Nonstructural Component Design in Seismic Design Category B

[skc8002]

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.

Thanks,
SKC

 

[skc8002]

I meant exempts from "Provisions of Chapter 13" and not Chapter 14. Sorry for the typo.

SKC

 

[ishvaaag]

I think it must not be necessary to apply the base shear of the mechanical equipment separately as a load for the general analysis of the structure. If the code accepts calculating the general structure without such precision, must mean that following the procedure is thought to be safe enough.

One must never forget that seismic codes do not establish general safety against all possible earthquakes, bust simply set a standard (seismic event or events) that must be complied with. Said short: they impose a mandatory opinion as to what level of earthquake the structure must resist.

However, clauses demanding the contemplation of the forces on the mechanical components and their attachments and connections with the general structure signify the connections are to be designed to stand the whole effect of the base shear of the equipment, mainly in framed structures meaning that other than sustaining the forces these must be spread efficiently to the supporting parts through dedicated members if necessary, and of course, the path of the forces to foundations must be ensured for all relevant hypotheses.