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Deflections under self weight

Deflections under self weight

Deflections under self weight

I had designed a piperack for a petrochemical plant. Few beams in this piperack had spans of 13m (size HEA360). There will be some deflection due to self weight. For small span beams (say upto 6m) this deflection will not be visible. But for such a long span beam deflection will be very much visible. How to overcome this problem? Castellated beam was one solution but was avoided due to some reasons.
Should a camber be provided for deflection against self weight? How difficult will it be to provide a camber in hot rolled member?


RE: Deflections under self weight

what is the proportion of piping load to the beam's self weight? I ask because if the piping is much higher the self weight deflection will be lost.
Cambering is difficult for small amounts and it is not precise. On a pipebridge, don't you have a problem with expansion of the piping?

You could moment connect a post between upper and lower beams to make a vierendeel truss.

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

RE: Deflections under self weight

Thanks Belbos & Padington green,
1. Pre-cambering of beam- Does amount of camber required is to be specified in design drawing (engineering drawing/ GA) or is it directly taken care by steel fabricator in his fabrication drawings? What is general practice?
3. Yes. We can design the member with fixed support but it is only possible in case of beam connected with column along major axis. This is very rare case since in piperacks generally in longitudinal direction, columns are oriented with minor axis. For beam to beam connection it will be difficult since such connection will add torsion to supporting beam and its connection will then have to be modified to take care of torsion. This will increase detailing & cost.

pipe operating load on this beam is much more than self weight of the beam.
Problem with using truss is that i have very less space available since this is a brownfield petrochemical plant & i m connecting this beam on an existing piperack.

RE: Deflections under self weight

We normally use 80% to 90% of the dead load deflection for the camber.

This is typical for commercial steel framed structures, and I feel it would translate well in your application.

RE: Deflections under self weight

I don't think I've seen anyone bother to camber a pipe rack beam.  If there's no occupancy, unless it's incredibly extreme, you'll likely be fine just shimming your support point's once your beam's in place, which you'll probably have to do anyway.

13 metres is slightly larger than I've ever spanned with a single pipe supporting beam (rather than something tying bents together), so this may not necessarily apply to your situation.  It'd take a lot to convince me that I want a camber on a pipe rack.

Do you have space constraints on depth, or another reason you're not using a deeper beam?  That span/depth ration seems pretty extreme.  It seems like it would be more efficient to use something deeper unless you've got a fair amount of lateral bracing and a light load.  However, if deflection under operational loads isn't controlling but you're worried about dead load deflection, it'd have to be a very small load indeed.

RE: Deflections under self weight

In figuring the camber, the dead load of the pipes must be added to the selfweight of the beam. It still might not be enough to be practical. I would be inclined to add in half the weight of the contents, they are there for almost 100% of the time

I have designed many piperacks and bridges and the the friction load from pipe expansion was always a bigger problem.

My concern with the camber is that it must be bent till the outer fibers become plastic, even then, there is springback when the forces are removed.

I guess I should go away, but this does not feel like any rack that I've worked on.

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

RE: Deflections under self weight

I imagine buckling might be your most serious concern.

As for deflection, ask your piping guy what he can tolerate.  

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