Caisson Foundation Design, Steel Requirements

[scott26]

I am designing a high-ecentricity caisson foundation two support a transmission line pole and am looking for some recommendations on the vertical steel requirements. Can anyone point me in the right direction. I can't find anything in the ACI that specifies a steel area for this type of foundation. I will treat it like a beam. I found one formula in a company standard that says the required steel area is As = (3.33 x Mult x 12)/ (fy x Ds)

Thanks!
Scott

 

[MikeDB]

It is basically a circular column with a high moment and a low axial load. The point of maximum moment will be below the ground line by several feet depending on the type of soil. The amount of steel required will be a function of the maximum moment, the concrete strength, and the diameter of the foundation. Usually the vertical steel is run inside of horizontal hoops and the hoops should have a 3 inches of clear cover. A good concrete design text might help with the rest.
Mike

 

[eric1037]

The geotechnical engineer should be able to provide an L-Pile (or other) analysis with a shear and moment diagram.

 

[miecz]

I often find that the minimum reinforcing is more than adequate to carry the maximum bending moment. I use 1/2% of the gross area as the minimum.

 

[GOTEK]

jmiec is correct to explain a bit further: "Section 10.9.1 of the ACI code states that the area of longitudonal reinforcement for concrete columns must be not less than 1 percent of the gross concrete area, Ag. If, however, the cross-section is larger than required by considerations of structural resistance, then Section 10.8.4 allows a reduced effective area, Ag', not less than one half the total area, to be used to determine the minimum reinforcement and design strength. This means that if the column has sufficient axial strength using only half the gross concrete area, Ag/2, then the longitudonal reinforcement ratio can be reduced to 0.5 percent of the gross concrete area, Ag."

The reference for the above quote is Drilled Shafts: Construction Procedures and Design Methods, Publication No. FHWA-IF-99-025, pg. 363. It goes on to say that the above criterio can be used often because drilled shafts are typically designed with large diameters in order to develop sufficient side friction and end bearing in both soil and soft rock.

 

[scott26]

Typically high-ecentricity caisson foundations for transmission structures are designed as beams as opposed to columns because of the requirement for a high-moment resistance. Thanks for all of your input!

Scott