Thrust Block Design 1

[a7x1984]

I will try to keep this short and sweet. I recently, and for the first time, designed a rectangular stress block for a 54" DIP pipe operating at a peak 250psi thrust. This resulted in a roughly 900kip factored thrust.

As, the structural engineer, I had no idea what thickness DIP was being used and decided that it was rational to provide 'hoop' steel around the pipe, in the thrust block, to tie the longitudinal bars (resisting soil pressure) at the penetration for continuity. The second reason was to resist the hoop stress that I presumed would be transferred from the DIP.

I was told that this was overkill. I thought my approach was rational and the hoops steel would be needed, especially close to the interface of the concrete and pipe.

Has anyone have thoughts about this? FYI, I used the Lame equations for a Thick-walled member, assuming a ring of concrete engaged around the pipe would behave as such. I do understand that if the strain in the very outside fibers of the pipe is near zero, strain compatibility requires the concrete adjacent to be the same.

Should I have assumed a maximum allowable strain in the DIP and checked the concrete from that assumption?

 

[BigInch]

I usually place straight bars vertically and at 45[°] around the hole at the face of the block concrete to complement the longitudinal bars and ensure that the load is transferred throughout the height of the block.

Nobody I know actually cares if any amount of reinforcement in an anchor block is overkill or not. As long as the aggregate will get through the grid.

From "BigInch's Extremely simple theory of everything."

 

[rconner]

What is the specific pipeline thrust focus you are in the process of blocking (from the shape of the block and your inquiry about a collar around the “pipe” you describe, it kind of sounds like you are restraining not a bend fitting but instead e.g. a closed valve, wall penetration or test bulkhead etc. inline with the pipe axis. However, at 250 psi PA, i.e. full bulkhead or closed valve thrust for 54” DIP with a 57.56” pressure diameter is about 650 kips per shown e.g. in the chart on page 17-13 at

http://www.american-usa.com/system/assets/151/original/section 17_RB.pdf,

not “900 kips”)?
If this is however e.g. a simple block or wall Pipe penetration, I think both USA manufacturers of this size pipe can supply a factory/welded-on “thrust collar”, with about a ¾” plate thickness and that extends outside the pipe and into the wall concrete. [As BigInch notes, it is also not unusual for such wall penetrations to be reinforced with a steel reinforcement lattice tied around same due to the high and localized shear loads at such penetrations.]

 

[MJCronin]

Hmmmmmm....54 inch diameter and 250 psig design pressure.

IMHO, You are nearing the limits of what can be done with UG thrust blocks.

Perhaps some other method of axial pipe restraint could be considered ?

http://www.ebaa.com/PDF/Brochure.1100.pdf

-MJC

 

[a7x1984]

Big Inch, I agree that would have achieved the same effect for transferring tension from the longitudinal bars. So, you didn't worry about hoop stress from the pipe carrying over into the concrete? Oddly enough, the Washington San. Commission did care about it; no clue as to why.

rconner, It was a factored value given to us from our civil guys. It is a straight pipe at a push-on style retrofitted joint, hence just a rectangular block. There was a thrust collar and that was easy to handle just as you would punching shear of a column. Again, this project was a first for me. My concern, and where my ignorance resided, was such a large pipe and high hoop pressure splitting the block. My assumption was that it (the pipe & some amount of concrete) were to behave like an old-fashioned wood barrel with iron straps binding the barrel together. Also, no shear bars, just lots of concrete thickness!

MJCronin: We were delegated the structural design work only. I had no choice but to design as such. I think the issue was the push-on joint was to existing precast pipe? Because I am not in that field , I do not know the particulars. Your attachment only went up to 48"

Thanks Guys

 

[rconner]

I believe that in the total absence of any surrounding restriction(that is e.g. the existing welded-on thrust collar, and I suspect massive surrounding reinforced concrete thrust block), the tensile stress in say a nominal thickness 54" PC250 DIP would be only about 10,900 psi. With a DI "E" of about 24,000,000 psi, I believe this would in turn result in only about 450 microinches of strain, I suspect even with no external boundary strength meaning only a couple hundredths of an inch expansion on the very large pipe diameter. While I guess the composite structure could be analyzed with FEA and/or reinforced concrete design/modular ratios etc., it appears if something were to crack (and I've not heard of that occurring in any sort of normal application), perhaps it thus couldn't crack much.