The WSDOT Bridge Design Manual, Section 3.9.4, modifies the calculation of live load distribution factors for the exterior girder of multi-girder bridges shall be as follows:

• For exterior girder design with deck slab cantilever length equal or less than 40 percent of the adjacent interior girder spacing, use the live load distribution factor for interior girder. The deck slab cantilever length is defined as the distance from the centerline of the exterior girder to the edge of the deck.
• For exterior girder design with deck slab cantilever length exceeding 40 percent of the adjacent interior girder spacing, use the lever rule with the multiple presence factor of 1.0 for single lane to determine the live load distribution. The live load used to design the exterior girder shall not be less than the live load used for the adjacent interior girder.
• The special analysis based on the conventional approximation of loads on piles as described in LRFD Article C4.6.2.2.2d shall not be used unless the effectiveness of diaphragms on the lateral distribution of truck load is investigated.