Pipe Support Beam - Analysis 1

[beanbag]

I am designing a simple span steel I-beam to support future piping (quantities, sizes and support fastening conditions of the piping will vary and are unknown at this time) and my client advised me to assume 800 PLF uniform (piping DL + LL) load acting across the full length of the beam. They also want the beam and piperack bents installed now. What unbraced-length-of-compression-flange (Lb) should I assume in designing the beam? One source that I read advises to assume 1/3 of the full beam length while another one advises to assume the full beam length when the piping support conditions are unknown. Does anyone have any advice on this? I don't want to design an unsafe beam but I also do not want to be overconservative, since there will be several such beams on this project and the construction cost will be significant.

 

[BAretired]

What is the beam span?

I would opt for the assumption that the beam is laterally unbraced over its full length unless you have good reason to assume otherwise.

BA

 

[delagina]

beams for piperack bents are moment connected to columns that's why you see some guidelines (fluor, etc..) saying 1/3 or 1/2 of the beam length as LTB length.

i usually use length though. the cost of your beam is minor compared to cost of the pipes and equipment.

 

[msquared48]

I agree - use the full length in the absence of further information.

Remember tht pipe locations can always be changed or removed too. So, even with definitive locations for the pipe, I would still use the full length.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[paddingtongreen]

That usually is not a problem when you apply the pipe expansion loads. By the time you have a top flange good for the friction loads, the effective length for vertical loads is covered.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[beanbag]

Thank you all for your replies. For uniform loading, distance between points of inflection equals approx 60% of the beam length when both ends are fixed, approx 80% of the beam length when both ends are partially fixed, and 100% of the beam length when both ends are pinned. In my case I would say that I have partially fixed ends. although the piping might not be machanically fastened to the top of the beam, would it be reasonable to assume that the causation of the 800 plf uniform loading, i.e. the weight and stiffness of the piping, would also partially resist LTB of the beam, and therefore it would be reasonable to assume something a bit less than 80% of the beam length for partially rigid end conditions, like say assume Lb equals 2/3 of the beam length in determining Fb in my case?

 

[BAretired]

The points of inflection are not braced points. Use the full length.

I asked you before but you have not responded, what is the span of the beam?

BA

 

[beanbag]

BA: Thanks for your feedback. I am considering assuming the PI locations as fictitous "brace" points because the compression flange is the bottom flange in the zones between each PI and each support column. Any idea why the Fluor guidelines (noted previously by one of the repsondents) would advise their engineers to assume LTB equal to 1/3 the beam span and not the full beam length?

There are several beams and bents involved in my project, all with different lengths but the same loading criteria. Some beams are approx 10 feet long, some are approx 20 feet long and some are approx 30 feet long, but I don't see how the length affects the "philosophy" that I am looking for. Thanks in advance for your explanation on this.

 

[delagina]

if some companies use 1/3L then 2/3L is more conservative than that. but i still prefer whole length to simplify my calcs.

if you use 2/3L, what unity ratio will you use so you dont over/under design? is it 1.0 or 0.9? remember 800plf is just an assummed load based on 8" pipes.

the best it to coordinate with other structural engineers in your project so you are all in the same page.

 

[structSU10]

If the PI locations are the inflection points then per code (AISC) they cannot be used a brace points - no exception. You could alter your LTB moment using the Cb factor that would be appropriate if the ends are fixed or partially fixed. I agree that full length is more appropriate, especially if no direct connection is being made between the pipe and the beam.

 

[dhengr]

Use the full length, since you don’t even know the loading with any certainty. And, consider Paddington’s comment about lateral loads at the top flange due to pipe movement.

Also, this is not the "philosophy" forum. And, “fictitous "brace" points” may cause real beam failures which is philosophically sometimes/usually/maybe not a good thing.

 

[BAretired]

The length does not affect the "philosophy" but for short spans, it makes little difference in cost whether you consider the full length or the half length. For long spans (30') perhaps you should provide third point braces.

BA

 

[JoshPlumSE]

Beanbag -

I have an old (1993) copy of the Fluor guidelines which says the same thing. I believe these were based on the AISC 9th edition steel code. That was before AISC had specifically codified the fact that points of inflection cannot be considered to be bracing. Since AISC has clarified this, it is more difficult to justify that assumption (unbraced length = 1/3 of span) than it used to be.

Pipe racks do have a lot of redundancy in them. So, you could argue that the pipes which are supported by the beam will do a lot to brace the top flange of the beam. That should limit the type of lateral torsional beam buckling that we're concerned about. But, only for the top flange. It shouldn't help you much in the negative moment region where the bottom flange is in compression.

Personally, I would use the full length of beam, but with a larger Cb factor.