LL Distribution Factor for CIP Box Girders

Memorandum

September 6, 2005

TO: All Design Section Staff

FROM: Jugesh Kapur

SUBJECT: LL Distribution Factor for CIP Box Girders

The current WSDOT BDM section 3.1.2-C for live load distribution factor for multi-cell Cast-In-Place (CIP) concrete box girders shall no longer be used. The load distribution factor for multicell CIP concrete box girders shall be per AASHTO LRFD Specifications for interior girders from Table 4.6.2.2.2b-1 for bending moment, and Table 4.6.2.2.3a-1 for shear. The live load distribution factor for interior girders shall then be multiplied by the number of webs to obtain the design live load for the entire superstructure. The correction factor for live load distribution for skewed support as specified in Tables 4.6.2.2.2e-1 for bending moment and 4.6.2.2.3c-1 for shear shall be considered.

DF = N_b x Df_i Live load distribution factor for multicell box girder

Where:

Df_i = Live load distribution factor for interior web

N_b = Number of webs

Background

In the current WSDOT BDM section 3.1.2-C, the live load distribution factor for multicell CIP concrete box girders, the number of design lanes, including fractional lanes, shall be taken as the curb-to-curb roadway width in feet divided by 14. The recent Caltrans research projects on the live load distribution in multicell box girders at the University of California, at Davis indicates that “the current AASHTO LRFD live load distribution factor formulae for multicell box girders provide a conservative estimate for bending moment and shear, even for bridges outside the range of applicability”. The attached parametric study shows that the current live load distribution factor per BDM/Standard Specs could be significantly over or under estimated depending on the span length and girder spacing.

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