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Re: Boggled Out By Load Combinations[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Boggled Out By Load Combinations
- From: BCainse(--nospam--at)aol.com
- Date: Sun, 11 Apr 2004 12:57:34 EDT
The delta-sub-s is the elastic deformation determined in a linear elastic model using the code level (read factored, not ASD) forces see CBC 1612.2 and 1630.1. It is an artificial way to use simple linear structural theory to get at deformation levels that reflect actual behaviors without using non-linear analyses.
The 0.7 x R x delta-sub-s "Maximum Inelastic Response Displacement:" is an attempt to determine a maximum deformation with yielding that will actually occur.
In CBC Section 1633.2.4 "Deformation compatibility" it states that "All structural framing elements and their connections, not required by design to be part of the lateral-force-resisting system, shall be designed and/or detiled to be adequate to maintain support of design of dead plus live loads when subjected to the expected deformations caused by seismic forces. P-delta effects on such elements shall be considered. Expected deformations shall be determined as THE GREATER OF [emphasis added] the Maximum Inelastic Response Displacement, delta-m, considering p-delta effects determined in accordance with Section 1630.9.2 or the deformation induced by a story drift of 0.0025 times the story height."
It goes on to say, " For elements not part of the lateral-force-resisting system, the forces induced by the expected deformation may be considered as ultimate or factored forces."
So, I would think you need to use the maximum of the allowable story drift, 0.0025 times the story height or delta-sub-m as your displacement in the PL/AE analysis. Then for ASD level forces, reduce that force by the 1.4 load factor.
If I understand your previous description, you are considering a non-structural architectural element. As such it would ordinarily be subject to the provisions of CBC Section 1632 but for the fact the force is generated by the deformation of the structures to which it is attached, not the mass of the element. As such, the code doesn't really give you any realistic or practical guidance in this section and I think you have appropriately looked to other sections of the code for guidance.
Bill Cain, SE
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