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cessna98j (Civil/Environmental) (OP)
11 Jun 12 21:35
We're designing some pretty substantial conveyor galleries (25' wide x 100' long intermediate span) for an industrial project, in which the galleries are supported by bents every 100' and anchored for longitudinal seismic load about every 600' at a transfer tower (pinned at one tower, allowed to slide at the other tower).

Based on our seismic parameters for the site, we're ending up with longitudinal seismic loads of around 1,000K (ASD) that will be transferred from the galleries at each anchor point (to the transfer tower). We planned to account for the omega overstrength factor at all of the gallery connections.

However, since the bottom chord of the gallery truss itself acts as a collector (this is where its pinned at each intermediate bent, and at the anchor point) and longitudinal EQ force is transferred from each 100' segment to the next, increasing in net force up to the anchor point.

Based on the verbiage of ASCE 7-10, 12.10.2.1, do we really need to include omega overstrength factor in the design of the truss chords for longitudinal seismic since they act as a collector element? Bumping up an additional 1000K of axial load in the chords for Long EQ is a substantial task, but I don't see any way around it unless I'm missing something.

Any thoughts or previous experience with something similar would be greatly appreciated!
ishvaaag (Structural)
12 Jun 12 17:27
Reading the code it seems clear they want the overstrength factor in place but as note in exception to 12.4.3.1 says

The value of Emh need not exceed the maximum force that can develop in the element as determined by a rational, plastic mechanism analysis or nonlinear response analysis utilizing realistic expected values of material strengths.

So if your 1000 kips are not yet from this detailed analysis, I would try the nonlinear response analysis just to see how the forces on the chords fare.

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