Shear wall lateral system - what is the minimum percentage

[EngrRC]

I am designing a building with lateral resisting system composed of shear walls to ASCE 7-16. Vertical resisting system uses frames and shear walls.
Some earthquake base shear is taken by the columns - is there a limit (minimum %) for which the "shear wall system" no longer becomes applicable and it should be designed to Dual System?

For dual system, I know that the moment frames shall be designed to carry at least 25% of the base shear. To clarify, my question is when should I comply with this- when my columns resist 5%, 10%, 25% or what percentage of the base shear? The code does not seem to explain this.

 

[HTURKAK]

I think the standard is ckear. Pls look 12.2.5.1

12.2.5.1 Dual System. For a dual system, the moment frames shall be capable of resisting at least 25% of the design seismic forces. The total seismic force resistance is to be provided by the combination of the moment frames and the shear walls or braced frames in proportion to their rigidities.

As specified in Section 12.2.5.1, and Table 12.2.1 the moment resisting frame in any of the dual systems must be capable of resisting at least 25% of the seismic base shear.If your system does not satisfy min. 25 % criteria , your system can not be considered dual system and needs to be designed as sinngle system.

 

[EngrRC]

My question stems from the code specification that if frames are participating in the lateral response, then they need to be designed for seismic provisions - this is applicable for dual systems where beams and columns are acting as moment frames. This can be a stringent requirement especially if special moment frame would be required.

Now, are you saying that if my columns resist only 24% of the base shear, then I can stick with the shear wall system and not design the columns/beams as moment frames? To me, the dual system requirement of 25% means that at a certain point (e.g. when columns resist at least 10% of the base shear) you need to amplify the forces to meet this requirement. What I would like to clarify is - what is this limit?

 

[HTURKAK]

Now, are you saying that if my columns resist only 24% of the base shear, then I can stick with the shear wall system and not design the columns/beams as moment frames?

I am not saying but the ASCE standard saying this. Explicitly says (For a dual system, the moment frames shall be capable of resisting at least 25% of the design seismic forces.). If the frames cannot resist at least 25% , the system will not be , no more dual system. Say the frames can resist only 24% of the base shear, the system will be single , shear wall system . You are expected to design the walls to resist total seismic loading. You can develop two systems ; the first one the frame columns gravity only , hinged at storey levels to find the design effects for shear walls , the second one , perform the analysis as dual system and the columns will resist to the design displacements.

 

[KootK]

To clarify, my question is when should I comply with this- when my columns resist 5%, 10%, 25% or what percentage of the base shear?

This will have nothing to do with how much shear your columns resist relative to your walls. Rather, in a high seismic, ASCE7 jurisdiction, it works like this:

1) You only have to go with a dual system if that is triggered by other code requirements (height limits etc). To my knowledge, the percentage of shear taken by the columns is irrelevant.

2) No matter how much shear is taken by your columns, you have two options for dealing with that:

a) Design the columns for the estimated shear.

b) Design the columns for the estimated deformation of the shear wall lateral system acting alone (most common approach for ELF).

In thinking about this, it may be useful to understand the motivation for dual system design.

The idea behind dual system is that you want a backup system to keep the building from flopping over after the shear wall system has been beat to hell and is probably leaning over by h/40. This, so that folks can successfully escape the building. The dual system provisions do not envision both systems being mobilized at the same time. In fact, the moment frame part of the dual system is not even intended to be utilized during the EQ. It is intended to be utilized immediately after the earthquake. This is why the percentage of shear being drawn to each system based on stiffness is immaterial as far as whether or not a dual system is a code requirement.