Seismic design of squat walls - response modification factor

[fracture_point]

I'm looking at the seismic design of a series of squat walls (h/l < 2.0) and determining the forces to design them with. Initially, the structure was intended to be designed with special reinforced walls using a response modification = 5 to reduce seismic forces by.

Following this through to design, it is clear that the walls are shear controlled even when there is only minimal flexure (longitudinal) reinforcement in the wall. By definition, the wall would fail in shear before flexural and therefore, by my understanding, the wall is clearly not able to develop enough ductility and energy dissipation required to reduce the seismic forces by R = 5.

Does anyone have any insight or resources to tackle this issue? We want to try and keep R=5 to reduce the demand on poor soil conditions.

Cheers

 

[Agent666]

If your failure mode is going to be shear based, then I'd say you are stuck with considering the elastic response (or what other loads are required based on your local codes in this situation) and the higher loads associated with this unless you can introduce some sort of flexural mechanism to achieve the energy dissipation required.

 

[hardbutmild]

I agree with agent666. As he said, you might not need to do pure elastic design, but reduction of 5 seems very unrealistic.
There might be a factor in your code that makes the R value smaller based on the h/L ratio (so it wouldn't be 5, but might be 2,5 or something).

You may also try to make them masonry, because in some codes h/L doesn't matter for masonry and their R value is the same for all h/L ratios (I don't know if that's the case in your code).

Maybe you could make your walls have higher h/L ratio by dividing one long wall into few shorter ones. You could use some non-bearing walls to fill up the space between them.

If nothing works, you could always perform a pushover analysis and determine the R value for your specific structure if your code allows for this.

 

[WARose]

The way I read Chapter 15 of ASCE 7-10 (don't know what code you are using), we are talking about a "R" value of 2 or 3 (depending on the detailing). A short RC wall isn't going to be able to form the hinges (like a moment frame) or otherwise have the ductility to give you a 5.

By the way (hate to make the situation worse), if this is SDC D or higher....you have to consider dynamic lateral earth pressures as well. (Depending on what code you are under.)