Too many shear walls 1

[Robbiee]

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.

 

[BAretired]

1. A rigid wall which is unconnected at the base will not be able to resist any lateral force other than dead load, but it will affect the relative deflections between levels.

Personally, I would connect all walls equally.

2. I don't know of any reason why the natural frequency should be a concern.

BA

 

[Robbiee]

Thanks BAretired,
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?

 

[Teguci]

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.
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.

 

[vandede427]

the two reasons i can think of to replace concrete walls with stud partitions would be 1) cost and 2) reducing the effective seismic weight.

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.

 

[rowingengineer]

How tall is the building, once you get to higher heights having having different stiffness in different direction will affect the wind load. Generally this happens if the building is sensitive to frequency for wind loading.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[DaveAtkins]

If your walls weigh enough to provide sufficient resisting moment, you theoretically do not need any dowels into the footings for overturning.

But you still have to justify transferring the shear into the footing, and for that you should have developed (or partially developed) dowels.

DaveAtkins

 

[Robbiee]

Again, the problem with too many shear wall is that the foundations, including the piles and the pile caps and the tie beams are all designed for higher than gravity loads. This is a 10-storey building and all shear walls have overturning moment that requires developing the bars to resist tension.

 

[hokie66]

Without the intermediate walls, you would have much thicker slabs. Same problem with mass. Agree with vandede, when you have adequate shear walls, don't try to give them back.

 

[msquared48]

The more shear walls, the less the stress in the walls and the smaller the vertical boundary elements, drag forces and greater the redundancy.

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

 

[ahmedhegazi]

For question #1 ,
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.

 

[vandede427]

teguci's response was the best

"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.

 

[Teguci]

Star? (Smile)

 

[Gumpmaster]

Like Teguci said, it will likely be a vertical and horizontal irregularity per ASCE 7-05 Tables 12.3-1 12.3-2. It probably qualifies for the Soft Story irregularity, the Weak Story irregularity, and the Out-of-Plane Offset irregularity.

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.

 

[JLNJ]

Maybe you can come up with a system which isolates the walls from the floors, thereby making certain that the shear does not make its way into the "non shear" walls. You will need some special details but I believe the code lets you do this.