Pile, Pile Caps and Compression laps and anchorage
Pile, Pile Caps and Compression laps and anchorage
(OP)
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,
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,






RE: Pile, Pile Caps and Compression laps and anchorage
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.
RE: Pile, Pile Caps and Compression laps and anchorage
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?
RE: Pile, Pile Caps and Compression laps and anchorage
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.
RE: Pile, Pile Caps and Compression laps and anchorage
RE: Pile, Pile Caps and Compression laps and anchorage
RE: Pile, Pile Caps and Compression laps and anchorage
>>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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RE: Pile, Pile Caps and Compression laps and anchorage
M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)
RE: Pile, Pile Caps and Compression laps and anchorage
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.
RE: Pile, Pile Caps and Compression laps and anchorage
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.
RE: Pile, Pile Caps and Compression laps and anchorage
RE: Pile, Pile Caps and Compression laps and anchorage
RE: Pile, Pile Caps and Compression laps and anchorage
>> 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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