Drilled Bell Pier Design
Drilled Bell Pier Design
(OP)
i am designing a Drilled Bell Pier for a grain silo which would support 204000lbs of gross weight on 4 piers and have designed it using practicality and came up with a 2ft diameter concrete pier with a 5ft diameter bell at the bottom with total embedment depth of 16ft with 8-1.693in longitudinal reinforcement and 0.375in spiral reinforcement at every 11in pitch. the soil is assumed to have a allowable bearing capacity of 3000psf. I have designed the embedment depth based on the overturning moment, and designed the section using circular concrete column analysis. Would hope that i could get an opinion on whether the section is just enough for the load or that its overdesigned.





RE: Drilled Bell Pier Design
RE: Drilled Bell Pier Design
yrevdoog
To assume the soil bearing capacity is 3000 psf and getting pretty close with almost 2600 psf on the 5' dia bell, with something of this magnitude, I think I would be more comfortable with some figures verifying the actual soil conditions.
I'm not Geotech or structural.....just interested in everyones projects in these fora
Haggis
RE: Drilled Bell Pier Design
Column analysis is not appropriate for a pier - as the columns assume no lateral bracing while piers have earth all around. Normally, you would assume an unbraced length for the pier of perhaps 3 or 4 feet and design accordingly.
Better to use more, smaller bars (I would stop at #10's or #9's as a size limit for a 24" shaft).
Also, why is there bending moment in the pier? With four piers, and overturning by the rigid silo, you would really only develop axial loading (tension/compression couples resisting the overturning).
RE: Drilled Bell Pier Design
RE: Drilled Bell Pier Design
Even though allowable is 3ksf, check the penetrometer readings of soils obtained from the bottom of the shaft.
Finally, have a Site Geotech measure the as-built diameters, and shine a torch to the bottom to see you have a clean hole, prior to concrete pour.
RE: Drilled Bell Pier Design
RE: Drilled Bell Pier Design
I would never design a foundation to support a significant structure without at least one boring. I strongly recommend that a boring be made in the area of the proposed structure and that the boring extend to at least 26 feet, 16 foot embedment plus two pier diameters. If you or the company doing the drilling have any concerns about the soil at 16 feet, the boring should be extended deeper.