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Beam Deflection in a fixed beam: Wrong deflection results compared to beam theory results

Creator
Creator

Hi,

I am doing a simple problem of a beam fixed at both ends and carrying a uniform distribiuted load, material is steel. The results given by NX 11.0 are different from the results I get using beam deflection formulas (Theoretical). 

In attachments I have attached the beam problem, Beam theory formulas used to calculate deflection and results given by NX.

Since the Moment of Inertia, Modulus of Elasticity values are taken as given by NX itself, and also values of load and beam dimensions are displayed by NX itself , it does not make much sense why both the results are different. 

Deflection (Y) given by beam theory is : 1.350E-5 mm and by NX is 2.286E-5 mm which is an error of 69 %. 

Whereas the results for Bending stress is coming exactly same from both theory formulas and NX. Can anybody explain why am I getting theses kind of results.

Thanks,

Ashwani Thakur.

8 REPLIES

Re: Beam Deflection in a fixed beam: Wrong deflection results compared to beam theory results

Siemens Phenom Siemens Phenom
Siemens Phenom

The difference is beam theory assumption. The hand equation that you show is for the bending deflection only, that assumes there is no shear deflection in the beam. The Nastran default when a PBEAML is used, is to include shear flexibility. If you add shear flexibility term to your equation it will match.

The other option is to set the shear flexibility in Nastran to zero and then the answer will match your current hand calculation.

 

Regards,

Joe

Re: Beam Deflection in a fixed beam: Wrong deflection results compared to beam theory results

Creator
Creator

Thanks,

 

Also is there any difference between PBeam and PBeamL element type. I have used PBeam for my problem. 

Re: Beam Deflection in a fixed beam: Wrong deflection results compared to beam theory results

Siemens Phenom Siemens Phenom
Siemens Phenom

There is no difference in the element. However, when using PBEAML, you enter section dimensions, the program calculates the properties based on the your dimensions and the selected shape.

With the PBEAM, you are calculating the section properties and can choose to leave some fields blank, or non-default like in your case make the shear factor zero to eliminate the shear deflection. So PBEAM allows the user to "customize" the beam behavior as desired.

 

Regards,

 

Joe

Re: Beam Deflection in a fixed beam: Wrong deflection results compared to beam theory results

Siemens Phenom Siemens Phenom
Siemens Phenom

PBEAM/PBEAML are just two different means to specify the properties. On PBEAM, all properties are explicitly entered. On PBEAML, you enter dimensions for a particular cross section shape, then Nastran computes the properties based on thin wall equations or the Pilkey finite element method.

 

You can see the properties values that were computed from PBEAML by echoing the bulk data. The PBEAM card that Nastran generated internally fromthe PBEAML dimensions will be printed to the .f06..

Re: Beam Deflection in a fixed beam: Wrong deflection results compared to beam theory results

Creator
Creator
Can I set the shear factor as zero in PBeam property so that I do not have any shear deflections in my problem.

Re: Beam Deflection in a fixed beam: Wrong deflection results compared to beam theory results

Creator
Creator

I tried setting the shear factor zero in PBeam section properties but I couldn't find an option to save the properties and when I come out of the section property window, the software automatically assigns the shear factors non zero values.

 

I have attached the snapshots of the section property that I edited. 

Re: Beam Deflection in a fixed beam: Wrong deflection results compared to beam theory results

Siemens Phenom Siemens Phenom
Siemens Phenom

The listing shows that you are still using a PBEAML I section.

 

You need to create a new PBEAM section and manually key in all of the data that is shown in your listing, minus the shear factors.

Re: Beam Deflection in a fixed beam: Wrong deflection results compared to beam theory results

Creator
Creator

Hi,

 

Should I use user defined properties in PBeam type and put K1 and K2 as zeroes.