Drilled Bell Pier Design 3

[yrevdoog]

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.

 

[yrevdoog]

Additional info silo drilled piles are spaced 14ft apart in a square arrangement. axial load = 51kips, bending moment = 357kip-ft.

 

[haggis]

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

 

[JAE]

Using 8 - #14 bars is excessive - way over designed.

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).

 

[tincan]

In drilled shafts with axial loading we normally use 1% vertical reinforcement.

 

[fndn]

The 2' diameter is plenty for the loads you have, and most deep foundation contractors carry 2' augers. Just watch out that the center to center of the group caisson is close to 4 ft-you want to avoid bell-bottoms being close, should all your depths be exactly 16 ft.

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.

 

[yrevdoog]

additional info, changed the design of the foundation, reinforcement wise to 8-#8 rebars, calculations shows 6-#8 rebars.i figure the overturning moment caused by the eccentricity of the silo would be counteracted by the depth of the foundation and section. the axial load is taken cared of by the reinforcements and section. one more question: what does allowable soil bearing capacity for dead load mean?

 

[GeoPaveTraffic]

What are the soil types? Bells can only be constructed in cohesive soils. Also, in the midwest (USA), it is common to limit the bell diameter to twice the shaft diameter. You may want to check with local contractors in your area to ensure that they are able to cut a 5 foot bell from a 2 foot shaft.

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.