturning
None of this, obviously, is of an exact nature. We just try to err on the conservative side while taking advantage of obvious weight that the shearwall would have to overcome to overturn. Using this method definitely decreases the number and size of holddowns.
RE: Shear wall dead weight & overturning
Also, when including the floor dead load and roof dead load as a part of the resisting moment, one must look at how conservative the loading was assumed in the beginning. For framing design, the design loads are often overestimated. But in overturning design, using this conservative dead load will result in an unconservative design for holdowns. Different codes have different factors to be applied to reduce the dead load resistance to overturning. UBC97 applies 0.9, previous versions has 0.85 for resistance to seismic overturning. JAE, in this case, would you take 0.9 of the accurate DL or 0.9 of the original conservatively estimated dead load?
RE: Shear wall dead weight & overturning
RE: Shear wall dead weight & overturning
RE: Shear wall dead weight & overturning
RE: Shear wall dead weight & overturning
RE: Shear wall dead weight & overturning
The dead load is reduced according to the applicable code (IBC went overly conservative with its 0.6 x dead load for seismic).
RE: Shear wall dead weight & overturning
weight of the foundation shall be considered for "overall" stability of the system but for anchorage/holdown design, it can not be included.
Re SacBleu's comment:
I agree that 0.6 dead load is very conservative. In the UBC, it used to be 0.85 for a long time, then it was changed to 0.9 in 1997. We'll have to wait and see what this factor will be in California for the next code cycle.
RE: Shear wall dead weight & overturning
If we multiply the applied lateral load by a factor to increase the overturn moment, and use a smaller reduction of dead load, we get a more uniform Factor of Safety against overturn, but the "bookkeeping" becomes a mess unless we use canned software or Excel.
RE: Shear wall dead weight & overturning
DBD
RE: Shear wall dead weight & overturning
We have a special detail that connects an interior shear wall to the exterior wall channel (Tee-condition). The number of nails transfers the tributary dead load of the exterior wall.