Registered: 12/13/99
Posts: 6226
Loc: Houston, Texas, USA
That depends on your project specifications. However, you can find a host of publications (ASCE #7 for example) that recommend a vertical seismic load.
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Regards, Richard Ay - Consultant
Generally piping system consider lateral direction only for seismic unless otherwise asked for special case(example; when hydraulic mass need to considered because of submerged pipe in a liquid). You can find it the design criteria or guid of nuclear power plants per US NRC Regulation and ASME III NB/NC.