Steel Pipe Bridge Beam - Span to Depth Ratio 1

[Boiler106]

Ive always wondered if there are any span to depth ratios for steel beams in pipe bridges similar to rules of thumb used for floor members? L/24 to L/30 seems conservative.

Any suggestions?

 

[MotorCity]

Its ok to start with a rule of thumb to get your design in the ballpark, but it really depends on how much movement the pipes and the pipe connections can tolerate. Welded pipe connections fair significantly better than threaded or coupling pipe connections when it comes to deflection

 

[bugbus]

L/650 incremental deflection after pipe installation was the criterion used on a recent bridge. These were fairly large water mains (up to 24"). I imagine it depends a lot on the type of pipe, asset owner requirements, etc.

 

[BAretired]

I'm actually going to disagree with a number of commenters here. The L/24 or L/30 "rules of thumb" were developed for typical steel flexural members, namely I-shaped beams. I beams are much more efficient in bending than hollow round shapes, so a pipe would need to be deeper than an I beam to compensate for that lower efficiency and provide the same strength and stiffness performance.

I don't remember ever using L/30 to select a trial beam; it was always L/24, so a 10" deep beam for a 20' span or a 15" deep beam for a 30' span. The "rule of thumb" was pretty good for floor and roof beams and I still use it as a starting point when selecting a trial section.

The rule of thumb applies equally to all symmetrical shapes, I beams, rectangular beams, hollow sections or pipes. Using Working Stress Design (WSD), the moment resistance of a beam occurred when the fiber stress at the top and bottom was equal to some stress f_a which was specified in the code.

If maximum bending stress is defined by code, then maximum unit strain 'e' is also defined at all fibers, so e=f_a/E at top and bottom. Curvature is 2e/d irrespective of shape. This does not say that all 10" beams will be adequate for a 20' span; they still must be checked for strength. It says that it passes the deflection test, but if the bending moment exceeds its capacity, it will not pass the strength test.

 

[human909]

Vortex shedding and vibration in the vertical direction would be one constraint, and I would expect most of the rules-of-thumb above to avoid that.

THIS.

I learnt this the 'hard' way earlier this year when two lightweight structure on one site had vibration problems triggered by bracing that was too slender. At the wrong wind speeds the the square hollow section bracing would vibrate like hell. I suspect a big part of the issue was the relatively open plains meaning clean laminar wind flow. Rectification was relatively easy given it was an industrial site, but a good lesson for me.


Regarding span to depth. My rule of thumb is between 20-30 span to depth. 30 would for light weight pedestrian structures. 20 for moderately loaded beams and for heavily loaded beams sized to suit strength all the way down to 10 or less.