Drilled Pier Minimum Steel
Drilled Pier Minimum Steel
(OP)
I have an interesting question. There has been some discussion among engineers I know on what the minimum steel requirements should be for a cast-in-drilled-hole concrete foundation for something that takes high lateral loads (and very little axial load), such as a light pole foundation, or a retaining wall on piers. I can calculate how much steel is required for design, but the problem comes from which sections of code to use for determining minimum steel. Since the loads are lateral, I typically use minimum steel for flexural members per ACI. But I have been told to use the minimum steel required for columns (As = 1% of gross area). Which is more appropriate? Any thoughts?





RE: Drilled Pier Minimum Steel
But just last week I completed a light pole design, and I used 0.5%, justifying it with that part of the Code that allows area of steel to be based on 50% of the area of the column, if the additional area of concrete is not required for the applied loads. I DID use the entire area of concrete and steel for checking bending, but the way I figure, the entire area is NOT required for axial loads, so the 0.5% applies.
With 1%, you end up with a lot of steel in a light pole's drilled pier.
DaveAtkins
RE: Drilled Pier Minimum Steel
"Structural Design Procedure: Minimum Longitudinal Reinforcement
In the case in which there are no moments or shears the minimum steel precentage can be less than the normal structural minimum of one percent. Section 10.9.1 of the ACI Code states that the area of longitudinal reinforcement for concrete columns must not be less than one percent of the gross concrete area Ag. If, however, the cross-section is larger than required by considerations of the structural resistance, then Section 10.8.4 allows a reduced effective area Ag', not less then 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 longitudinal reinforcement ratio can be reduced to 0.5 percent of the gross concrete area, Ag. That is, (rho)min(%)=Ag'/Ag>=0.5 when Ag'/Ag<1, where (rho) refers to percentage of steel. In fact, in many cases in which drilled shafts are designed with large diameters in order to develop enough side and base area to produce adequate geotechnical resistance in soils and in some soft rocks, this criterion can be used."
I realize that the very first sentence of this quote does not agree with your situation of a laterally loaded member. However, the ACI code and commentary does not include this restriction to cases where there are no moments or shears.
Is the diameter of the drilled shaft dictated by the bending structural resistance of the shaft, or to provide the required resistance by the soil? If the diameter of the shaft is dictated by the required soil resistance, then there is extra structural capacity and the above exception applies, in my opinion. If the bending resistance of the shaft controls the diameter, then the situation is more ambiguous.
RE: Drilled Pier Minimum Steel
RE: Drilled Pier Minimum Steel
RE: Drilled Pier Minimum Steel
Drilled piers are covered under ACI 336.
Link: ACI 336 Link
RE: Drilled Pier Minimum Steel
RE: Drilled Pier Minimum Steel
ACI 318: 22.2.3 commentary
"Structural elements such as cip concrete piles and piers in ground...are not covered by this code. Such elements are covered by the general building code"
So, I go to IBC 1810.1.2.1, which gives rho >= 0.0025 for seismic design category C .
and, IBC 1810.1.2.2, which gives rho >= 0.005 for siesmic design cat D,E, or F.
RE: Drilled Pier Minimum Steel
RE: Drilled Pier Minimum Steel
I don't think the plastic mesh will do any good, but a short pier like you have can probably be plain concrete.