Steel Beam (theoretical question) 7

[tony1851]

If an I beam is loaded uniformly along its span, the middle of the span is the position of maximum b.m. but zero shear. It is normally assumed that all the compressive stress is carried by the top flange, and all the tensile stress by the bottom flange. Assuming the load pattern never changed, would it be possible to cut a vertical slot down the web at the centre of the span,leaving only the top and bottom flanges intact? Theory seems to say 'yes',but somehow doesn't seem right to me.

 

[steellion]

Yes, that is theoretically true. AISC has established design guidelines for adding web openings to I-shaped beams, which is sometimes necessary.

 

[IDS]

The beam would have almost zero shear capacity, but if there was zero shear its load capacity would be almost unchanged.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[JAE]

Look at AISC's Design Guide 2 - Beams with Web Openings.

Found here: AISC Design Guide 2 link

 

[rb1957]

aren't your bending stresses at the remote fibers based on the I of the section ?

sure you could recalc I to remove the web contribution, and sure the change would be smallish, but not negligible (not clearly negligible).

and there's a small matter of stress concentration (the web would feel the cap tension stress (near enough). but if the loading never changes, there'd be no fatigue either ...

 

[spats]

It is not assumed that the flanges take all the tension/compression. The full section carries the load. How wide is your slot? No matter how narrow, bending stresses are introduced into the flanges because there is a change in shear over the half-width of the slot. The wider the slot, the more bending in the flanges. The slot wouldn't have to be all that wide for the flanges to fail. There's also the issue of transition of stresses from the full section to the flanges only, which to me could cause some stress concentrations in the flanges (not uniformly compressed/tensioned). Besides, "uniform" loads would NEVER be that balanced... put a concentrated load at one end, and the whole concept falls apart.

 

[BAretired]

The beam would have almost zero shear capacity, but if there was zero shear its load capacity would be almost unchanged.

Actually, the two flanges will have considerable shear capacity. It is normally ignored when a web is present, but in this case it should be considered. The flanges are weak in bending, however.

It is normally assumed that all the compressive stress is carried by the top flange, and all the tensile stress by the bottom flange.

This is an approximation sometimes made when trying to zero in on a beam size but, as stated by others, the web does carry a small portion of the bending moment.

With a vertical slot at midspan, secondary stresses would have to be considered, but it is quite true that the flanges alone could carry almost the full bending moment of the full section if the slot is not too wide.

BA