Friction load transfered to foundation from piperack

[allimuthug]

Hi ,

In most of the design basis it is written that for foundation design 10%( laterally) of the pipe operating load is to be considered for foundation design.
Please let me know the basis for it .

 

[KootK]

Even in a structure not exposed to wind or earthquake loads, you need to design for some lateral load to account for:

1) Out of plumbness in your vertical members which generates overturning moments.
2) Other imperfections that might initiate instability (buckling) failures.

In the AISC code, #1 is dealt with using the notional load concept which assume a 0.2% out of plumbness in columns (h/500). I was taught to design indoor mezzanines in low seismic areas for a lateral load of at least 5% of the gravity load. Pipe racks in industrial settings are not constructed with great precision so perhaps 10% makes sense there.

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.

 

[IFRs]

"Even in a structure not exposed to wind or earthquake loads" - are yours subject to either or both?

 

[FixitFelix]

I'm guessing that's friction loads coming from the thermal expansion and contraction of pipes.

From PIP STC01015 Structural Design Criteria,

4.1.7.7 For pipe racks supporting multiple pipes, 10% of the total piping
weight shall be used as an estimated horizontal friction load applied
only to local supporting beams. However, an estimated friction load
equal to 5% of the total piping weight shall be accumulated and
carried into pipe rack struts, columns, braced anchor frames, and
foundations.


Process Industry Practices (PIP) only suggests 5% of friction load to be used in designing foundations.

 

[IFRs]

Friction loads can be huge if the pipes are insulated, especially if they are steam lines. Heavy oil is heated and those pipes can be large leading to massive loads. I thought the OP was looking at "lateral" loads which may be large if the pipes are heated and just after a turn.

 

[allimuthug]

Hi guys,
I got the answer for friction loads transferred to foundation.
Yes as per PIP 10% of operating loads are transferred to struts, column , bracing if the transverse beam supports multiple pipes of almost same size . It was recommended since not all the pipe would expand at same time some may expand and some would contract which intern would cancel the effect of friction (30%) to 10%.
However if there are only few pipes you need to understand the direction of expansion of pipes to justify the exact friction loads in the foundation. It is recommended that friction load transferred to foundation is highest of 40% of operating load of larger diameter pipe or 10% of the operating load of entire pipes in that rack.
Note the transverse beam must be checked for 30% of operating load for friction locally if the pipes are >12 Inch.

 

[TLHS]

The basis is decades of common practice. Honestly, I've looked into it and there aren't really any studies that quantify the reduced thermal friction values as you go through the load path of the structure. It's generally, as you say, to account for the fact that pipes won't all expand from the same virtual anchor and they won't all see thermal effects at the same time. The thermal effects are generally gradual in the case of environmental thermal loads, or more sudden in the case of start-up or shut-down. In the former case, they have time to slide a bit, in the latter they're unlikely to happen at the same time.

Also, depending on how your lateral system is detailed, things will cancel out as you go down the structure. Your net thermal force on a large rack system is going to be close to zero, since the pipe has to push somewhere to pull somewhere else. Depending on how long your modules are in comparison to expansion loop locations and other similar things, you might have almost no thermal induced lateral force by the time you get to the footings. On the other hand, if you have a lot of independent frames, this may not be the case.

You've also got an extra level of comfort in your back pocket given that thermal loads tend to be self limiting. If you under-design your structure it will tend to move and won't suffer a catastrophic failure unless you've done a bad job of detailing.

PIP is what I would usually use as my basis, but I've seen similar design methodology in reasonably old document as well. It likely started in some oil company or large design firm's standards at some point.

If you've got a bunch of independent frames, watch out that you don't have the possibility of a non-ductile failure of some sort. You generally want at least one set of longitudinal tie beams and some ability to move. If you have a concrete moment frame in one direction with no longitudinal tie beams in the other, for instance, you'd likely use more conservative forces throughout your system.