Support reactions given by software

[pepeuy]

Hi everyone.

I'm having some discussions with some colleagues about forces transmitted to static structures linked to pipe supports.

We have a horizontal run of a 30" pipe, with a saddle support welded to it. The base plate of the saddle will be the one in contact with the static structure (the base plate will be the one wearing off, instead of the pipe wall).

The actual question is:

Using autopipe to study flexibility and restraining reactions (using a friction coef. of 0,3 for steel on steel) the axial forces are one third of the vertical loads, that's obvious.

The argument arises when designing the static structures, since I want to consider the axial forces as given by autopipe, and my colleagues (based on their experience) say this forces will actually be smaller, so the structure proposed is weaker at the end.

What should i do?

Thanks in advanced.

 

[ColonelSanders83]

I would document the loads at each support and formally send them to the structural department. As the pipe stress analyst your job is to document the reactions at the supports and ensure the structure can handle the imparted loads based on the allowable’s of whatever code you are working to. Anything that is under designed because of "experience" leaves you with an open liability in the future. I personally like to point out the sections of the code with state my support structure allowable stress limits. This is a hard requirement and I simply state that if the structure can't handle the calculated loads, then it doesn't meet code and I won't be able to release the piping. My management usually sides with this type of argument because I use the Code as my basis.

I generally use 0.4 for steel on steel, per machinery's handbook. Any tampering with the axial forces on your part must be technically justified. "Experience shows", or "we've always done it that way" won't be of any use if there is a failure.

 

[SNORGY]

pepeuy:

I think what others might be trying to say is that the structure will deflect somewhat under the applied load, so that at equilibrium (at the end of the displacement) the load will be somewhat less than than that computed by the piping stress program. In other words, relaxation of thrust occurs through reaching equilibrium with the "F=kx" behaviour of the structure.

I haven't used Autopipe but I do use CII. There are, presumably, provisions in the software to model the structure with the piping system. Alternatively, you could gap your axial restraint (if such exists) or input a nodal displacement by the amount which you (or the structural folks) judge the structure is likely to deflect under load.

That said, if the structural folks are suggesting to design to these "reduced actual" loads - especially if they are just the frictional loads - I would suggest that there is probably too much underdesign going on. I also agree with ColonelSanders83: use 0.4 for the coefficient of friction for steel-on-steel and report the loads as calculated directly from the pipe stress software.

Regards,

SNORGY.

 

[DSB123]

pepeuy,
I think you should revisit your friction factor. ).3 is low and after several years if the piping is not thoroughly maintained then the friction factor of steel on steel can be much higher then 0.3

 

[BigInch]

As Snorgy says, your colleges are considering the support as infinitely rigid, but if it actually deflects under load, then final loads would be reduced.

If your structural support deflects, it should not be modeled as an ifinitely rigid anchor, but as a spring pipe support with the same lbs/in spring coefficient of the structure. The forces between pipe and structure will then balance correctly.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

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