Staad gives axial load in girder of a simple bridge?

[koo]

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

 

[structuresguy]

The answer is actually quite simple and even you stated the reason for the axial loads in your question. The reason you have axial loads in a "simply supported" beam is that you used pin type connections at both ends. As the beam deflects, the ends pull together since the beam support to support distance shortens. However, since you restrained the beam at both ends, the supports stop the shortening by inducing high axial loads in the beam and lateral in support. The behaviour would be similar to a very tight catenary cable, if you ignore the initial tension. The sag of the cable induces further high magnitude axial loads. What you actually have modeled is an indeterminate structure.

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.

 

[koo]

Structuresguy,

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

 

[structuresguy]

Hmmm, interesting response from STAAD support. I usually don't trust those guys. I usually find the language barrier to be a real problem with staad tech support people. I don't speak Indian and they barely speak English (usually), so describing a technical problem is even more difficult than general conversation would be.

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.

 

[koo]

I modeled the beam as a frame member. He said the axial is actually tension. The moving load is generated by a string of static loads with a increment between. I don't see a horizontal component though it's possible after the deck deflects, I think. The support guy I talked to is very knowledgeable. But he couldn't explain why the lateral forces at supports well, only said it's possilbe.

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

 

[lippie]

The axial load is most likely caused by the Poisson effect. When the web of the girder is subjected to an inplane loading, Poisson effect will cause the web to undergo a deformation along the axis of the beam. Depending on how the supports are positioned, it can induce an axial force. To avoid axial stress, you need to ensure that the nodes of the girder end are free to deform in an appropriate manner.

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.

 

[structuresguy]

koo: To answer your question, I use STAAD for buildings, as we do not design bridges. YOur problem is interesting. I hope you figure it out. I am glad you got in touch with someone knowledgeable at STAAD. I have not always been as lucky in the past.

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.

 

[koo]

Structuresguy and lippie,

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

 

[structuresguy]

Hi koo, thanks for the tip, I will wait till afternoon EST to call next time.

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.

 

[edward1]

Koo, You can get good forces on composite steel girders if you will RELEASE the connecting beam from girder to deck in the longitudinal direction using FPZ .999 at START and END. Also this gives good deck moments which added to the girder moment will equal the total composite moment. This reduces the axial loads to almost nothing. This model does not give accurate deflections; you must not release these members for deflections. Another way to model this is use ELEMENTS for deck and run girders at center of ELEMENTS and then use MEMBER OFFSET START LOCAL 0 0 -z and END LOCAL 0 0 -z with the offset z equal to the distance from center of deck to centroid of girder. Ed McFadden