Dual system.

[shin25]

I am trying to hand check the structural analysis of an eight storey dual system (shear wall and moment frame) building. For the lateral load, how should I assign loads to moment frames and shear walls? I assume they will be shared by moment frames and shear walls based on their relative stiffness. Is there any easy method to figure out this distribution?

 

[msquared48]

Worked on a high rise in the very early 80's with a prestressed concrete core and exterior moment frames. If my memory serves me right, with a dual system, we took the entire shear to the central concrete core, and an extra 25% to the exterior moment frames. From there, we did our "adjusting" for story drift and axial deflection of the moment frame columns.

Mike McCann
McCann Engineering

 

[Lion06]

one way to do this is to model the moment frame and apply a 1 kip point load to the top and determine the drift. Use that to compare to the stiffness of the shearwalls and distribute the loads accordingly (since you can calc the bending stiffness of the shearwalls directly).

 

[prsconsultant]

The load distribution should be based on relative stiffness of the frames and shear walls. As a minimum, the moment frames should be able to resist 25% of the lateral forces. This does not mean that the frames can not resist a bigger portion of the lateral force. Depending on the building geometry (plan / vertical irregularity), location of the frames and shear walls and relative stiffnesses there could be significant differences in how the load get distributed.

Unless it is very obvious that shear walls would resist significant portion of lateral force, I would suggest creating 3-D model to get proper load distribution.

 

[shin25]

The extra 25% appears to be the requirements for structures in seismic design category D and above.

 

[Lion06]

Can someone help me out with this 25% number? Where does it come from and why is it important? Why is it necessary that the moment frames resist 25% of the lateral force?
Is this a seismic requirement only, as shin references?

 

[shin25]

The design should consider for seismic design category D and above the larger of-
1. Force effect in the moment frame considering the relative stiffness of the shear wall and the moment frame, and
2. 25% of the total lateral force (seismic) acting on the moment frame alone.

 

[OldPaperMaker]

In ASCE 7-05, Section 12.2, "Structural System Selection", item 12.2.51 "Dual System" directs that the moment frame shall be capable of resisting at least 25% of the design seismic forces.
Of course, the rest of this section has the myriad of other trivia that some code writing committee added to the "to-do" list.

 

[connect2]

OldPaper Maker, nice reply re: re code writing committee(s). Relative rigidity being relative between dual systems and its individual system elements, the process being iterative could I guess then yield situations where the MF doesn't capture 25% of the lateral laod, then in any case the MF system shall be designed not less thn 25% of the lateral load regardless. At the end of the day it becomes a cautionary lower bound in the face of uncertainty.