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# Seismic Design Force

## Seismic Design Force

(OP)
First, I must say that this is not a criticism of Compress, but just an observation.  Compress does a fantastic job, and is an excellent piece of software.

I did, however, notice something peculiar about Compress and the way it calculates the seismic design force, Fp, using equation 13.3-1 from ASCE 7-05.  In regards to the variable Wp, or the component operating weight, Compress uses the vessel's new operating weight, which is correct so as long as all of the external vertical forces placed on a vessel are weights; e.g, forces due to a mass and acceleration.  However, if the forces acting on the vessel are massless, such as thermal nozzle loads, and are not due to the acceleration of gravity acting on a mass, then they should not be considered as contributing to the variable Wp since Wp is meant to represent the weight of a mass distribution.

Again, Compress is not doing anything wrong.  The software specifies that what you are placing on the vessel is a load, which intrinsically means it is a weight, although this is not spelled out anywhere.  I believe many users commonly, and incorrectly, place external loads on vessels to represent massless forces such as thermal loads.  This practice is incorrect, is it not?

### RE: Seismic Design Force

The COMPRESS Users Manual indicates that vertical loads are treated as resulting from a mass.

"Vertical loads are considered to be masses. Adding a vertical load to a vessel affects the vessel weight and period of vibration. The load may be attached to the outside or the inside of a vessel. If eccentrically positioned, the vertical load induces a bending moment. An example of a vertical load would be an overhead condenser."

Lateral forces are treated as a force without any associated mass.

The mass attributed to the vertical load will be considered when determining the vessel natural frequency.

"Although this forum is monitored by Codeware it is not intended as a venue for technical support and should not be used as the primary means of technical support."

Tom Barsh
Codeware Technical Support
www.codeware.com

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