Bending moment on braced legs for horizontal loads.

[capablanca]

Dear Sirs:
I am reviewing a seismic analysis of a vessel supported on four legs done with Compress. The legs are X braced ;the bracings are located at midheight of the legs.Thus,below the bracings and above the bracings there are segments of legs that are submitted to bending due to the seismic horizontal shears.
I cannot visualize this bending effect in Compress output.
How this is accounted in the software? Also ,when calculating the Periods ,using Raleigh method ,how this effect is considered.
Thanks for any tip on this.
Capablanca

 

[TomBarsh]

In COMPRESS, the bracing for the legs serves only to prevent sidesway of the vessel. The COMPRESS method is not intended to model structures with several "bays" of bracing.

The centroid of the brace is assumed to pass through the base plate and through the connection of leg to the shell; in this case there is no bending moment imposed at an intermediate location along the length of the leg.

You might be able to "fudge" the KL value by adjusting the leg length entered. But of course this will change the vessel height and exposure to the wind.



"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

 

[capablanca]

Tom:
Thanks for your reply.Please comment following points to be sure that I understood properly your communication:
a)If somebody uses the y'parameter, "vertical offset distance" (page 12-15 of Compress Help Manual) Compress stress analysis of the leg will not account for the bending moments due to shear between y' and the base plate elevation. Care shall be then exercised when using this option. The same applies for the bending moment on the leg above the bracing up to the connection point with the shell.
b)Regarding the period calculated by Compress also the effect of the bending of legs is not accounted in a similar situation, using a nonzero value of y'. In this case the period calculated by Compress, for sway modes, will be shorter than the period if the effect were accounted.
Above considerations can be of importance for tanks supported in slender legs in a high seismicity zone.
In summary the Compress Manual words fully applies:"Compress will help you design vessels but will not think for you"
Regards Capablanca