Plate Deflection Results wrong

Question

I am trying modelling a simply supported plate model in staad with 10&times;10 mesh with density 25 Knm2 and E2510^6 and comparing it with results of other finite element applications and also with numerical matrix analysis by hand calculations. Both fem app and hand calculations results match but those of staad pro doesn't. Though I was sure of correct modelling and also checked for equilibrium each time, I tried models of other dimensions l b and h as well. Each time fem results and hand calculations matched perfectly. 
 For example I tried modeling 35&times;50 m plate with 2m depth. Hand and fem calculation results were 30 and 33 mm at centre resp. But staad gives 300mm. For other models also the result are far too variable like this case only. Please I need urgent support. Any reference would be appreciated.

Atul Sharma · Accepted Answer

Yes, into 10*10 divisions each. Hand calculations reference is Timoshenko plate theory on which most FEM applications of today are based on. Model is accurate. The main blunder is calculations differences. 
 
 For 5*5 m plate of 100mm thickness with SS corners and same mesh divisions and density and E also is giving this very variation. Staad gives 16.8mm approx at center due to self wt but other Fem applications ( ETABS ) and hand calcs gives 2.7mm approx. 
 I have many more models showing this high variation of results for various aspect ratios. 
 Explain? Are staad pro plate deflection calculations results expressed in different norms than conventional?

Modestas · Answer

I think the difference might be due to different support conditions. Make sure that STAAD.Pro model uses the same support conditions for the slab as considered in the manual calculation.

Modestas · Answer

As we understand you are following one problem as described in Timoshenko&rsquo;s Theory of plates & shells Chapter 5 which deals with Simply Supported Rectangular plates: 
 As highlighted above, there are two boundary conditions (Support conditions in STAAD.Pro): 
 
 First one to provide simply support along two plate edges at global X = 0 and X = 2 m (in the model a = 2 m). 
 Second one to provide simply support along two plate edges at global Z = 0 and Z = 1 m (in the model b = 1 m). 
 
 Once both boundary condition are defined in the model (all edges of the plates have simple supports), the max Y displacement at center reduces to 1.198 mm (approx. 1.2 mm) which is a close match to the result you are expecting. 
 Please make sure the problem modeled in STAAD.Pro is the same as described in any book. Most of the times, during hand calculations we use formula, completely forgetting the assumptions based on which these formulae are derived. But in STAADPro no such assumption is made. While modeling, we must ensure correct assumptions are incorporated using different modeling techniques, to achieve correct results. 
 Plate_Model.STD

Modestas · Answer

In the Timoshenko's book the boundary conditions are such, that each edge of the slab is simply supported (it is highlighted in the previous screenshot which we posted). This is modeled in STAAD.Pro by providing simple supports at each edge of the slab (each node of meshed plate). By doing that, the results between Timoshenko's book and STAAD are reasonably close.

Modestas · Answer

If the boundary condition are the same, then the analysis results between different FEM applications should be similar. If the results are different, then there must be a different modeling approach used between those applications. Can you attach a screenshot of the support conditions or at least the deflected shaped of one of those FEM applications? 
 The attached screenshots refer to an example in the verification manual where a similar problem has been solved in STAAD.

Plate Deflection Results wrong

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