Staad gives axial load in girder of a simple bridge?
Staad gives axial load in girder of a simple bridge?
(OP)
I modeled a simple composite steel bridge in Staadpro 2002. Deck is modeled as plates, girders as frames, bolster as frames. HS20 moving load. But the oupput gave me large axial Fx load in girders and lateral reactions at supports (pinned), even larger than vertical reactions! Where could they come from? And the mid span moment is much less than AASHTO appendix A values. Only the vertical reations at supports are close to the supposed values. What's going on?
I read people here, like edward1..., said they got good results with similar model. Any thing I did wrong? Thanks.
Koo
I read people here, like edward1..., said they got good results with similar model. Any thing I did wrong? Thanks.
Koo





RE: Staad gives axial load in girder of a simple bridge?
If you release the axial restraint at one end, you will obtain your true simply supported condition. THe reactions, deflections, and bending moments will be what you expect to see.
Hope this helps.
RE: Staad gives axial load in girder of a simple bridge?
Yes, I tried to release axial moment at one end, still not much change. Staad support offered me the explanation that the lateral reactions may come from diff. deflection of slab (but the net lateral force by summing all supports gives 0), and the axial force in beam is actually the net force of tension and compression. It seems make sense for a frame member.
But how would one come up the final mement? The one on the beam is way low. Did anyone encounter this before?
Koo
RE: Staad gives axial load in girder of a simple bridge?
Did you model the top and bottom flanges as separate elements in STAAD?
Is it possible that the slab is retraining the beam in the model and creating an axial force?
Dumb question, maybe. Does the moving load have any horizontal component? Just out of curiosity, do you get the same behavior with a static vertical load? (I have never designed with HS20 moving loads)
I have used staad for about 5 years and have not seen the behaviour you are describing unless the support conditions are incorrect, or as would be the case of a truss, the top and bottom flanges are modeled as separate elements.
RE: Staad gives axial load in girder of a simple bridge?
But after removing the plate elements, the beams will give good results. no lateral or axial, just like a hand calc.
Do you use staad for bridge or building?\
Koo
RE: Staad gives axial load in girder of a simple bridge?
As for slceng and structuresguy's comments, they are completely unnecessary and quite derogatory. They are not suitable comments for this discussion board. I work with an Indian guy here who read this and was quite disturbed.
RE: Staad gives axial load in girder of a simple bridge?
slceng: I don't agree with your comments. A competent foreign engineer should be trusted just as any competent local engineer, regardless of nationality. Actually I am technically a foreigner here in the States as well. I was born and raised in Canada. So are you saying I should not be trusted?
In response to lippie: Read what I said. I never said anything negative about Indian people. All I said was that trying to communicate a technical problem with someone who does not speak english well (in this case Indian as primary language) is difficult at best, and I have had very poor results with STAAD's particular Indian speaking tech support reps, so I don't trust the answers they have given me because I don't trust they fully understood the problem. I also don' trust their anwers because most of the time it does not fix my problems with STAAD.
I however, do resent to a great extent being forced to limit my usage of tech support for software I pay a great deal for becuase the American company I bought the software from chooses to hire tech support reps who don't speak English well enough to understand what people are saying over the telephone. Had I traveled to some foreign country and bought their locally produced and marketed software, well I would expect a language problem to exist.
We are not talking about software for virtually painting pretty hair on your daughter's barbie dolls. Peoples lives at are stake when we use these analysis packages to design structures. Expecting tech support from someone who is fully able to understand the problem is not too much to ask.
RE: Staad gives axial load in girder of a simple bridge?
Thanks for your input. Let's get back to the technical...I had a long talk with Staad. What I understand from it is that the axial force is due to the bending of the composite section (tensile on bottom, comp on top), however, the section modeled in Staad is not treated truely "composite" and there are also bending moments on individual sections (girder, deck). The total moment should be a sum of all these. Lateral forces at support will dissapear if one end is free in both lateral directions. Not quite intuitive but understandable.
Don't you have similar composite section issues in building modeling?
Tips about STaad support: try to call them after 11:30am EST and you are more likely to talk to a US based person, and communicating by email works as well.
Koo
RE: Staad gives axial load in girder of a simple bridge?
We do use composite sections quite often, however, they are usually infill beams, which I don't generally model in STAAD. The main beams on gridlines are often moment connected, so the composite action can not be counted on, since moment at support governs. I personnaly dont find STAAD composite beam analysis to be user friendly, so I use other software to analyse composite beams.
RE: Staad gives axial load in girder of a simple bridge?