Design Memorandum



TO:                  All Design Section Staff

FROM:            J. Kapur
DATE:             July 22, 2004
SUBJECT:       Shaft Design and Detailing Recommendations
                        Pile Detailing

May 14, 2004

This design memorandum summarizes the latest WSDOT-ADSC recommendations on design, detailing, construction of drilled shafts in wet or dry conditions, and cast-in-place concrete pile detailing. This Design Memorandum supersedes Design Memorandum dated March 19,1999.

Transverse Reinforcement:

Shaft transverse reinforcement shall be designed for the lesser of plastic shear or elastic seismic shear of the column above. The volumetric ratio and spacing requirements for transverse reinforcement of the AASHTO LRFD article 5.13.4.6.3 for confinement need not be met. Shaft transverse reinforcement may be constructed as hoops or spirals. The clear spacing between spirals shall not be less than 6" minimum or more than 9 maximum. Spiral reinforcement is preferred for shaft transverse reinforcement, however if #6 spirals at 6 clear do not satisfy the shear design, then hoops may be used. Full welded splices as shown in the attached detail shall be used for spiral or hoop splices.

Longitudinal Reinforcement:

Longitudinal reinforcement shall be provided for the full length of shaft. Longitudinal reinforcement on top of the shaft shall be the larger of 0.75%Ag of the shaft or 1.0%Ag of the attached column. The minimum longitudinal reinforcement beyond the top of the shaft shall be 0.75%Ag of the shaft per AASHTO LRFD article 5.13.4.6.3d. The clear spacing between longitudinal reinforcement shall not be less than 6" minimum or more than 9 maximum. If unable to meet this requirement, a larger diameter drilled shaft shall be considered.

Shaft Concrete:

Concrete Class 4000P shall be specified for the entire length of the shaft, wet or dry conditions of placement. A reduced concrete strength of 0.85 fc for structural design, and increased strength of 1.5 fc as an upper bound for stiffness analysis for Ec calculation shall be used for all drilled shafts.

Typical shaft reinforcement and casing details are shown on the attached drawing. The details may vary, depending on the soil conditions and specific design and construction methods and equipment that apply.

Pile Detailing:

To be consistent with information given for shafts, pile bar lists shall be included in the pile sheets. This list will include the bars mark, size, number, length, bend type and total weight. These quantities shall be calculated based on estimated pile tip elevations. A note stating that the quantities are based on estimated pile tip elevations shall be added to the pile bar list.

Background:

Drilled shaft reinforcing shall be detailed to minimize congestion, facilitate concrete placement, and maximize consolidation of concrete. Drilled shafts for bridge foundation shall be reinforced for their entire length.

The top of the shaft in typical WSDOT column-to-shaft connection, due to larger shaft diameter, remains in elastic conditions under seismic loads. Therefore, the AASHTO LRFD requirements for confinement of plastic hinging zones become irrelevant. The minimum drilled shaft transverse reinforcing shall not be less than that required for the forces resulting from an elastic analysis nor that required to develop the plastic capacity (1.3Mn=Mp) of the column above, whichever is less. The column-shaft splice zone shall meet the requirements outlined in the TRAC Report titled "NONCONTACT LAP SPLICES IN BRIDGE COLUMN-SHAFT CONNECTIONS".

Longitudinal reinforcing in drilled shaft should be straight with no hooks. To facilitate concrete placement and removal of casing, hooks in vertical shaft reinforcing should be avoided. If hooks are necessary to develop moment at the top of a drilled shaft, the hooks should be turned toward the center of the shaft while leaving enough opening to allow concrete placement with a tremie. Shaft longitudinal reinforcing is designed for the plastic moment (1.3Mn) induced by the column or the elastic seismic moment (R=1), whichever is less. This applies to all seismic zones in Washington State.

If you have any questions regarding these issues, please contact Patrick Clarke at 705-7220 or Bijan Khaleghi at 705-7181.

JK:dsb

BK
Attachment

cc:   J. A. Weigel, Bridge and Structures - 47340

       F. Higgins, Bridge and Structures 47340