Calculating crushing imposed by pipe supports 2

[Alex303]

Hi, I have a 600NB Std Wt pipe that is full with water and supported every 5m. My question is whether or not the pipe requires a seat or whether it can just lay flat on a concrete pedestal? Obviously if it is sitting in a rounded pipe seat it would be better due to greater contact support on the pipe, but I am wondering how much better it is, more so out of interest?

If the pipe is lying flat on a plate or concrete pedestal you are basically allowing the pipe to deform and increase it's contact area on the plate as it does so. This means that the initial stress would be quite high, but then it would peter out as the contact point flattens. I would like to know whether the pipe has yielded in this zone (ie dented the pipe). I would also be interested in whether it was possible for the pipe to buckle in this loading case?

I have looked at table 14.1 on Roarks formulas which kind of describes my loading case, however that is only for a cylinder, not a pipe. I know shell analysis can get quite complex, but I was wondering whether someone had experience on how to calculate such a thing?

Unfortunately I don't have experience using any stress analysis programs otherwise I would just do that

Thanks

 

[Alex303]

Actually, the contact stress formulas detail the use of an infinite diameter for a flat support so I am pretty confident the formulas provide a good approximation of the maximum stresses for contact on a flat surface.

You have to keep in mind that the formulas are actually already conservative because they assume a perfectly rounded cylinder.

AFAIK the stress distribution is something like this:

So due to the fact that these are compression forces acting in the same direction to hoop stresses which are tensile forces, an internal pressure actually provides a strengthening quality to contact stresses and can therefore be negated (IMO).

 

[rconner]

I have not before seen such a pipe support depiction. I have however seen a somewhat more detailed unit elemental depiction of an empty and unpressurized steel pipe on a "rigid, level, foundation" that was proferred by an MIT M.E. grad gentleman who went on for the further education and notable experience as explained at

http://www.colorado.edu/engineering/deaa/cgi-bin/display.pl?id=36.

While not saying that his analysis is directly applicable to the OP (the full length support per Mr. Parmakian"s model is after all intuitively some better than spaced piers), I will attach herewith for your perusal his depiction of that sort of basic line loading that was contained in the article "Minimum thickness for handling steel pipes" more than three decades ago in the June 1982 issue of the Water Power & Dam Construction periodical.
Notice specifically the location and direction of Parmakian's bending moment M, and also "T = tensile stress resultant in pipe shell, positive as shown in Figure 1." Parmakian developed a differential equation via substitution involving five equations with five unknowns involved, and with solution of that equation and further study he concluded "The pipe shell stresses which result at the bottom of the pipe are due almost entirely as a result of the bending moment M."
Again, I am not primarily talking about the "contact" or shear stress, but instead (basically I believe Parmakian's) tensile bending stress at the inside surface of the pipe as a result of the bending moment at that location, and that weight, hoop stress due to eventual filling and internal pressure (not included in Parmakian's model), and maybe even other installation and service loads, might under some conditions be at least some additive to e.g. Parmakian's initial tensile bending stress (as of course hoop stress is also tensile, and also "positive", at the same location as well as elsewhere around the pipe).
While certainly not saying it will occur with your steel pipe and spans, it might be more descriptive to visualize e.g. a non-ductile (brittle) pipe ring say being crushed by loading between two flat plates -- if you are watching the test, I believe you would eventually see (as I believe Parmakian's model predicts) distress or a crack first open up on the inside surface of the pipe or lining very near the (clock) locations of load application.