ASCE 7-05, Section 15.3.2.2

[sbigham]

Section 15.3.2.2 requires non-rigid non-building structures being supported by another structure to be modeled together. Does anyone know the underlying basis for this requirement? In the past suppliers have furnished loads and we have designed the support structures to carry equipment. Now, according to this, everything would have to be modeled together. The equipment suppliers will not give out their computer models, so it will force us out of the support strucutre business.

 

[phuduhudu]

There is a good explanation of this in a document supplied by FEMA called FEMA451 which has design examples specifically for nonbuilding structures as well as others and a bit of commentary. All their documents can be downloaded free - I think from the EHRP and they are the basis for ASCE7.

They only need to be modeled together if they form more than 25% of the seismic weight of the structure. Presumably the reason is that if they are non rigid e.g. they can't just be treated as a solid lump of mass and form part of the structure. You can no more look at them separately for a seismic analysis than you can look at the top 2 stories of a building separately from the lower 8 stories of a building.