Engineering and Construction

The downdrag load on a pile group or pier group is more complicated. Two possible downdrag loading conditions for the group must be calculated, as follows:

1. Large spacing of piles. If the piles are spaced far apart, then it is likely that the downdrag will act on each pile separately. Equations 6.21 and 6.22 can be used to determine the downdrag load for each pile. The downdrag of each pile in the group would then be summed up in order to calculate the total downdrag for a given pile group.

2. Small spacing of piles. When the pile spacing is small, the downdrag load may act effectively on the entire perimeter of the group, in which case the following equations are utilized:

For square pile groups in cohesionless soil subjected to downdrag:

For square pile groups in cohesive soil subjected to downdrag:

As a conservative approach, the downdrag load should be calculated for both cases outlined above and the maximum value used for design.

Example Problem 6.9 Consider the same situation as the previous example, except that there is a 2 by 2 pier group. The piers are spaced 3 pier diameters (centerline to centerline). Determine the downdrag load on the pier group.

Solution Since there are four piers in the 2 by 2 group and assuming each pier is separately subjected to a downdrag load, or:
The second downdrag condition is to use Eq. 6.24, or:
Hence the governing condition would be assuming the downdrag over the entire perimeter of the pier group, or QD = 960,000 lb (4300 kN).