BDM, Section 9.8.5, C 4 Final Revisions

March 18, 1999

Reinforcing and Concrete Strength

Due to soil conditions and varying construction methods, it should be assumed that the drilled shaft concrete will be placed in wet or dry conditions. A reduced concrete design strength of 0.85 f’c shall be used for all drilled shafts, wet or dry conditions of placement. For high seismic zones (EPA equal to or greater than 0.2g), a concrete strength of 1.5 f’c should be used to calculate Ec for the shaft concrete as an upper bound for stiffness analysis. Concrete Class 4000P is should be used for drilled shafts for the entire length of the shaft, wet or dry conditions of placement.

Drilled shaft reinforcing shall be detailed to minimize congestion, facilitate concrete placement, a d maximize consolidation of concrete. 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 (for example, a shaft-supported footing), the hooks should be turned toward the center of the shaft while, leaving enough opening to allow concrete placement with a tremie.

The amount of transverse reinforcing used in the drilled shaft meet all design force requirements, but need not meet AASHTO "volumetric" requirement s for concrete columns. The minimum drilled shaft transverse reinforcing used shall not be less than that required for the forces resulting from an elastic analysis nor that require to develop the plastic capacity (1.3Mn) of the column above, whichever is less. Drilled shafts for bridge foundation support shall be reinforced for their entire length.

The concrete cover for drilled shafts shall be in accordance with the following:

Shaft Diameter                                                                          Concrete Cover*

                                                    Uncased                                 Casing   Remains                  CasingWithdrawn

Less Than 4’- 0"                                3"                                                  3"                                          4"

Less Than 5’- 0"                                4"                                                  4"                                          4"

5’- 0" or Larger                                  6"                                                  6"                                          6"

*Note: See the current version of the "Shafts" special provision to assure that the latest concrete cover requirements are utilized.

Design for plastic shear induced by column or elastic shear (R=1), whichever is less utilizing the column shear and transverse reinforcement provisions in AASHTO Division 1-A Seismic Design Specification 7.6.2(C).

The column-shaft splice zone shall meet the requirements outlined in the TRAC Report titled "NONCONTACT LAP SPLICES IN BRIDGE COLUMN-SHAFT CONNECTIONS".

The minimum clear spacing between spirals should be no less than 2.5". However, 3.5" minimum clear spacing is preferable.

Design the drilled shaft 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.

The minimum reinforcing shall be 0.8% of the shaft’s gross section or 1.0% of the attached column’s gross section (if applicable), whichever is less.

The minimum clear spacing between bars shall be no less than 2.5". However, 3.5" minimum clear spacing is preferable. IF you are unable to attain this requirement, a larger diameter drilled shaft should be used.

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

(REFER TO THE ATTACHED STANDARD DETAILS)