hemiv
Structural
- Dec 7, 2018
- 78
Hi all, I'm working on checking the adequacy of a 104" x 3/4" steel casing jacked under, at most, 50' of overburden. Using 113pcf unit weight, 150psf cohesion, no groundwater. Medium dense sands with silts, some gravelly.
I've never done this type of design before, and I'm using the American Lifelines Association "Guidelines for the Design of Buried Steel Pipe", 2001. For such a depth, I'm ignoring all surcharge effects. Coming up with 27.2 psi on the pipe just from soil pressure minus cohesion effects.
My problem is in checking the deflection with the modified Iowa formula. With E' as 2,000psi, I'm coming up with 3.16% deflection. Which is under some recommendations I've seen to keep steel pipe under 5%. But when I move on to section 4.2.2 for Through-Wall Bending, the check fails. I return a 26.4ksi stress, whereas allowable is 35ksi/2 = 17.5ksi. So, naturally I think I can maybe bump up a wall thickness and it'll all be good. But the higher the wall thickness, the higher the stress. E.g., at a 2" w.t. I'm getting 35ksi. By this logic, I would have to decrease my w.t. to 3/8".
Now, this partly makes sense to me. The thicker pipe walls will be stiffer, deflect less, and thus have less support from the surrounding soil. But also, it just seems odd that (1) I would be picking bones about spec'ing a smaller pipe than the field guys want to use, and (2) that I can meet the recommended 5% deflection limit, but still fail the stress criteria by almost 10ksi margin / 50% overstress. Not to mention that, if I want to pass the stress criteria, the only option is to go with a thinner pipe.
Is there something I'm missing?
I've never done this type of design before, and I'm using the American Lifelines Association "Guidelines for the Design of Buried Steel Pipe", 2001. For such a depth, I'm ignoring all surcharge effects. Coming up with 27.2 psi on the pipe just from soil pressure minus cohesion effects.
My problem is in checking the deflection with the modified Iowa formula. With E' as 2,000psi, I'm coming up with 3.16% deflection. Which is under some recommendations I've seen to keep steel pipe under 5%. But when I move on to section 4.2.2 for Through-Wall Bending, the check fails. I return a 26.4ksi stress, whereas allowable is 35ksi/2 = 17.5ksi. So, naturally I think I can maybe bump up a wall thickness and it'll all be good. But the higher the wall thickness, the higher the stress. E.g., at a 2" w.t. I'm getting 35ksi. By this logic, I would have to decrease my w.t. to 3/8".
Now, this partly makes sense to me. The thicker pipe walls will be stiffer, deflect less, and thus have less support from the surrounding soil. But also, it just seems odd that (1) I would be picking bones about spec'ing a smaller pipe than the field guys want to use, and (2) that I can meet the recommended 5% deflection limit, but still fail the stress criteria by almost 10ksi margin / 50% overstress. Not to mention that, if I want to pass the stress criteria, the only option is to go with a thinner pipe.
Is there something I'm missing?
