Too many shear walls
Too many shear walls
(OP)
In a concrete residence mid-rise building, all the partitions between the units are concrete walls that continue from foundations to the top. The building is narrow and long, and the wall are in the short direction, which makes it very stiff in this direction. Questions:
1. By providing minimum rebars in some of these walls and not developing the rebars at the base for tension, are we eliminating them from the lateral resisting system, and
2. Is it OK to have the natural frequency in the short direction 3 time greater than the long direction as long as the storey drifts are within the allowablelimits?
Thanks for the input.
1. By providing minimum rebars in some of these walls and not developing the rebars at the base for tension, are we eliminating them from the lateral resisting system, and
2. Is it OK to have the natural frequency in the short direction 3 time greater than the long direction as long as the storey drifts are within the allowablelimits?
Thanks for the input.






RE: Too many shear walls
Personally, I would connect all walls equally.
2. I don't know of any reason why the natural frequency should be a concern.
BA
RE: Too many shear walls
We did connect all the walls, but, There are no columns here, only walls, therefore, the foundation system is now totally designed to resist loads due to earthquake, which made it very expensive, especially becasue the building is located in a high seismic area. And regarding the NF, you mean it is OK for the building to be much more flexible in one direction than the other?
RE: Too many shear walls
2. The natural frequency should not be too much of a concern for a rigid shearwall type building. If you have a flexible moment frame in the other direction you might consider a modal analysis. This might be a good idea for the high seismic area anyhow.
RE: Too many shear walls
i did a condo or two for a developer who required full-length concrete walls between all units and marketed that fact to buyers.
but seriously, don't make us talk you out of too many shearwalls 8). usually as SE's were fighting for more.
RE: Too many shear walls
Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it
RE: Too many shear walls
But you still have to justify transferring the shear into the footing, and for that you should have developed (or partially developed) dowels.
DaveAtkins
RE: Too many shear walls
RE: Too many shear walls
RE: Too many shear walls
Redundancy is particularly important now with all the major earthquakes we are seeing, and will continue to see for some time to come.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Too many shear walls
you can eliminate whatever you do not need from lateral load resisting system ,just load will go to the stiff elements
For questions#2 ,
check building acceleration under wind for human comfortable factor(Serviceability limit),i think this one is critical.
Check interstory drift under sseismisc load ,and overall drift under wind,the guys who are doing the external building envelope ,facade or whatever should be happy about drift under wind load.
For development length , you can reduce them if the bars not highly stressed, ACI 318 clause 12.2.5
last thing ,what is the stiffness of slab compared to walls for lateral stability?,in some type of buildings as the one you describe ,slab and shear walls form like a frame so you will find the slab is part of lateral force resisting system ,just want to make sure you considered this point.
RE: Too many shear walls
"1. Just because the walls are pinned at the foundation doesn't eliminate the lateral loads the shearwalls attract on the way down. What will happen is that all of the lateral loads in the other shearwalls will have to redistribute through the diaphragms to the rigid shearwalls within the last two or three stories. The code penalizes this type of lateral redistribution."
he's dead on with this point. the walls will attract load from top to bottom, don't know they don't have dowels into the foundation, then have no where to go with it and be forced to redistribute. you don't want this.
RE: Too many shear walls
RE: Too many shear walls
If it's allowed by all of the referenced sections in the table, then your forces will increase greatly and you may have to do a modal response spectrum analysis taking into account the actual story stiffnesses. It may not even be allowed based on your seismic design category.
RE: Too many shear walls