Pile, Pile Caps and Compression laps and anchorage

[sybie99]

I am trying to get my head around the requirement for compression bars in piles, in other words, what length of bar should stick out the top of a pile?

The concept of tension lap I fully understand, but not that of a compression lap. Say for example the continuity of rebar in a column is broken, and you do not lap the bars but place them end to end, will the concrete not transfer the compression stresses?

Say you place an external force onto the end of a column, in such a case the force is transferred into column by the concrete. Also where for example you have precast columns with corbels picking up beams, with no physical connection between the beam and column, the force is transferred by contact between elements, and is transferred into column steel, without any laps.

So why are compression laps needed for piles, usually the extend into the pilecap, unless there is a moment tranfer, or shear transfer, why is it needed?

Regards,

 

[DST148]

@sybie99: Your 1st para - Where does this requirement come from? What type of pile is it? Typically concrete piles not enclosed by a structural steel pipe or tube are reinforced when subject to uplift or where the required moment strength exceeds the design cracking moment strength of the pile.
2nd para - Ref ACI 318-05: The code does not permit use of structural plain concrete for columns but allows for vertical components like walls and pedestals. The code also allows end bearing splices in columns with certain restrictions - End bearing splices used to splice column bars always in compression should have a tension capacity of 25 percent of fy of the steel area on each face of the column, either by staggering the end-bearing splices or by adding additional steel through the splice location.
3rd para - I agree, here the load is transferred through bearing.

 

[sybie99]

I am referring to Concrete Continuous Flight Auger Piles. A reinforced cage is dropped into the wet concrete. Then pile is cut to correct level and the bars in the pile are left to stick out and into the pile cap. Precast driven piles are also reinforced. In either case, if there is no shear, or moment transfer to the pile, is it necessary for the rebar to stick out into the pilecap?

The second part of my question, which relates to the above, is for a single pile supporting a column, what is the minimum depth one can make the pile cap? And what influences this depth?

 

[JacksPanic]

You can design it not to have moment transfer, but given the tolerances to which those piles are installed, there will be some moment transferred. This should not exceed the moment capacity of the pile and hence you need the pile rebar sufficie tly anchored into pile cap.
Single column can theoretically start right on top of pile, but it is more practical to have a nominally sized pile cap to ensure you have a flat surface when placing column shutters. Also to account for eccentricity of piles being installed out of position, but within tolerance.

 

[sybie99]

Thanks jackspanic, what about compression lap, am I right in thinking no compression lap is needed? To put it differently, what is the minimum length the pile bars should stick out into pilecap?

 

[JacksPanic]

just like column rebar needs to anchored in footing/ pile cap, so does pile rebar need to be anchored in pile cap. not sure i understand your question, but why would you not want to extend the pile rebar into the pile cap?

 

[amec2004]

My reply to your questions as the followings

>>In either case, if there is no shear, or moment transfer to the pile, is it necessary for the rebar to stick out into the pilecap?

For pile design there two part you need to check: pile strength and pile head serviceability.

For strength part you have to check 1) Geotech vertical downward and uplift capacity 2) Pile shaft section vertical compression & tension capacity 3) Pile shaft section flexural and combined axial tension/compression with flexural capacity

For serviceability part you have to check 1) pile vertical settlement due to axial compression 2) pile head lateral displacement

If your pile has tensile uplift load case, your pile head rebar definitely need to extend into pilecap. More than that, you have to check the pile head rebar anchorage length and ensure you have enough anchorage length as rebar development length ld (straight bar case ACI 318 12.2.1) or ldh (90 degree hook case ACI 318 12.5.1) to mobile the full tensile stress equal to rebar Fy. You are allowed to have a shorter development length and more number of rebar so that the rebar will only mobile partial tensile stress less than yield stress Fy as per ACI 318 12.2.5. In any case the total resistance Tr = phi xnxAsxFy shall be larger than the factored tensile load on pile.

For pile has no tensile load case, you still need to extend you pile head (normally 4” ) and pile rebar into the pilecap to achieve some sort of pile head fixty. In the pile lateral capacity check we have pile head condition as PIN head and FIXED head and there exists big capacity difference between PIN and FIXED head condition. For this case your pile head and rebar anchorage is more to restrain the pile head rotation and achieve a fixed head condition.


>>The second part of my question, which relates to the above, is for a single pile supporting a column,
>> what is the minimum depth one can make the pile cap? And what influences this depth?
For single pilecap you still need to provide the head and rebar anchorage as normally 3” construction tolerance is allowed for pile construction, so there would be a pile head moment caused by this eccentricity. We need the pile head and rebar anchorage to restrain pile head rotation due to the moment.

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[IsaacStructural]

Just a thought here, if the pile cape doesn't extend below the frost line, it could potentially lift upward with frost heave if it wasn't tied into the pile reinforcing. It wouldn't likely come completely displaced from the pile below, but the upward movement could damage the structure above.

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[sybie99]

Thanks Amec 2004, that all makes sense.

With regards to the last part of your answer, can I go about it as follows:

1. Calculate moment (pile tolerance, say 75mm multiplied by axial force from column above = moment to pile.
2. From this moment I can calculate the tension in the reinforcement, and from this tensile force I can determine what is the anchorage length of the reinforcement required.

Another related question. In a case where piles support the superstruture, but the ground slab is a surface bed (i. e. not lateral restraint to the pilecap or pile head) one must brace the pile caps (or pile heads. If there are 3 piles this is not an issue, if two piles they must be braced in one direction. Now if you have a single pile supporting columns, am I right in saying this pile cap or pile head must be braced in 2 directions using ground beams to other single piles? I would assume so. In South Africa single piles are not braced/tied at the top in cases where the ground slab is not connected to the pilecap. This is risky in my opinion, but it seems engineers do this to save money for the client. I have never heard of any issues but it seems like bad/dangerous practice. In reality, the surrounding soil must provide considerable restraint to the pile, hence I guess there have been no issues thus far.

 

[JacksPanic]

risky? yes.
but the cap and pile should be sized so that it will have the capacity to deal with the eccentricity if it is installed within the contractor's allowable tolerance.
if a single or double pile group is installed out of tolerance, the need for a ground beam or additional piles arises.

 

[JacksPanic]

these additional piles/ground beams is obviously for the contractor's account.

 

[sybie99]

To come back to compression laps, where for example column starter bars sit in a base, if the bars are stressed to capacity, is the full code specified anchorage required? The reason I ask is because often a column base has say 25mm starters yet is only 500mm deep, so the straight part of the bar penetrating into the base can only be about 450mm allowing for cover. This is less than the 1m anchorage specified for a y25 bar in the BS. Surely the hook does not count in compression. This is just a general discussion on compression anchorage of column starters into foundations

 

[amec2004]

My reply as the followings

>> Calculate moment (pile tolerance, say 75mm multiplied by axial force from column above = moment to pile.

The more reasonable approach is only to count permanent dead load plus partial live load, and definitely not full live load

>> From this moment I can calculate the tension in the reinforcement, and from this tensile force I can determine what is the anchorage length of the reinforcement required.

Yes if you can accurately work out the tension + compression couple from the moment.
One more moment to take into account is that when your lateral load acting at column base move to the pile head, there is another moment = lateral load x distance from base plate to pile head as well

>>Now if you have a single pile supporting columns, am I right in saying this pile cap or pile >>head must be braced in 2 directions using ground beams to other single piles?


It depends on what is your design assumption on this single pilecap.
If it’s PIN head assumption, that means you already take the pile head rotation into account and allow more lateral deflection, just leave it without extra restraint

If it’s FIXED head assumption you have to provide restraints to achive your assumption.

>>In reality, the surrounding soil must provide considerable restraint to the pile, hence I guess >>there have been no issues thus far.

You shall not count the soil as soil is way much weaker to play a role in resistance. We never count soil passive resistance in pilecap lateral resistance as the required lateral displacement to mobilize the soil passive resistance is out of allowable serviceability requirement. The structure will collapse before that soil passive resistance mobilized.

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