huangyhg

precast concerete pile cap and embedded plate
our client has asked us to look at an alternative precast pile cap design consisting of welded pipe pile to an embedded plate. they are looking at alternatives to speed up construction on a rush job. looking for some reference material or advise on how to deal with this connect detail. i would assume this connection (weld and plate embedded shear studs) would have to be designed to ensure moment capacity has been met, and the pipe pile to cap weld would require testing to ensure that this is achieved. can this type of connection be considered a true moment connection, similar to a pile embedded in a cast in place cap.
i would question how much faster it will really be. yes, the caps' concrete will be full-strength when they arrive on site, so concrete strength is not a factor. but:
- how long will the correct welding take?
- will you be precasting caps with a hole in them to fit over the previously driven piles, or will you be driving the piles through the caps?
- how will you provide cover over the embed and the weld to prevent corrosion?
- if you are not driving the piles thru the caps, what do you do if a pile isn't located quite right and does not align with the holes in the caps?
make all pile caps the same size so the contractor can gang-form them (or keep to a minimum number of sizes to ensure re-use). the forms could be set, rebar installed (pre-assemble the rebar), concrete placed, and maybe even forms removed in one day. then reused the following day. be sure to require a membrane curing compound for the caps.

ralph
structures consulting
northeast usa
my mistake for not clarifying, the pile cap beam (1 x 1 x 9 m) is installed 5m above ground. the construction sequence is to drive 5 x 610 mm piles with around a 2m spacing. install cages and concrete to just below pile cut off elevation. install precast cap beam with 5 x embedded plates on the driven piles. weld top of pile to the plate with full penetration weld. driven piles will need to precise to ensure they line up with embedded plates.
i'm not sure i have a clear picture in my head.
why "install cages and concrete to just below? pile cut off elevation" if you are installing cap beams?
sounds like too many steps, but a picture would help.
ralph
structures consulting
northeast usa
i've found a drawing for a similar type of concept, the piles shown are rock sockets, while at our site the piles are down 20m in cohesive soils. the design is for a mid span pier.
ah .. now i see. we're discussing a bridge pier not a building foundation. the nomenclature for bridge work does not always mean the same as for building construction.
i would expect the weld design to be very critical. executing the welding in the field would be difficult, as i expect it will be an overhead condition (to plates embedded in the bottom of your precast pier cap beams). i can't advise too much on the design aspects of your condition, but from a constructability perspective i see it as a real challenge to ensure a good weld and to provide for durability of the finished product. any coating on the pipe pile would have to be removed to ensure a good weld, resulting in some kind of field-applied coating to prevent corrosion. if durability takes a back seat to fast construction, then i suspect it could be accomplished.
but given the critical nature of the welding, will you really save time?
given enough time and money, anything is possible.

ralph
structures consulting
northeast usa
to be honest we have never really dealt with this type of connection for a cap/pile connection, usually embedded piles in cast in place caps. the attached detail was from a tender drawing i picked up on another job. i'll check on quality control requirements for the project. again our client had proposed it because he had completed a similar type of design just recently.
thanks for the input
how is the pile cap kept stable while the welder is underneath it. this method would not be acceptable with h&s in the uk.
also, the tops of piles will not line up evenly no matter how hard they try. it is possible to grind them all down accurately but this takes time....
how are the pile tops to be prepared for the butt welds?

i am told that the cap remains attached to the crane while the worker tacks it in place. again trying to find someone who has dealt with this type of system.

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