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RE: Uplift on Belled Drilled Pier

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I would like to hear some more comments on this topic if anyone can add to my questions below (and would like to share your knowledge and expertise). I have been asked to put some preliminary numbers together for some cost comparisons using drilled piers versus one large mass of concrete to anchor a structure against overturning (the structures are bents that are supporting 2000 L.F. of a continuous overhead conveying system located outdoors that will be located about 40 feet above ground). There will be about 25 of these bents (2 columns per bent, and the bent is 8' center to center of columns) spaced in the 2000 L.F., so that is the owner?s reasoning behind spending the time to do an analysis. The owner wants to see if using drilled piers would offset the cost of excavation and forming of spread footings (the spread footings would also rely on the soil overburden to resist uplift). The soils report and recommendations are in progress, so for now I am using some low-end skin friction values that the geotech recommended. (This is my first design experience with drilled piers).

The equipment is available to do a belled pier, so I am looking at both a straight-shaft or a belled pier. However, the reference I was using (Principles of Foundation Engineering by Das) stated that "For the majority of drilled (belled) piers constructed in the United States, the entire load-carrying capacity is assigned to the end bearing only. However, in certain circumstances, the end bearing capacity and the side friction are taken into account."

Question #1: What and where would I find out what these certain circumstances are? Would the Bowles reference cover this?

Question #2: The owner?s design shows the 2 columns from the bent setting directly on the drilled pier, (there will be no cap connecting the two piers together, so they will act independently), and the anchor bolts will be cast directly into the drilled pier. This tells me that I will need to run vertical steel the full length of the pier due to the lateral loads (correct me if I am wrong). Is this typical to run vertical steel the full length of the drilled pier? My preliminary numbers are showing that what the owner is proposing to do is not going to be cost effective based on the quotes he received for the unit cost of concrete to excavate and form vs. the unit cost to do a drilled pier. However, the owner is asking for an analysis of what he wants to do, and not anyone?s opinion.

Angela Baughman, P.E.
Wausau, WI

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