Drilled Pier Minimum Concrete Strength

[Jpr1216]

IBC Table 1808.8.1 specifies that the minimum compressive strength for socketed drilled piers is 4,000 psi. I've routinely seen drilled piers specified as 3,000 psi when longitudinally reinforced with a minimum 0.5% steel cage. This section of IBC does not specify if it is in reference to all drilled piers or only unreinforced driller piers. Reinforcing is only required in piers in high seismic and in expansive soils so it seems plausible that this section is intended for unreinforced piers. In my opinion, the word "socketed" is key and implies that the code's intent is to guard against the pier pulling apart due to the active soil pressures at the top of the pier lifting up on the pier while the bottom of the pier is socketed into the rock. In which case, providing a reinforcing cage meets the tensile requirement and thus a lower concrete 3,000 psi strength could be utilized.

What do your firms use for drilled pier concrete strength and do you agree/disagree with my interpretation of the code's intent regarding the minimum compressive strength of drilled piers?

Thanks for your help!

 

[Aesur]

We use Caissons fairly often for cantilever columns on canopies, etc. and use 3000 psi concrete; correct me if I'm wrong, but isn't socketed where you have a steel member, typically a post that is "socketed" into bedrock and the concrete is poured around the caisson above the bedrock. This would be different from a typical caisson and may be why it's common to see standard caissons with less concrete strength as they aren't "socketed".

 

[Ron]

implies that the code's intent is to guard against the pier pulling apart due to the active soil pressures at the top of the pier lifting up on the pier while the bottom of the pier is socketed into the rock.

Huh? Active soil pressure doesn't typically work in that direction. Please explain.

 

[JLNJ]

Drilled piers certainly can be socketed into rock. Socketing makes sure you are bearing on something sound and yields the greatest bearing capacities.

If you limit the pier capacity to .25*f'c and you spec 3000 psi concrete, the pier limit is 54 tons. I have seen socketed rock bearing capacities well in excess of 50 tons, so maybe the IBC is trying to keep the concrete strength from being the limiting capacity.

 

[Jpr1216]

Ron, expansive soils swell laterally and upwards simultaneously essentially grabbing onto the perimeter of the pier and heaving it upwards. If properly socketed, the pier will remain in place but will develop a tensile force in the pier. Most geotechnical reports that I've seen address this behavior by giving the structural engineer an active skin friction pressure that must be applied to the upper 10' or so of the pier to estimate the active soil forces on the pier. The upward force is resolved through the reinforcing caged and skin friction with the rock at the bottom of the pier. The weight of the pier also contributes to offsetting the upward force. The tensile force is used to calculate the minimum penetration depth into the bearing strata so that that the pier cannot pull out of the rock, analogous to development length for rebar.

 

[dauwerda]

I disagree that the code would be using a higher strength concrete to increase the tensile strength of unreinforced concrete. If your pier has uplift forces (whether caused by structural loads or expansive soils), it needs to be designed for such and have longitudinal reinforcement in it.
Section 1810.3.9.6 requires all socketed drilled shafts to have reinforcement or a structural steel core, so I really don't see how using reinforcement could justify using lower compressive strength.

The commentary doesn't give much help on the minimum numbers used, It simply states, "The code's intent is that concrete or grout used as structural material for foundation construction will be of sufficient strength and durability to satisfy safety requirements."

My past experiences have all used 4000 psi concrete as a minimum for structural concrete.

 

[RabitPete]

My past experiences have all used 4000 psi concrete as a minimum for structural concrete.

On the same page here. Never used 3 ksi for any structural projects, a few times for indoor residential slabs if customers (typically landlords) insist on saving every cent. The cost savings are barely noticeable as formwork and ironwork labor are much more than materials

 

[Aesur]

@dauwerda and @RabitPete - You guys must not be doing residential or commercial in low seismic areas, or maybe specialty engineering? In my area we design for 2500 psi, to avoid SSI where possible, but specify 3000 psi minimum, the few times we actually needed 4000 psi the contractors freaked out and started asking for VE options, although I am in agreement that I don't believe the cost difference from 3000 to 4000 psi is much if anything at all.

 

[Jpr1216]

I agree with Aesur, I see it all the time. I just don't know how it's justified given Table 1808.8.1. (Aesur, What does SSI stand for?) Dauwerda, fair point. The tensile strength difference between 3 & 4 ksi is negligible (maybe 100 psi at best) so probably not what the code is after. But why not just say "Drilled Piers" instead of "Socketed Drilled Piers" in the table? Not all drilled piers are socketed. It just seems odd that the code would dictate a minimum compressive strength. If I design it for the compressive loads and reinforce it for tension and shear, why shouldn't I be allowed to use 3,000 psi concrete?

 

[dauwerda]

Aesur, you are correct, my background is in industrial structures.
If I recall correctly, it is around $10-$15 per yard increase to go up from 3000 to 4000 psi.

Most drilled piers in residential design also aren't socketed are they? The IBC's definition of a socketed drilled shaft is, "A drilled shaft with permanent pipe or tube casing that extends down to bedrock and an uncased socket drilled into the bedrock."
So, if you have a drilled shaft that isn't cased or uses temporary casing that is removed, it is not considered a "socketed drilled shaft" (per the IBC), even if it does extend into bedrock.

 

[Aesur]

@Jpr1216, SSI is Special Structural Inspections, when it comes to concrete inspections if you design for 2500 psi and meet other requirements, you can sometimes get away without needing to have these inspections completed during construction, it's typically a money saver for the owner and a time saver for the contractor.

I did a little digging and found the following documents: Link From a quick glance, it appears that 4,000 psi is the minimum you would see in bedrock and therefore is the minimum concrete strength for "socketed" drilled piers to provide similar friction values to transfer loads to the bedrock. Based on this, it appears that sometimes the capacity is not just end bearing but relies on friction between the concrete socket and the bedrock. See section 5.

 

[Jpr1216]

Ok, so it is plausible that the code's intent could be to ensure adequate load transfer to the rock in skin friction by requiring concrete with similar stiffness & strength properties as the surrounding rock. Makes sense. But, @dauwerda, you also bring up a good point: IBC very specifically excludes from it's definition drilled shafts without permanent casing, which are routinely socketed as well and thus also rely upon skin friction load transfer. What do you guys think: Is this perhaps an oversight on the code's part and it should require all drilled shafts with or without permanent casing to have the higher 4,000 psi strength? Or is there something about the permanent casing that necessitates the higher concrete strength? My guess: an oversight.

 

[Jpr1216]

I'm not a bridge guy, but it may be that the language in the IBC regarding drilled shafts was borrowed from the LRFD Bridge Design Manual and not meshed well with the other sections of the IBC. I don't have the Bridge Manual in front of me, but I found Missouri's online version and it sounds very similar to the IBC. It requires permanent casing and 4,000 psi concrete for drilled shafts. I suspect permanent casing is required for bridges due to the limited number and critical nature that piers serve for bridges and because they often are placed in saturated areas where scour and erosion are likely. Permanent casing is almost never used for buildings and it should probably be removed from the IBC definition of "socketed drilled shafts".

https://epg.modot.org/index.php/751.37_Drilled_Shafts

 

[dauwerda]

The 2006 and prior IBC called these foundations, "Caisson piles", in 2009 edition it changed to "socketed drilled shaft".

Going back and looking at the 2006 edition, "Caisson piles" had their own design section (1810.7) that required a min compressive strength of 4000 psi concrete. Other drilled piers did not have this requirement (cased or uncased). The proposed/accepted changes can all be found here:

https://www.iccsafe.org/cs/codes/Documents/2007-08cycle/FAA/IBC-S142-S209.pdf

Based on this, I don't believe the code is intending to require 4000 psi concrete for traditional drilled shaft foundations.


From what I can tell, the foundation that the IBC is now calling a "socketed drilled shaft" is the same thing that ACI 543R-00 call a Drilled-in Caisson, which has the following definition:
"A drilled-in caisson is a special
type of CIP concrete pile that is installed as a high-capacity
unit carried down to and socketed into bedrock. These foundation
units are formed by driving an open-ended, heavywalled
pipe to bedrock, cleaning out the pipe, and drilling a
socket into the bedrock. A structural steel section (caisson
core) is inserted, extending from the bottom of the rock
socket to either the top or part way up the pipe. The entire
socket and the pipe are then filled with concrete. The depth
of the socket depends on the design capacity, the pipe
diameter, and the nature of the rock."

As can be seen in the above definition, this is very different than a tradition drilled pier foundation.