Lx,Ly and UNB,UNT in STAAD

[subbu1985]

Hi!!!!

In STAAD if we specify Lx,Ly and UNB,UNT commands to a full member (even when it has not been split up by nodes) how will STAAD respond??

 

[SteveGregory]

It uses the values for the parameters that you specify regardless of the member length.

 

[subbu1985]

Thaks Steve,

But in that case, using UNB,UNT for beams instead of Lx,Ly should result in a different Interaction Ratio right??
Because UNB,UNT commands in general prove to be critical for beams...

I've attached two STAAD files for your reference..
Both with similar loading..

One file has secondary members modeled in it with both UNB,UNT & Lx,Ly commands for member 1. (The comments in the editor file will specify which is member 1)
When you analyse with Lx,Ly commands it'll give some interaction ratios for members.
When you analyse with UNB,UNT commands you'll get higher interaction ratios... UNB,UNT will be critical.

The second file does not have secondary members but still has UNB,UNT & Lx,Ly commands for member 1.
But in analysis of file 2, both the commands give almost similar interaction ratios..

Can u kindly look into it and explain the reason??

 

[SteveGregory]

Lx and Ly is used for calculating the allowable compression in a member. UNB and UNT is used for calculating the allowable bending stress in a member.

 

[muh2008]

It should be Ly & Lz. these are the local axes where Lx is not right.

 

[SteveGregory]

You are right! LZ and LY

 

[subbu1985]

Agreed Guys... It's Ly,Lz only.. I had used Lx,Ly as just indicative parameters..

Anyway.. Did u check the STAAD Models that i had sent????
Any observations on the behavior in the two models..
Think it'll throw some light...