Compression Piles in AASHTO 1

[WARose]

Question for someone who does a lot of bridge design........


In the IBC 2012, there is still a way for you to have the reinforcement in a cast-in-place, concrete pile terminate at a point (and just have a single bar going down the center) where the pile will always be in compression only and has little moment. (I've typically fought this approach.)

But in the AASHTO code, I really don't see a way to do that. It appears to me that you need the 1-0.5% minimum at just about all cross sections. For those of you familiar with this code (and I don't have the latest): is that how you see it? Do you need to have "the cage" at all cross sections? Is that what you typically see for anything getting traffic loads?

Thanks.

 

[BridgeSmith]

Assuming a cast-in-place concrete pile is what the AASHTO spec calls a drilled shaft, Subsection 10.8.3.9.3 and the commentary for that section seems to be the only ones that really address the reinforcement for drilled shafts. I don't believe it's changed much in the last few versions. If you'll let me know what edition you're looking at, I can let you know if anything's changed.

Anyway, according to my reading of it, if there's no expected lateral loading, it still must be reinforced as a column to the depth where it is laterally supported (defined in the commentary), reinforced adequately to carry the axial loads from there to 10 feet below fixity (there are several views on what constitutes fixity), and no reinforcement is required below that point. I'd have to dig into Section 5 for the reinforcing required for columns and for sections with only axial load.

 

[WARose]

This is really more of a practice question than anything else. What I am looking for is: do [you or anyone else that does bridge deign] ever see a pile/drilled shaft with that single bar in the middle for compression (only) loads (where it is supporting moving loads)?

 

[Ingenuity]

...pile/drilled shaft with that single bar in the middle for compression (only) loads (where it is supporting moving loads)?

Can you argue that micropiles are piles/drilled shafts? Micropiles are often used and designed to AASHTO specs for bridges and over the uncased length, all they have is a single threadbar core.

 

[BridgeSmith]

Almost all of our drilled shafts are reinforced the same all the way down. We've done a few where we actually added more reinforcement in the first 10 ft to handle the moment at the interface with an abutment wall, but then dropped bars out below the region that could be vibrated in order to meet the minimum spacing requirements.

To answer your question, no we haven't seen, nor would we consider, a drilled shaft without at least some type of reinforcing cage the full depth. Granted, pretty much all of our drilled shafts fall into that "significant lateral loading" category.

With the minimal material cost of rebar, I can't imagine there would be much, if any, savings by changing from a cage in the top of the shaft to a single bar further down.

 

[WARose]

Can you argue that micropiles are piles/drilled shafts? Micropiles are often used and designed to AASHTO specs for bridges and over the uncased length, all they have is a single threadbar core.

Probably could....but I never seal micropiles. I want to keep the focus here on RC piles. (As per AASHTO and practice.)

 

[WARose]

Thanks HotRod10.....that's the info I'm looking for.

 

[BridgeSmith]

"...all they have is a single threadbar core."

In that case, that threadbar is the structural support. Micropiles don't count on unconfined concrete as a structural support component.

 

[WARose]

.... reinforced adequately to carry the axial loads from there to 10 feet below fixity (there are several views on what constitutes fixity), and no reinforcement is required below that point. I'd have to dig into Section 5 for the reinforcing required for columns and for sections with only axial load.

I checked my AASHTO (again; 16th edition) and it clearly (to me) says that all cross sections require 1% steel. (Regardless of loading condition, including for drilled shafts.) It also calls out a minimum of 6-#5's for a circular section. (See p.185, Division I, Sect. 8.18.1.2)
--------------------------------------------------------


[red]EDIT:[/red] The Section on structural design of drilled shafts (i.e. Sect. 4.6.6.2) put me into Sect. 8.18.1.2. BUT the wording in Sect. 4.6.6.2 kind of throws me for a loop: Where the potential for lateral loading is insignificant, drilled shafts need to be reinforced for axial loads only. Those portions of drilled shafts that are not supported laterally shall be designed as reinforced concrete columns in accordance with Articles 8.15.4 and 8.16.4, and the reinforcing steel shall extend a minimum of 10 feet below the plane where the soil provides adequate lateral restraint.

.....

The design of longitudinal and spiral reinforcement shall be shall be in conformance with the requirements of Articles 8.18.1 and 8.18.2.2, respectively.

That part about "not supported laterally" got me wondering about well, what if it IS supported laterally? Do any requirements go out? That last sentence makes it look like the answer is "no".

 

[BridgeSmith]

Ok, you're going waaaay back. I'm in the 8th Edition LRFD spec. I may not even have a 16th Edition Standard Spec. I have the 17th Ed. on the shelf, so I'll take a look at that.

 

[WARose]

Right. It's not just having a current code.....it's understanding the code. As someone who doesn't do AASHTO hardly ever, I want to be sure i am understanding this right.

You've already given me backing by standard practice.....now I am thinking about code.

 

[BridgeSmith]

A drilled shaft without significant lateral loading would be an anomaly here, so I haven't looked too closely at the reinforcing requirements for that situation. I'll try to take a peek at it tomorrow.

 

[WARose]

Thanks. I assume from that you've seen my edited post a couple posts back. I kind of state my confusion with this.

But it's great to get some feedback from a practicing bridge guy.

 

[WARose]

I borrowed the 17th edition from a friend today.....here is what it says: (in Sect. 4.5.17.4) Cast-in-place piles, carrying axial loads only where the possibility of lateral forces being applied to the piles is insignificant, need not be reinforced where the soil provides adequate lateral support.

I'm not sure if the 17th ed. is the latest....but there it is.

I'm a little surprised that AASHTO would allow that for a couple of reasons. First off we are talking something (vital) that is getting vibrating loads from traffic (in spite of the fact IIRC impact loads are not applicable to foundations) and not to mention you'd think in certain seismic zones that wouldn't fly.

I'll keep looking and see if I can find something in the seismic section that supersedes that.

 

[BridgeSmith]

Yes, the 17th edition of the Standard Specifications is the latest, although that's not saying much. The last revision was in 2002. Any highway structures designed now using federal funds must be according to at least the 7th Edition of the LRFD, published in 2014.

The exemption on reinforcing only applies to shafts which do not have the potential for any significant lateral loading, so anything subjected to seismic loading would not qualify. The only configuration I can envision that would qualify would be where the shaft was attached to a cap or footing that was buried well below the surface, so that the soil around the footing would accommodate whatever shear was applied.

Perhaps it was being interpreted to allow for shafts to be left unreinforced where they shouldn't be, so that's why the more recent LRFD spec. requires that reinforcing extends 10 ft below fixity. Fixity occurs usually well below the point of lateral support.

 

[WARose]

The wording of this throws me for a loop. Thee 17th edition has that "10' below fixity" statement as well, but how can that coexist with the "where the possibility of lateral forces being applied to the piles is insignificant" statement? If you have the latter you don't have the former (by default).

 

[BridgeSmith]

I believe the second statement, where it states "...reinforcing steel shall extend 10 feet below the plane where soil provides adequate lateral restraint." defines where the soil is assumed to provide "adequate lateral support".

If you somehow have a shaft that is fully laterally supported, with only insignificant potential for lateral loading (which would seem to be a nearly impossible scenario) then it would not need to be reinforced at all. However, if there is a portion of the shaft that is "not supported laterally", it has the potential for lateral loading due to the assumed initial eccentricity described in Sect. 8.16.5.2.8, and the reinforcing must be carried to 10' below the point of "adequate lateral restraint".

Again, the newer LRFD spec versions have more restrictive and more prescriptive requirements, expounded on in greater and clearer detail in the commentary for Sect. 10.8.3.9.3. I encourage you to look at a 4th Edition or newer AASHTO LRFD Bridge Design spec.

 

[WARose]

Again, the newer LRFD spec versions have more restrictive and more prescriptive requirements, expounded on in greater and clearer detail in the commentary for Sect. 10.8.3.9.3. I encourage you to look at a 4th Edition or newer AASHTO LRFD Bridge Design spec.

I don't have those codes. But if there is anything in them that would prohibit someone from omitting re-bar in compression loaded (only) shafts.....let me know.

 

[BridgeSmith]

The way I read it, if the shaft is fully laterally supported, with 'insignificant lateral loading potential', then it wouldn't require reinforcement. If it's not fully laterally supported, there is automatically the potential for lateral loading, and reinforcement would be required to a depth of either 10' below lateral soil restraint (Standard spec.) or 10' below fixity (LRFD).

For reference, the text of the LRFD spec:

>>>Where the potential for lateral loading is insignificant, drilled shafts may be reinforced for axial loads only. Those portions of drilled shafts that are not supported laterally shall be designed as reinforced concrete columns in accordance with Article 5.6.4. Reinforcing steel shall extend a minimum of 10.0 ft below the plane where the soil provides fixity.

Where the potential for lateral loading is significant, the unsupported portion of the shaft shall be designed in accordance with Article 5.11.<<<

and the commentary I referred to:

>>>Shafts constructed using generally accepted procedures are not normally stressed to levels such that the allowable concrete stress is exceeded. Exceptions include:

Shafts with sockets in hard rock,
Shafts subjected to lateral loads,
Shafts subjected to uplift loads from expansive soils or direct application of uplift loads, and
Shafts with unreinforced bells.
...
A shaft can be considered laterally supported:
 below the zone of liquefaction or seismic loads,
 in rock, or
 5.0 ft below the ground surface or the lowest anticipated scour elevation.

Laterally supported does not mean fixed. Fixity would occur somewhat below this location and depends on the stiffness of the supporting soil.<<<

 

[WARose]

Thanks for the info HotRod. (I'd give you another star, but I think you can only give one per thread per poster. )

I wonder how they justify the statement "A shaft can be considered laterally supported....below the zone of liquefaction or seismic loads,...". Below the level of seismic loads would indicate to me a level without Rayleigh, shear, etc waves from a seismic event. And that kind of depth is hundreds of feet down at very common frequencies during seismic events. It's almost (by default) telling you to reinforce the whole thing.