Design Memorandum



TO:                  All Design Section Staff

FROM:            Bijan Khaleghi
DATE:             July 18, 2008
SUBJECT:       Column-Shaft connection Design and Detailing Recommendation


This design memorandum is an amendment to BDM Articles 7.4.4 and 7.8.2 addressing design and detailing issues related to single column/single shaft configurations.

The Shaft transverse reinforcement within the noncontact lap splice zone shall not be less than #6 @ 6 pitch.

The requirements of the AASHTO Guide Specifications for LRFD Seismic Bridge Design Article 8.8.12 on

Lateral Confinement for Oversized Shafts for SDC C and D applies. The volumetric ratio of lateral reinforcement
in an oversized shaft shall be 50% of the confinement at the base of the column provided the shaft is designed
for a flexural expected nominal capacity equal to 1.25 times the moment demand generated by the overstrength
moment of the embedded column. The lateral confinement shall extend along the shaft until the embedded
column cage is terminated. The spacing of the oversized shaft confinement may be doubled beyond the column
cage termination length, but not exceeding 9 pitch.

The development length of column longitudinal reinforcement in drilled shafts shall be per BDM Article 7.4.4 per TRAC Report WA-RD 417.1 titled "Noncontact Lap Splices In Bridge Column-Shaft Connections". The requirements of the AASHTO Guide Specifications for LRFD Seismic Bridge Design Article 8.8.10 for development length of column bars extended into oversized pile shafts for SDC C and D may not be used.

The requirements of the AASHTO Guide Specifications for LRFD Seismic Bridge Design Article 8.8.13 shall be used for Lateral Confinement for Non-Oversized Strengthened Shafts for SDC C and D.


Background:

The requirement for noncontact lap splice of the TRAC Report titled "Noncontact Lap Splices in Bridge Column-Shaft Connections" is based on the assumption that all column longitudinal bars are in direct tension. This may not be true for column to shaft connection at plastic hinging state that only portion of column bars within the tensile region of the neutral axis are in tension. This ratio is within 40% to 60% and could be safely approximated to 50%. It should be mentioned that the strength contribution of steel casing is completely ignored per WSDOT practice.


If you have any questions, please contact Bijan Khaleghi at 705-7181.

cc:   Mohammad Sheikhizadeh, Bridge Construction - 47354

       F. Posner, Bridge and Structures 47340