It is a real a good practicabel idea two load cases XY and ZY define ?
Or its not better allways one XYZ load case
to calculatet ?
Ok, for the stress calculation in the pipe we can become by maximum from the XY or ZY load cases lower stress as by XYZ load case.
But what is with the stress in the restraint structures or nozzles on tanks ?
To be realy sure by the stress calculation for this components we must 2 calculations for the load case XY and for the ZY make.
Or we must take the maximum moments and force from both load cases XY , ZY.
And then ist the advantage for this components
off.
How to do this normaly ?

Typically, the codes tell you to compute a horizontal "g" factor. The load case to be examined consists of this horizontal load, plus 2/3 of this load in the vertical direction.

A seismic event is really a wave moving through the soil, so there are components in all three directions.

If the piping system is predominately in one direction (either X or Z), many analysts would run two cases (XY and ZY), where the Y value is 2/3 the horizontal. If the system didn't have a predominate direction, and you wanted to look at a direction of 45deg in the horizontal plane, then you could setup a case such as (.707X, .667Y, .707Z).

I would then have CAESAR II summarize the restraint loads from all my cases (using the "MAX" combination method) for support design.

You would not want to run a case such as (X, .667Y, Z), since this results in a horizontal load 1.4 times larger than necessary.

Herr Ohliger,

The B31 codes do not provide specific guidance on this subject they direct you to consider and take these inertia loads into account during the design process. Ususally specics of G values are per the local building code authority.

Having said all this inertia loads and their contribution to piping failures are macht nichts. Much about nothing. Most failures are caused by seismic anchor motion or SAM. When a line travels from one structure to a separate structure both structures have different periods and will probably displace differently in time. If the line is fixed in both buildings these differential displacements may cause pipe failures to occur. So adequate flexibilty or modification of restraints are important considerations, more so than G loads.

The B31 codes talk about this and also tell you to consider it in your work. So G loads for building code authorities and SAM for the pipe!