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?

 

[SWComposites]

Wouldn’t it also be a function of pipe size and weight?

 

[Boiler106]

yes, certainly. but lets say a minimum span to depth ratio.

 

[JLNJ]

I don't really use span-to-depth rules of thumb. I simply calculate the deflection and use my judgment from there.

 

[phamENG]

I've never heard of any. As SWComposites and JLNJ mentioned, different types of pipes have different tolerances for deflection so you need to design it specifically for your case.

Would L/24 really be conservative? A 20ft span would be 0.833ft or 10 inches. Would you really want to use an 8 or a 6 inch beam to span 20ft? Might be able to, but a light 10 would likely be more efficient for material, handling, and connection geometry.

 

[HTURKAK]

The following acceptable criteria for piperack is;

Dmax = L/240 L = the Span Length
Drift limit ≤ H/100
(where H = pipe rack height).

Suggest you to look to the following doc.








Tim was so learned that he could name a
horse in nine languages: so ignorant that he bought a cow to ride on.
(BENJAMIN FRANKLIN )

 

[SlideRuleEra]

Boiler106 - AISC Modern Steel Construction had an article on "Rules of Thumb for Steel Design", structural depth (L/20 thru L/28) for "general purpose" beams is included.

This rule of thumb business may seem like "bunk" to a younger engineer... however, the author does a good job of explaining how rules of this type predate the use of computers. Rules like this remain useful to make sure software is not providing mindless garbage answers.

My opinion on your question is L/24 is conservative; L/30 is conditionally acceptable, if deflection, bending stress during pipeline hydro testing, etc. are ok.

 

[BAretired]

Lots of information in this document, particularly for water pipeline bridges.

 

[dik]

... and what did my wife say about 'rule of thumb'... beats me!

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[phamENG]

oh, dik...no...

 

[JLNJ]

SlideRuleEra certainly has a point about garbage out from garbage computer analysis.

Just last week I was asked to comment on a pipe support using 3.5" tubes to span 25' (carrying 6" pipes).

 

[Enginerdad]

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.

 

[phamENG]

Enginerdad - I'm pretty sure the OP was referring to steel beams used in bridges to support pipes, not made out of pipes. That's what my answers were based on, anyway.

 

[gte447f]

Enginerdad, it's not clear to me whether the OP is asking about a beam that is a hollow round shape or a beam that supports a hollow round shape, as in a pipe rack structure. The wording of the thread title seems a little confusing (to me at least). Initially I was thinking the same as you (i.e. hollow round section beam), but after reading the OP and some of the responses, now I think the OP is asking about a beam or bridge structure that supports steel pipe, but is probably constructed out of some other structural steel shapes.

 

[Boiler106]

to clarify: wide flanges used to support utilities. After meeting deflection and strength criteria, i wondered if anyone set a (perhaps, arbitrary) limit on span to depth ratio based on experience.

 

[phamENG]

After meeting deflection and strength criteria, i wondered if anyone set a (perhaps, arbitrary) limit on span to depth ratio based on experience.

If anything, it would be the other way around. Those span to depth rules of thumb would be used as a first iteration in the design process.

If you're using an auto-sizing feature in something like RISA (which I don't like unless you put your initial design in first), then it would be used as SRE suggested: a check to make sure you got a decent result. If it doesn't match up, then hand calcs to verify the computer results would be in order.

 

[SlideRuleEra]

Boiler106 - The L/D ratio is (properly) used for two conditions:

1) Selecting an initial trial size beam.
2) Sanity check on final calcs.

An example of #1, say you are designing a "general purpose" beam, by hand.
31' simple span
loads are known
adequate lateral bracing

Available beam sizes:
W10
W12
W14
W16
W18
W21
W24

Which would you choose for the initial trial?

W10 (L/D = 37)
W12 (L/D = 31)
W14 (L/D = 27)
W16 (L/D = 23)
W18 (L/D = 21)
W21 (L/D = 18)
W24 (L/D = 16)

I would try a W14, with a "light" W16 as second choice.

 

[Boiler106]

my initial question was poorly worded. Im just looking for any self imposed limitations on span to depth ratio that others have employed in the past. This comes up in our office when we have lightly loaded beams and even HSS headers that span long distances. Im aware of rules of thumbs and their uses.

Contrary to someones previous suggestion, im not a young engineer. I just like to pique others brains here on things ive done for years

 

[SlideRuleEra]

1) I'm just looking for any self imposed limitations on span to depth ratio that others have employed in the past.

2) Contrary to someones previous suggestion, I'm not a young engineer.

1) No, depends on the situation. I've use span/depth ratio (successfully) of 72:1
(HP10x42 with simple span of 60', no lateral bracing)

2) That was me, I didn't mean it that way but sure reads like I did... sincere apology offered.

 

[JStephen]

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.