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TO: All Design Section Staff
FROM: J. Kapur
DATE: May 14, 2001
SUBJECT: Design Policy Memo – C-I-P Load Distribution

The AASHTO-LRFD live load distribution factor as specified in tables,,, and shall not be used when designing cast-in-place multicell box girders.  Instead, the load distribution for both moment and shear shall be taken as specified in the WSDOT BDM article 5.3.1D-1 to D-3.  The number of design lanes, including fractional lanes, shall be taken as the curb-to-curb roadway width in feet divided by 14, and the multiple presence factor shall not be applied to the live load distribution factor. The correction factor for skewed bridges as specified in AASHTO LRFD table shall be applied to the shear distribution factor. 


The live load distribution factors as specified in AASHTO LRFD Specifications tables,,, and are for web as individual girder design.  This requirement results in unnecessary reinforcement congestion in the exterior cells.  It is both practical and economical to design the cast-in-place multicell box girders as a unit rather than individual girders.  The unit design method permits uniform reinforcement distribution throughout the width of superstructure.

 If you have any questions regarding this design issue, please contact Bijan Khaleghi at 360-705-7181.





            cc:        J. A. Weigel, Bridge and Structures - 47340
                        E. H. Henley, Bridge and Structures - 47340
                        H. L. Coffman, Bridge Preservation - 47341
                        F. Higgins, Brid  ge and Structures - 47340
                        R. A. Lewis, Construction  - 47354
                        Z. Y. Fahoum, Bridge and Structures - 47340