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seismic support of horizontal roof gas lines

seismic support of horizontal roof gas lines

seismic support of horizontal roof gas lines

(OP)
Is there any reason for seismic support of horizontal gas lines that sit on blocking on top of a roof membrane?  I have not found any references for supporting this type of gas line for seismic loads.  The seismic load of the pipes is very small, however, adding connections to this type of line is not practical.  It would penetrate and void the warranty on the roof membrane and the connector would need to be at least 8 inches long.  Does anyone know if this type of piping is typically exempted from seismic requirements because it cannot sway and even if it does move it will only slide on top of the roof membrane?  The connections to the mechanical equipment are flexible and the vertical risers are supported.  

RE: 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.

RE: seismic support of horizontal roof gas lines

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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