Design of Concrete encasement around penstock

[aakalim152]

We are designing the concrete encasement around a steel pen stock for internal loads. Although the pen stock is capable of taking the loads, but we need to find how much load will be transferred to the concrete encasement. Maximum internal water pressure in 6.5 ksi. The steel pipe material is ASTM A 537 class 1, and the concrete lining is 16 inch thick. Concrete strength is 3ksi.

I have been asked to make a computer model for finding the stress in concrete lining. I prepared a model in sap2000 using layered shell element and then applying internal load. But the answer i am getting are not satisfactory. The resulting stresses in steel are 250kis and in concrete 30ksi (in the S11 direction i.e. local 1 axis), which i think are too high. I am not sure weather a 6.5 ksi internal water pressure can cause such high pressures in the shell.

Also if someone can provide reference to any standard or manual for the design of such concrete lining, it would be really appreciated.

Regards,

 

[JStephen]

6.5 ksi = 6500 psi = 15000' of head? That's more like hydraulic tubing rather than penstock material.

In some cases, concrete encasement would just be treated as fill, rather than as part of a composite assembly. This assumes it can freely crack without damaging a more extensive structure.

 

[aakalim152]

The head is actually only 200' but after adding the effect of transient loading, load rejection and water hammer, the maximum hoop stresses come out equal to 6500 psi.

And yes initially we did not take the concrete encasement to be part of the composite assembly, since the penstock was capable of taking all the loads. But our review consultants insisted that we should design the concrete encasement considering composite action.

 

[cvg]

650 psi?

you only have 80 psi static and you have 6,500 psi transient?

hoop stress does not equal internal water pressure

The hoop stress can be expressed as:

σh = p d / 2 t (1)

where

σh = hoop stress (MPa, psi)

p = internal pressure in the tube or cylinder (MPa, psi)

d = internal diameter of tube or cylinder (mm, in)

t = tube or cylinder wall thickness (mm, in)

 

[JStephen]

A hoop stress of 6,500 psi is entirely reasonable, but the 250ksi sounds like you're applying a radial pressure of 6,500 psi, which is something entirely different. If calculated hoop stress is 6,500 psi without the concrete, then it should come out somewhat lower when figuring a composite structure.

 

[aakalim152]

I got it. I was applying hoop stress as the internal pressure, which gave very high pressures.(quit a fundamental mistake i guess). I now applied the internal pressure of 80 psi and the results i am getting are quite reasonable.

@Jstephen
You are right in that the resulting stress is coming out lower than that calculated for steel only. Thanks for explaining that also.

@cvg
Well the internal stress is only about 80 psi but considering that our pen stock diameter is 75 inch and thickness 0.5 inch, we do eventually get such high hoop stresses.

Thanks a lot for your input guys. It really helped.

 

[hokie66]

Where is the concrete? Inside or outside the pipe? Your title says the concrete is an encasement, but in the thread it is called a lining. Two different things. In either case, just use the steel for tension, as concrete is weak in tension.

 

[aakalim152]

Concrete is outside the pipe. And since we are using reinforced concrete so it will take some tension, but yes majority of the tensile stress will go to the penstock steel.

 

[hokie66]

If the steel is only stressed to 6500 psi in hoop stress, forget the concrete taking tension. If it is reinforced concrete, it is the reinforcing which would take tension, not the concrete. The concrete would crack.

 

[saplanti]

I think you have to correct your terminology first. Is 6500 psi the internal pressure of the pipe or the hoop stress in the steel?

If it is pressure you need to provide the other parameters at least for the geometry in addition to pressure. Probably you need to talk about the value of the surge pressure as well.

Otherwise, all the answers are imaginary and there is no base for the discussion.

Regards,

Ibrahim Demir

 

[aakalim152]

@saplanti

Thanks for the input.
The maximum internal pressure including the effect of surge is about 80 psi and the resulting hoop stress in the penstock steel is 6500psi. You are right that initially i got my terminology wrong since i was wrongly assuming the steel hoop stress as the internal pressure. But now i have corrected my terminology, and applied the internal pressure in the model, not the hoop stress.