External Vertical Force Included in Base Shear?

[uscme1997]

I am applying an external vertical force to the vessel. Is this external force included in determining the weight of the vessel to be used in calculation of the seismic force Fp?

 

[TomBarsh]

COMPRESS treats vertical loads as "weight" and considers them to act in downward direction only. The vertical loads are treated as having an associated mass. Thus vertical loads will affect the natural frequency of the vessel for the wind and seismic code analyses, as well as being considered in base shear for seismic analysis.

Horizontal loads are treated as a "force" without an associated mass. The magnitude of the resultant force is combined with the calculated base shear for both wind and seismic.




"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

http://www.codeware.com

 

[uscme1997]

Thanks for the help. I have another related question. If I enter nozzle loads into the WRC 107 portion of COMPRESS, will these nozzle loads be treated as external forces and be considered in calculation of the base shear and overturning moment in seismic?

 

[jte]

A good question. I had one vessel I designed long ago which I had to revise when the piping stress guys came up with the nozzle loads. It had an unusually large diameter overhead line. Forgot if I increased the size or number of anchor bolts, but I remember having to deal with it. Not a frequent situation, but worth considering.

jt

 

[TomBarsh]

The nozzle loads are applied only in the local stress analysis of the shell or head using the WRC-107 analysis. They are not applied to the vessel in a global sense; that is, they are not considered in the design of the vessel supports nor are they considered in the longitudinal stress calculations of the shell components (cylinders and cones).

If the loads on nozzles or clip/lug attachments are of such large magnitude that it is desired to study their effect on the vessel as a whole, including supports and longitudinal stresses in the shell due to compression and bending, then the equivalent loads must also be entered in the "Loads" menu as lateral forces and vertical loads.

At one time we actually considered applying nozzle loads to the entire vessel, this is easy to do with a computer program. However, this would needlessly complicate the user-interface and the amount of information that the designer would have to enter: it would be important that all nozzle loads are entered in strict orientation, and what loads acted concurrently, for very little gain or benefit in the reliability of the analysis.




"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

http://www.codeware.com

 

[TomBarsh]

Actually, it would be simple to confirm whether or not the nozzle loads are included in the foundation loads. Sometimes the best approach to solving a problem (or answering a question) is to tinker around with the toy, er, I mean tool, to see how it works...this is all that I really do myself.

For example, make a simple vessel model. No wind or seismic. Add a nozzle. Run calculations and save the base ring report. Then add loads to the nozzle, check the base ring report again, it will not have changed. Conclusion: nozzle loads do not impact the foundation loads in COMPRESS.



"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

http://www.codeware.com