Simple beam but large displacements 2

[penpal97]

I have a simply supported W27x281, 10ft long, 100kip/ft. Basic beam equations give a displacement of 0.065in.

I model the beam in STAAD (single element), and get 0.065in. However, once I model the beam as ten 1ft long elements, I get a deflection of 0.109in.

Anyone else have this sort of problem? Or suggestions on how to fix the issue?

 

[IDS]

The difference is shear deflection. The basic beam equation accounts for only flexural deflection, and it seems that STAAD uses the same equation to calculate intermediate deflections along a beam.

When you subdivide the beam the program is calculating deflections at the beam ends including shear deflections. You should get the same result with just two beams, with a node at mid-span. Because you have a short span (in relation to the beam depth) and a high load the shear deflections are significant.

If you switch off shear deflections (assuming STAAD allows you to do that) you should find the result agrees exactly with the basic bending formula.

Of course in this case the calculation including the shear deflection is more correct (assuming the support at one end allows lateral movement).

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[BridgeEI]

You can type "set shear" as the second line in the code and it will ignore shear deformation.

 

[penpal97]

You guys are brilliant. Thank you.
For my final calculation, I will have to account for the shear deflection. Thanks again.

 

[rb1957]

wot, no "LPS"s for the guys who helped you out ?

we get paid for them (in a later life, that is).

 

[msquared48]

Of course rb was referring to the Libations for Public Servants we all imbibe in at the end of the day...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask