seismic support of horizontal roof gas lines

The need for seismic supports should be based on a rational analysis of the piping system. ASME B31.8 is the pertinent specification for "Gas Transmission and Distribution Piping Systems". Paragraph 833.4c of that spec indicates that bending stresses due to external loads, "such as weight of pipe and contents, wind, etc" when combined with longitudinal pressure stress shall not exceed 0.75S (where S is the specified minimum yield stress). I assume that the "etc." would include seismic loads.

Seismic loads can be determined based on ASCE 7, "Minimum Design Loads for Buildings and Other Structures", Section 9.6.3, "Mechanical and Electrical Componet Design".

If, after you analyze this system, there is a need for seismic restraints, then you will need to figure out a way of accomplishing it.

Many a support has been placed on a membrane roof. The key here is to prevent moisture penetration. This is often accomplished by using a "tar box" around the pentration.

Steve1 brings up a good point. The ASME codes limit the stress to the elastic range for pipework.

However, buildings and structures are typically designed with a response reduction factor Rw of some value greater than 1. ie. The structure is designed to absorb energy by plastic deformation.

We are therefore sitting an elastically designed pipe onto a plastically designed structure. I have suggested that we could use the design plastic deformation of the structure in the pipe design but not surprisingly, this suggestion didn't quite get off the ground.

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