Simply supported beam/plate

[penpal97]

I have modeled a simply supported beam 10' long, 5kip point/line load at midspan. Midspan moment = 12.5k-ft. For a 1ft thick concrete slab (and assuming no cracking), there should be ~500psi compression on top, ~500psi tension on the bottom.

I run the model and good results for principal major stresses in compression and tension (pg 3 and pg 4). I get ~zero psi for principal minor stress in the bottom (pg 5).

Why is there 500psi of tension in the principal minor direction at the top (pg 2)? I would expect there to be ~0psi on the top, just like the bottom.

For full disclosure, I modeled the beam on the 45degree line, not along the x-axis, because I have to do some other modeling later on. Meaning the beam goes from 0,0 to 1,1 to 2,2 to 3,3 etc. I do not think the element orientation should affect its results though.

 

[sasa2k]

Principal stresses are given for the local axis of the elements. Try global stresses instead.

 

[penpal97]

Thanks for the reply.
It is intentionally skewed so that I cannot use global stresses. My final model will not be oriented along the major axes.

 

[crisb]

What program are you using, are they shell elements? Are you sure all the stress outputs are principal stresses, and that some are not stresses on member local axes. I would look at the help files carefully?

 

[penpal97]

I'm using STAAD plate elements. The stress outputs are the local principal stresses.

 

[rb1957]

other than top max principal stress things look good, no ? top min principal and bttm max principal look good to your hand calc ... bttm min looks more sensible (considering poission effects) than top max ...

 

[crisb]

For a plate element I would only expect to see results for 4 principal stresses (max/min at top and bottom), as the out of plane stresses will be zero.
Are you sure you are not confusing some of the results with shear stresses, or von Mises stresses etc. These are the results I believe to be available for Staad plate elements.

 

[penpal97]

Thank you for all your responses thus far.

crisb: If you look at the top of each index (upper left) of each page, you can see the title of the stresses that are displayed. They are the max/min of the top/bottom; not shear, von mis, etc.

 

[crisb]

Maybe try the Staad forum, or ask support if it is a bug?

http://www.eng-tips.com/threadminder.cfm?pid=750

 

[penpal97]

Thanks.