Punching shear
Punching shear
(OP)
My company requested a load drop analysis on a reinforced concrete slab to check a 30" pipe 7' below grade for failure. The vendor gives us a report that the pipe will fail. The concrete is subjected to punching shear failure. When I look at the calculation, I notice they do not take into account the rebar in the concrete for energy dissipation. They tell me it does no work. This can't be true! I find it hard to believe but they insist it provides no resistance. We really want to avoid costly modifications to the slab. Any suggestions on rebar contribution to punching shear strength? Thanks for your inputs.






RE: Punching shear
I do not clearly understand your problem. If you could give some more information, perhaps we could help further.
RE: Punching shear
RE: Punching shear
The concrete will locally be broken apart to the extent that the rebar will become of no consequence. Sorry, but my guts tell me that the pipe will fail too.
Mike McCann
MMC Engineering
RE: Punching shear
RE: Punching shear
RE: Punching shear
However, except local damage need be suspected locally, the localization of the same and the 7 ft downwards distance for what is a 1 m diameter applied loads may mean that an analyisis of one stabilized pipe underground for a load applied in a sheared circle 1 m diameter with the load (or may be, on half circle, to allow for the eccentrical application), i.e., the effects of such load in the stabilized pipe underground may be enough to reveal the effects in the pipe.
The required understanding is that the 360 kips loads subsumes all the dynamic effects. Then you take a half circle footing 104 cm in diameter and load it with 360 kip. You set a 3D geotechnical model that include the soil layers and your pipe, joints included. You ascertain if the stresses in the pipe or connections are permissible, or if leakage occurs.
You can also make a dynamic analysis including alternatively the slab and the force growing to the (dynamic) design load level of 360 kips ccording to some impact law and look for the same problems.
Inspection of falling load impact books would also be clarifying, to just find simplified statements of the effects; I may be looking for that later.
RE: Punching shear
http:
one can ascertain that the falling object needs be stopped in an average downwards stopping length of 15.2 mm if the average impact force has to equal its weight. Even if this does not warrant on StVenant dissipation of localized effects the local effects can be averaged at even the 7 feet depth (giving our estimate of at least a 104 cm diameter circle being the averaged front of the impact, but it may be bigger and so the impact effects might not be averaged at the depth), it gives us what the average deformation needs be at the surface (be it obtained from rupture or deformation). So we have now both an impact force and an average deformation that we need to match in our model.
RE: Punching shear
RE: Punching shear
RE: Punching shear
RE: Punching shear
RE: Punching shear
We derive the notional central load P by making the deflection equal to 2mm.
RE: Punching shear
In short, by these calculations, if the pipe is buried in thoroughly compacted material on as much resistant deeper layers of soil, and the thickness of the steel pipe is not too thin, not severy tight to limit strength without consideration of the accidental loading, these calculations give the pipe some chance of survival.
Later we will try to confirm more or less the same with a (still, non geotechnical) FEM 3D model.
RE: Punching shear
RE: Punching shear
RE: Punching shear
All along the FEM analysis materials have been rendered weightless to just assess the added stresses of the force at the surface over the slab.
So more or less the insight above stands, if corrected, and if the impact is of the imparted magnitude.
Remaining tasks:
Proper assessment of impact force.
Proper modelling in geotechnical package.
But I think what above gives an idea on how to attack the problem with just FEM packages dealing with continuous materials.
RE: Punching shear
RE: Punching shear
Another insight is that the physics statement on the web site of the force is not quite likely good enough to portrait the imparted force even for one indeformable body along time, particularly at max value, from an engineering viewpoint (if not flawed, what I have not examined). We may try to examine the question from a quantity of movement viewpoint, then by the still demurred examination of impact formula statements from enginering design tests. This I plan to do later.
RE: Punching shear
Since however the conclusion of that damage to the buried pipe is by now well established, I suspend any further work on this humbling and yet to me interesting thread.