4 Story Wood Building With Masonry Stair and Elevator Shafts 2

[lanejoe10]

I have a four story wood building that is 'L' in shape. There is a masonry stair tower at each end of the building and a masonry elevator shaft at the center of the building. The building is located in California in a very high seismic zone.

I would like to utilize only wood shearwalls to resist the seismic forces, so that I can use a response modification factor of 6.5. I would design the diaphram as flexible and account for the additional weight of the CMU walls.

Does anyone see any problems with this approch of not utilizing the masonry walls to resist the lateral forces?

Thanks

 

[jdgengineer]

Yes, I don't think the code allows this. Per Section 2305.1.5 (I'm looking at 2007 CBC right now, 2010 may have slightly changed Section #) "Wood shear walls, diaphragms, horizontal trusses and other members shall not be used to resist horizontal seismic forces contributed by masonry or concrete walls over one story in height".

What's your reasoning for not wanting to use the masonry walls? Seems to me like it's free very rigid elements. Is it because you haven't designed in masonry before?

 

[HuckleberryFinn]

If you're wanting to keep the Wood's R factor without doing all the special detailing for the masonry, could you detail the diaphragm connection to the walls such that the Stair/Shaft was isolated from the rest of the diaphragm and laterally supported itself? Granted, that you are still subject to whatever seismic design category you find yourself in with respect to the masonry....(I don't have my copy of ASCE-7 Handy but I think high seismic zones require special masonry detailing).

Just a thought...

 

[hawkaz]

If you want to ignore the masonry, then design the shafts as free standing boxes- and provide a seismic separation between them and the wood building.

Physics don't care that you don't want the masonry to take the load. Since the masonry is more rigid than the wood stud walls, it will attract the load whether you want it to or not unless it is independent.