Problem with truss 'Staad Pro' model 1

[clarke1973]

Hello,

Hoping someone can help me with a problem I am having with a truss staad model. it is a simple 2d model of a single span flat truss supported at either end by columns. The span is 15m and depth is 2m. Top and bottom chords are british UC sections, vertical and diagonal webs are equal angles.

I have applied a single central point load to the bottom chord and run the analysis. the problem is that there is no resulting axial force in either the top or bottom chord, which of course is not possible. The webs have the expected tension and compression forces and the overall deflected shape is as expected. It is as if the contribution of the chords is being ignored. All of the webs have their ends pinned, so i'm sure the end restraints are input correctly. other than that I cannot work out what's wrong.
I am relatively new to staad so i'm sure it is something simple that I am overlooking.
anyone had a similar problem before, any advice?

Thanks

 

[Bagman2524]

Make sure all members have rotation free at their ends.
Try analyzing the truss by hand and see what forces you get in the top & bottom chords.

"Look for 3 things in a person intelligence, energy and integrity. If they don't have the last one, don't even bother with the first 2. W. Buffet

 

[TehMightyEngineer]

I'm fairly familiar with STAAD but I'm having trouble coming up with something that could result in no axial load in the chords.

Are your truss web members connected to the chords properly? Go to tools > check multiple structures so see if parts are missing.

Check the deflected shape which will 90% of the time reveal modeling errors.

As Bagman said, check that all members are released at their ends in the MY and MZ directions (MX is torsional in STAAD, don't release that unless you have no torsional restraint on that end).

If those are all fine then I suspect the force is there but it's not being displayed properly. Under the post-processing tab (results) you can go to graphs and the Fx graph will show axial loads. Also, clicking on the Fx button will display axial force diagrams on the model (compression in red, tension in blue).

Maine EIT, Civil/Structural.

 

[BAretired]

Check your end supports. If they are both hinged at the level of the bottom chord, the resulting axial stress in the bottom chord will be zero. One end must be given a degree of freedom horizontally.

BA

 

[TehMightyEngineer]

I experimented with an old truss model and gave it fixed ends and compared that to the released end. While it does definitely screw up the axial loads in the truss (the top chord flips from compression to tension near the ends) both the top and bottom chord still carry axial load.

I've attached the staad input file for the truss I just checked so the OP can run it as a double check to their model. See if you have the same error or maybe you can use it to figure out what went wrong.

Maine EIT, Civil/Structural.

 

[clarke1973]

Thanks for all the replies. Well I have rechecked all the member end restraints, supports etc but still cannot figure out what is wrong with the model. I have attached the file, would really appreciate if someone could have a look at it and let me know what the problem is...?

 

[RWW0002]

For plane structures, the plane needs to be in the y, and x (or whatever you have defined as the "up" and "to the right") directions. When plane structures have nodes with coordinates in the "out of the page" (z) direction you get a warning similar to the following and the structure does not behave correctly.

WARNING : IT IS ADVISABLE TO ANALYZE THIS STRUCTURE
USING THE COMMAND STAAD SPACE INSTEAD OF STAAD PLANE.

This warning shows up in your output file. Switch your "x" and "z" coordinates and see if the results make more sense.

Hope this helps.

 

[TehMightyEngineer]

Yep, that'll do it.

Maine EIT, Civil/Structural.

 

[clarke1973]

RWW002,thanks a lot! the truss is actually part of a separate 3d model. it is a transverse member supporting trusses spanning in the opposite direction, I have just isolated it in as a separate model for this purpose to make it easier to examine. The original 3d model was set up as a plane frame and then modified (by copying and rotating etc) to create the final 3d model.

So my question is, when running the analysis is there a way to specifically analyse the model as a 'space frame' rather than a 'plane'? or alternatively do the fundamental properties of the model have to be changed in the model settings for example. I have looked through it but cannot figure it out...?

thanks

 

[RWW0002]

At the very beginning of the input file, you simply change STAAD PLANE to STAAD SPACE. You will have to pay a bit more attention to releases and supports, but otherwise there should be no significant change required other than that initial command.

Also, I might note that currently the chords are modeled as continuous members instead of pinned-pinned members of traditional truss analysis. There is a current discussion on this issue at

http://www.eng-tips.com/viewthread.cfm?qid=366939.

 

[clarke1973]

Great, that did it!

Cheers