CMU core as shear wall

[PowersPE80]

I have a CMU elevator core that I am using to resist lateral loads. I have been assuming that the 2 walls in the x-direction resist the loads in the x-direction, and the 2 walls in the y-direction resist the loads in the y-direction.

Can I assume that all 4 walls of the core are acting as a "tube" to resist my lateral loads? Is the strength of the "tube" greater than the strength of the 2 individual walls in the direction of the load? If so, what is the best way to analyze the "tube"?

Thanks.

 

[ron9876]

Check ACI 530 for the allowable overhanging flange for masonry walls. Probably can't use the full section. Also the wall at the elevator door won't be as stiff as the opposite wall.

 

[dik]

I've done that often... you might check with local practice regarding using the flange... the wall is stiffened by the floor at each level and may not fall into the outstanding leg category.

Dik

 

[DST148]

@PowersPE80: Ref ACI 318-05, section 21.7.5.2 - Effective flange widths of flanged sections shall be smaller of one-half the distance to an adjacent wall web and 25 percent of the total wall height. You may read commentary for more detailed info.
Ref ACI 530-05, section 1.9.4.2.3 - Effective flange width shall be lesser of 6 times the flange thickness or the actual flange on either side of the web wall. Unlike concrete structures, achieving a shear stress transfer at the interface of the web and the flange is a bit difficult in case of masonry structures even with a combination of interlocking units, running bond, and horizontal reinforcing . The limitations given in the code are more of a traditional nature.
Flanges can be included for one direction of loading and ignored for other direction of loading where the effect is small. For a single elevator shaft I would consider an open C-section or C-Section with lips if return walls are available on the door side. On the door side the shaft walls are joined across openings by spandrels which produces coupling. The coupling may be considered in concrete structures. However, in masonry structures this coupling is generally ignored since it is very difficult to achieve shear and ductility demands for these coupling beams.
The final decision about modeling rests with the designer.