Effective Length Factor for Piers

[StructuralR2]

Hi. I am wondering what effective length factor to use in the longitudinal and transverse direction for the piles of a pile bent pier on a bridge with integral abutments. The pier has 1 row of 11 piles. Does the fact that there are integral abutments affect what effective length factor I should use in each direction? Would the load combination affect what effective length factor I should use in each direction? i.e. Strength I vs Strength III vs Extreme Event I, etc.

Any documentation or references that you have would be greatly appreciated.

Thanks for the help

 

[BridgeSmith]

In the direction perpendicular to the centerline of the abutment (and the row of piles), where the cap is not considered restrained from translation or rotation (AISC LRFD 2nd Ed, Table C-C2.1, case "e"), we would use a k value of 2.1. You could limit the secondary moment based on the eccentricity limit imposed by the the bearings (calculate the P-delta moment with a limiting value for delta).

For bending parallel to the centerline of the cap, where the piles are restrained from rotation, but still free to translate, we would use a k value of 1.2 (AISC, case "c").

Of course, if the pile bent is braced by horizontal and diagonal bracing, which they typically are, you could be justified in using a k value of .65 (case "a") parallel to the bent. Although, the AASHTO Bridge Design spec. seems to indicate a value of .75 for that case is appropriate.

Where opinions differ amongst designers is in the determination of the unbraced length. For the end of the unbraced length at the bottom where the piles are driven into soil, some take it as the ground line, some take it as 5' below the groundline, and still others take it as the point of fixity from a P-y analysis.

Rod Smith, P.E., The artist formerly known as HotRod10