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Beam Theory

Beam Theory

Beam Theory

(OP)
At what span to depth ratio does beam theory go out the window in plate girder design?  

RE: Beam Theory

I've always used a span to depth ratio of 10 as the cutoff.  If you span is longer than that then shear deformation is not an issue a simple beam theory is solid.  

For pratical purposes you can probably have spans half that limit without experiences large differences from beam theory.  

 

RE: Beam Theory

(OP)
9ft deep girder spanning 40' has me concerned.  

RE: Beam Theory

I seem to recall my advanced analysis professor mentioning that when your L/d approaches 4, you really can't neglect shear deformations.  

RE: Beam Theory

I know I should be quoting something out of AISC or even Salmon and Johnson but I was recently reading Roarks and he suggest that good'ole o=Mc/I or linear stress distribution gives "sufficiently accurate results" for span/depth ratios down to "about" 3. After that there are significant stress INCREASES  as opposed to decreases as previously thought by none other than Timoshenko (or atleast thats who he references, but I don't believe it).

I'm not sure how that applies to plate girder design.

EIT

RE: Beam Theory

(OP)
Josh-
Does STAAD consider shear deflections/deformations in a girder analysis?  

RE: Beam Theory

I ran a couple of quick tests.  

I tested a couple of different types of loading (single point load vs. distributed load) on both a simply supported beam and a fixed - fixed beam. I ran RISA models with shear deformation turned on and with shear deformation turned off.  

I got a difference in deflection of about 30% based on shear deformation when I used a similar span to depth ratio. However, the moments and shear values were much closer (within about 7% of each other).    

That should give you some idea about how accurate (or inaccurate)  traditional beam theory would be.   

RE: Beam Theory

You guys and your computer programs again...
I think Roark gives some equations based to account for the shear deflection and also a stress increase multiplier.
 

EIT

RE: Beam Theory

RFreund the thing is that we may have more at hand the computer by now than the book; for example, Roark's is 5 ft from me, the computer 0 ft, and one book full of tables for deep beam check and design at 20 ft in a cabinet so overburdened by clutter to access I rarely open it twice a year. This does also speaks on how well one may manage to organize his home if he chances to be one structural designer, sometimes.

RE: Beam Theory

Ishvaaag - haha, nice.  I would probably do the same thing, although I don't trust my Risa skills that much.

EIT

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