High axial forces in longitudinal in struts

[Ninjako44]

Dear all,
For the attached long. frame , I took Temperature effect by taking max temp.=+35 and min. temp.=-35, it applied large values of axial load on the struts and the brace which made the model not safe ,consequently i changed the profiles to larger , the axial force increased and so on, reducing the profiles is not good for slenderness. I don't want to depend on the bolt hole tolerances , any suggestions ?

 

[KootK]

Could you brace each independent structure only near the middle?

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[Ninjako44]

KootK,
Thanks for your reply. I could do this, but the load path will be very long. What is the normal practice in such situations ?

 

[BAretired]

You could add another expansion joint.

BA

 

[KootK]

If you go with a very long load path, be sure to give some account of collector elongation and bolt slip in your P-delta analysis.

Can you exert any control over the ambient temperature at the time of installation? I've used that approach successfully in the past. You may need scheduling information of course.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JAE]

I know you said you couldn't, but having braces separated like that is usually not a good idea due to the problems you are now facing.
I don't have any answers beyond what KootK mentions but for craneways and such (similar type situation) you usually try to brace at the center and let the ends expand and contract with temperature changes.


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[delagina]

Is this a piperack? I've not seen anyone add any temperature load on pipe racks. It's just not normal practice. Or did you mean pipe friction loads?

 

[BUGGAR]

What do your pipers say about the expansion joints? How are they dealing with temperatures?

 

[delagina]

that's the pipe friction loads, structural engineers put that load. but the forces caused by the structure expanding is "normally" not added. TS should get PIP piperack design guide if this is indeed a piperack. his expansion joint location is also longer than usual. it's usually 1 brace frame in the middle and 1 expansion joint per 100 feet. his is 3 brace frame on 84 meter piperack (not usual).

 

[KootK]

You could ditch the bracing and make the whole thing a bolted, moment connected frame. That would reduce the axial induced shear at the ends.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Teguci]

For thermal, we are talking about 1 cycle of loading per year so fatigue won't be an issue. I think the strategy should be to allow for the deflection with a flexible system such as shown in this sketch.

I'm showing an additional framing member since this will give you the ability to size it for just the bracing loads and not need to stiffen it up for any beam action. I'd think that laying a wide flange flat would be a good choice for this eccentric member.

 

[WARose]

I think someone has already suggested moment frame(s).....seems like I handled a problem like this with that as well.

Another suggestion: have you thought about trying slotted (horizontally) connections at the middle bay?

 

[TLHS]

The typical method for pipe racks is to have a single brace bay per rack. If you've got a long run, you break it into multiple structures as necessary by leaving out a strut between bents. Chevron braces are typical, since they won't develop significant stresses within the brace bay under thermal loads.

Moment frames aren't typical in the longitudinal direction of a rack, since braced frames are significantly more efficient and easier to build.

And yes, if you're in an area with significant temperature swings you should very much be designing to thermal loading if your design doesn't inherently mitigate the thermal expansion issues. If you allow it to expand out you're fine (one brace bay per structure with an intelligent bracing arrangement). If you lock it in with multiple brace bays you've got to analyze that and see if you have a problem.

 

[KootK]

I stumbled across a couple of excellent articles that address this to some extent:

Link

Link





I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[delagina]

@kootk, that's a good information. I've always not consider thermal effects on piperack structure as this has always been the practice. That link gave a good explanation why it should not be considered.