Dual System Seismic Drift 25% IMF 1

[wonderasap]

I am using a "Dual System with Intermediate moment frames..." and "Special Steel Concentrically Braced Frames". The building is a two story classroom building classified in seismic design category D. There is a large base shear of 682K. Also, W8 columns are to be used to "fit" within the cmu partitions. W14x82 columns are located at the braced frames which are along the perimeter of the building.

While designing my moment frames for the 25% of my design seismic forces I cannot accommodate my inelastic drift limit. Though, based upon strength I have an adequate design.

ASCE7-05 is somewhat vague in it's Dual System Requirements - "IMF shall be capable of resisting at least 25% of the design seismic forces." Finally, does this statement include the drift limits of 12.12.1.1?

 

[ishvaaag]

I would say no, and you would have an adequate design. It only says forces and you have them controlled for that. Seismic drift contrarily will be driven by the conjoint behaviour and if you have it such way controlled to permissible level, your design would be right in my viewpoint. The CBF would be limiting the rotational demands at the IMF connections, and neither the IMF would be as weak as not to be classificable as moment frames that not have some significant stiffness respect the total.

In short, that I see seismic drift control an issue for the whole structure, specially when diaphragm action is warranted enough; if not the IMF could work somewhat independently and then logically should keep ward off themselves of their otherwise uncontrolled seismic drift.

 

[80smetalfm]

I would say for this building height if regular, the issue of lateral drift is not a major issue.

 

[bazflexure]

@wonderasap
How are you modeling for 25 % lateral loads
by taking dead weight multiplier to 0.25 ??

 

[wonderasap]

I am using RAM Structural System and have created two models. The second model has only the moment frames for the lateral system. I reduced the Ss and S1 values by 25% in the seismic load generator. I verified that my base shear and story shears are 25% of the true values.