One of the concepts students in geotechnical engineering courses seem to have the most trouble with is estimating stresses in concrete piles during pick-up and setting them in place to drive. The basic problem is that it’s sometimes hard to get our heads around the analytical simplification of the actual situation. Let’s start by looking at the operation itself. These first photos come from a job in Delaware in 1998, using a Vulcan 530 to drive cylinder piles.
Depending upon the configuration of the pile, it’s also possible to have two- and three-point pickup, as we can see from these photos, taken at the construction of a terminal in Portsmouth, VA, in 2005-6. The contractor is Weeks Marine, the same contractor that got Sully’s plane out of the Hudson after his famous “landing” in the river.
So how to we solve problems like this? Basically we assume that the pile is a horizontal beam, simply supported at the pickup points (or in the case of one-point pickup, at the pickup point and at the furthest end from the pick-up point) with the weight of the pile as the only load. One thing that can be done is to raise the distributed load of the weight by a factor for inertial effects during handling. An example of this is a 60′ long 12″ square concrete pile with a 50% increase for inertial effects with single point pick-up. We used the CFRAME program from the U.S. Army Corps of Engineers to analyse the beam, although most any beam software (or in some cases tables or hand calculation) can be used for this computation.
In this case we are displaying the output of CFRAME which shows each section of the beam/pile (i.e., one one side of the pick-up point and the other.)
According to the Prestressed Concrete Institute’s Recommended Practice for
Design, Manufacture and Installation of Prestressed Concrete Piling (1993), the maximum permissible stress (tension) for transient loads such as handling loads is as follows
(US Units, psi for both variables)
For SI units, this works out to
(SI Units, MPa for both variables)
Some specifications allow the prestress of the pile to be added to , with the same units as the other variables. Obviously with precast concrete piles (rare in the US but used elsewhere) the prestress does not apply.
Other piles sizes and lengths can be computed using the methods described above.