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RE: Seismic Drift Loading

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Thanks for your response. It makes sense to me.


The 1” drift is the seismic drift whereas there is a reduced (1/2” per floor) specified wind drift. Therefore, the seismic drift is controlling the design as far as the forces being created within the frame members from an applied displacement loading. That is why I wanted to understand if the 0.7E factor was appropriate for determining the forces within the frame from a displacement loading due to an earthquake or if the forces generated from the full displacement are required.







From: Jake Watson [mailto:jake.watson1(--nospam--at)]
Sent: Monday, January 07, 2008 8:45 PM
To: seaint(--nospam--at)
Subject: Re: Seismic Drift Loading


It sounds like your frame will be dragged along with the main tower.  With that said, the 0.7E is appropriate so long as you are using the ASD load combinations.  You are complying with the "deformation compatibility" provisions.  The forces are only less on elements that can tolerate the movement through a slip joint or have gone plastic themselves.

A brief example:  assume a braced frame structure.  Gravity columns tolerate the drift because they are assumed pinned top and bottom.  Good details will also allow for interior partitions to slip at the top because the walls are fixed at the base.  Meanwhile the force in the braces is limited because they (hopefully) go plastic and stretch/buckle inelastically.  All these elements still move the same distance but have vastly different loading.

Did that help?

Jake Watson, S.E.
Salt Lake City, UT

P.S. Have you considered that the drift is wind related and not seismic?

On Jan 7, 2008 1:56 PM, Josh Buckholt < josh.buckholt(--nospam--at)> wrote:


I am designing a frame that will lean on the main tower. The specified interstory drift given from the building EOR is 1" per floor and the frame that I am designing must accommodate this ultimate, factored, amplified story drift of the main building. I know that the building codes, ASCE7, etc. try to address the fact that, because of inelastic response, design forces will be lower than the elastic response and deflections will be larger.

I have incorporated these story drifts at the building interfaces as support displacements. My question is if I apply the full ultimate, factored, amplified deflection as an ASD (AISC 13th Ed.) load case, can I use the 0.7E factor in my load combinations or must I use the full displacement because that is what will happen? I know that the full deflection will occur and must be designed for, but I know that the forces will be less if the frame becomes inelastic. The inelastic response may be a different issue than the load combination issue.

If anyone has any code or other references, it would also be appreciated. This is an east coast project.

Thank you,