Steel U Frame Footbridge Design 1

[Mac75]

I have modelled a U Frame footbridge as a 3D grillage but for some reasons i am getting high axial forces and moments at the bottom chord which is failing the check for combined axial & bending.The deck is at bottom chord level.

The bridge is simply supported and Span is 20m,height is 1.8m and width is 3.0m.
I have modelled all the elements cross member,bracing end posts as fixed in the model.Is there any guidance notes or book i can do a simple 2D check on the U frame analysis.
Any help would be great.

 

[ishvaaag]

Forfeiting any contribution to tensile strength of the frame within the 3 m floor, it should be M=ql^2/8 and then T=M/z

Moments you can neglect for this check if you are using lateral trusses. If you are using a Vierendeel setup the inconvenient moment may be coming from there. And, if you have some continuous true web, the Vierendeel-like setup is not convenient because will allow for the apparition of such nonexistent moments; hence if such is the case, model the webs with steel plate elements.

 

[Mac75]

Thanks ishvaaaq.

The footbridge is designed with CHS for top & bottom chord & also bracings.The cross members are SHS. the longitudinal members to support the decking are channel section & unequal angles. Is there any guidance to check this bridge with a simple 2D model?

Please help me.

 

[ishvaaag]

Well, if you dismiss any tensile forces passed to inner longitudinal members you would have essentially two trusses. However, this is essentially what is called a pony truss bridge, and then the vertical members in the trusses and their fixity to the corresponding beams at floor level need to coerce the lateral buckling of the top compression member. So typically the texts (see Galambos' both 4th and 5th editions) care particularly of ensuring the lateral stability of the top chord, and hence of the transversal U frames at panel planes.

http://books.google.es/books?id=W_a...esult&ct=result&resnum=1#v=onepage&q=&f=false

But something in your present design, maybe bottom chords underdesigned or the software detecting the U frame intervention on the overall stability and not counted when first sizing the bottom chord is making that your bottom chord is being the critical member. It also may come for the bottom chord members taking loads outside panel points, this might also contribute to them not meeting requirements.

Respect checking, if you want to acknowledge the -shear lag transmitted- tensile stress transmitted if something to logitudinal inner members and as well all the overall effects of lateral stability you better adhere to 3D models. You may directly model some out of straightness according to tolerances and potential vandalism effects and directly then discern if everything is OK.

Since these two effects may be affecting your stresses, I don't see that, except some gross misevaluation of something, maybe detectable just by T=M/z, you can benefit from some 2D analysis, and I see more promising investigating why your 3D model is giving whatg you think unexpected results. Start by looking at the deformation at some big scale, it maybe some required connectivity is not being read as you planned, this is a common cause of unexpected results; then correct the connectivity if such is the case.

 

[kelowna]

How did you model the supports?

Some time ago a young engineer working for me and doing a simple bridge model fixed both supports and introduced the bridge camber on the model. The program, not knowing any better, gave the results for a very, very shallow arch.

I would suggest you carry out a paper and pencil analysis. That should give you an idea if you are on the right track. If you can not make a hand calculation analysis of the problem you should not be using a computer. Kind of learning how to walk before driving a Ferrary.

 

[Mac75]

The supports have been modelled as Fixed at one end & Rocker at other end.

 

[kelowna]

Then I would suggest you do a hand calculation to check the computer output.

You have a very simpla structure. Choose a member with 'suspect' results and cut the structure there. The external loads applied should be in equilibrium with the support reactions, dead weight and member reactions through the cut.