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koo (Structural)
9 Feb 04 9:03
Hi,sap people,

I just started modeling a steel girder composite deck bridge in sap 7.12v.  Say simple span, 4 girders 60ft long with 6ft spacing, 8 inch concrete deck.  I saw posts here discussing modeling the deck with shell element, but I read the sap reference manual and it says that the lane load is applied to frame elements directly, not to shell and cautions using of shell element.  I would like to know how would you apply a lane load to shell element, and how would you attach the shell to the girder frame elements?  Is this more accurate? what about the confliction with the ref. manual?  Is there a good sap book with comprehensive samples other than those on CSI's website? Thanks for answering.

Koo
Helpful Member!(2)  pulshakha (Structural)
24 Feb 04 10:46
Koo,

Shells are difficult elements. So avoid them as far as you can. In bridge moving load analysis you have to apply the loads on the frames (girders). Calculate the eccentricity of the CG of the axle loads with respect to the girder center-line then define lane eccentricity to that girder.

You cannot apply lane loads (or even point loads) to the shells but can add the shells to your grid which will make your analysis more accurate since they contribute towards transverse rigidity.

SAP does not make an automatic meshing of shell with the frames. The shells are joined ONLY at nodes (joints). For your girders with spacing 6' and length 60' I suggest the following:

Insert shells in the gaps. Then divide the frames into 10 segments, each 6' long (Edit menu). Mesh the shells into 6 by 1, so that they are square and exactly fit with the divided frames (also Edit menu). One should always try to keep the aspect ratio of a rectangular shell near or equal to 1 for better results. Meshing of shells into smaller segments is important so that the forces are well transfered to the attached frames.

I hope this helps. I have also collected some examples in my website as follows.
wwwDOTgeocitiesDOTcom/sapclub2003
(please replace the "DOT"s with actual dots, because i think putting a website address in a post violates the rules of eng-tips.)
Regards
Helpful Member!  koo (Structural)
24 Feb 04 12:01
Pulshakha,

Thanks for replying.  I tried to do it this way.  The results did give me a much smaller deflection (more rigidity).  But some questions: 1. I divided the plated into 2x2ft. squares.  Do you know any rule-of-thumb thing about the element sizes, say 6'x6' is good enough or 1'x1' could give a result 25% better?  2. The elements generated this way are tied into the nodes of frame elements (girders), in a sectional view, the centroid of the plates are tied to the centroid of the girder web.  Hence, the deck is modeled not as on top of girders, but in the middle.  I think this affect the accuracy.  What do you think?  3. I post the same question on the bridge forum. One persone suggested to use grillage model, but I don't know the exact procedures.  People also suggested to use a rigid link or small beams to link the plate elements to frame elements.  I am thinking to do it this way: set up the girder frames, then set up the plate elements in a layer up from the frame plane where the centroid of the deck is supposed to be.  Set nodal constraints (deflection constraints) to bond frame elements nodes to plate nodes above, so that the plate and the girder deflect the same. Then run it. Do you have any experience or commments?  BTW, how does sap do the load distribution if it applies load to girders directly?  

Regards,

Koo
pulshakha (Structural)
25 Feb 04 3:40
Koo

As you know the FEM gives approximate solution. Although i haven't done any rigorous experiments but i think reducing the size of elements obviously should give more accuracy. But on the other hand, more elements means more number of interpolation functions (function that defines stresses within the nodes), which may again result in loss of accuracy. So i think making the elements very small doesn't have desired effect. It should be within a reasonable limit so as to transfer the forces to the entire frame or vice-versa. 2x2ft size is quite reasonable. But also don't forget to divide the frames into 2' long segments!

You are right about the connection points of the shells with the frames. I think in SAP2000 version 8 onwards, they have made a provision for fixing this problem. But remember, it is only a mathematical model with geometric and material properties. Although we see it as a 3D section, it is in fact only an abstract grillage of line or plane with no dimentions.

Instead of defining frame sections with predefined shapes you can also define a general frame with the same geometric properties and get the same results. At the joints, only the properties are summed up, not the physical dimensions. So, i think we don't have to worry much about that unless we are interested in the micro details of the individual element.

<<<People also suggested to use a rigid link or small beams to link the plate elements to frame elements.  I am thinking to do it this way: set up the girder frames, then set up the plate elements in a layer up from the frame plane where the centroid of the deck is supposed to be.  Set nodal constraints (deflection constraints) to bond frame elements nodes to plate nodes above, so that the plate and the girder deflect the same.>>>

Don't mess it up. Keep it simple! I think layer of shells is not the right idea because in that case the frame and the shell will be positioned in two different planes. Remember, these are only the dimensionless lines and planes.

<<<BTW, how does sap do the load distribution if it applies load to girders directly?>>>

Unless there are cross (transverse) girders between the supports the load applied in one girder is not distributed. In that case there must be the deck slab (shell).

Phew, a long post. i really should learn to write in short.

Regards.
koo (Structural)
25 Feb 04 9:01
Pulshakha,

I wish I would have time to do a comparison analysis.  The zip downloads on your web seem useful, but it's better to give them descriptive titles vs. a, b, c...I'll plow my way thru anyway. Thanks for share your thoughts.

Koo

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