Drilled Pier Minimum Steel 1

[gr8gonzo0]

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?

 

[DaveAtkins]

I have read that the 1% is appropriate.

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

 

[Panars]

Drilled shafts often have much more cross-section area than is required to support axial loads. I have been taught, and it is also in an FHWA publication, that the one percent reinforcing requirement can be reduced when the cross-section is larger than required to structurally support the axial loads. The text from the FHWA publication FHWA-IF-99-025, "Drilled Shafts: Construction Procedures and Design Methods" (page 362) reads as follows:

"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.

 

[civilperson]

If designed for flexure, a minimum steel will be obtained for one load condition. When investigating all load conditions, (wind from all compass points), then the flexural reinforcement will be repeated in many orientations. Use the interaction diagrams for columns to assure that moment strength is achieved in all orientations and thus a minimum 1% will also be assured.

 

[gkm123]

For light-pole bases the 0.5% for the main steel is the right way to go since the diameter is going to be as required by the soil pressure requirements. What do you guys think about the confining reinforcing?

 

[JAE]

Hey - remember that ACI 318 does NOT apply to drilled piers (see section 1.1.5)

Drilled piers are covered under ACI 336.

Link: ACI 336 Link

 

[Panars]

Portions of ACI 318 do apply to drilled shafts (piers) because ACI 336.3 incorporates them by reference. Specifically, ACI 336.3 Section 3.3 references ACI 318 10.8.4 and 10.9, which are the sections that relate to minimum steel under discussion here.

 

[kipfoot]

Here are a couple other provisions to chew on:

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.

 

[scottiesei]

Sorry to pull an old thread, but how about us in non-seismic regions. I would think I could use an unreinforced design per IBC section 1812.8. I have analyzed a few sections with a couple kips point lateral load at the top of a 3' diameter pier to depths of around 4'-6"-6'. The moment in the pier (assuming it has lateral support from the soil) is very small. The resulting stress is also very small especially when compared to the modulus of rupture of the concrete. I have a project that I am working on designing piers for 6' walls. I was going to spec some fibermesh for cracking to keep the pier together, put not provide any main steel. Any comments?

 

[hokie66]

Scottie,

I don't think the plastic mesh will do any good, but a short pier like you have can probably be plain concrete.