September 6, 2005
FROM: Jugesh Kapur
SUBJECT: LL Distribution Factor for CIP Box Girders
TO: All Design Section Staff
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
184.108.40.206.2b-1 for bending moment, and Table 220.127.116.11.3a-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 18.104.22.168.2e-1 for bending moment and 22.214.171.124.3c-1
for shear shall be considered.
DF = Nb x Dfi Live load
distribution factor for multicell box girder
= Live load distribution factor for
Number of webs
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
If you have any questions
regarding these issues, please contact Bijan Khaleghi at 705-7181.