Design of Concrete encasement around penstock
Design of Concrete encasement around penstock
(OP)
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,
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,






RE: Design of Concrete encasement around penstock
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.
RE: Design of Concrete encasement around penstock
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.
RE: Design of Concrete encasement around penstock
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)
RE: Design of Concrete encasement around penstock
RE: Design of Concrete encasement around penstock
@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.
RE: Design of Concrete encasement around penstock
RE: Design of Concrete encasement around penstock
RE: Design of Concrete encasement around penstock
RE: Design of Concrete encasement around penstock
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
RE: Design of Concrete encasement around penstock
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.