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Nonstructural Component Design in Seismic Design Category B
- To: seaint(--nospam--at)seaint.org
- Subject: Nonstructural Component Design in Seismic Design Category B
- From: Sushil Chauhan <sxchauhan(--nospam--at)gmail.com>
- Date: Sun, 7 Jun 2009 11:31:06 -0500
Please publish following message on SEAINT mailing list
I have had discussions on this in the past and would like to know how other people do it. The scenario is that there is a steel structure (industrial, no floor decking, braced vertically and horizontally) that supports multiple containers/vessels (mechanical components) at upper floors. This is related to seismic design of these Nonstructural Components in Seismic Design Category B (low seismic) where wind design governs mostly. These do not fall into the category of nonbuilding structures because the weight of steel structure is significantly higher. Clause 13.1.4 item 2 of ASCE 7-05 completely exempts (no strings attached) electrical and mechanical components in SDC A & B from the provisions of Chapter 14. I understand it like this - The ground motion in SDC B is pretty low and hence the amplification of ground motion on upper floors is not significant and hence no need to worry about (1+2z/h) amplification factor. Similarly the difference between actual response of the component as compared to the response of the floor it gets attached to is not that significant in SDC B because of low ground motion and hence we do not need to worry about ap and Rp factors.
So one will calculate base shear for the component(that is seated on an upper floor)using usual Equivalent Lateral Force method (ELF) and apply it on the structure's floor anchorage points as an external seismic load to the structure. The anchorage points on the floor will be designed for the this base shear. This base shear will be added to the ELF story shears for the structure (obtained by applying ELF only on structure). This approach generally will give a higher total base shear for the system (structure + mechanical components) as compared to lumping the component mass to structure's floor mass and applying Equivalent lateral force method on entire system.
I appreciate your comments on the approach.