TxDOT deviates from the AASHTO LRFD Specifications for several girder types. The paragraphs below contain excerpts from the TxDOT LRFD Bridge Specifications, Revised April, 2007:

Section 3.5: Precast Concrete I Beams

Live load distribution factors must conform to AASHTO LRFD Bridge Design Specifications, Article 4.6.2.2.2 for flexural moment and Article 4.6.2.2.3 for shear, except as noted below:

• For exterior beam design with a deck slab cantilever length equal to or less than one-half of the adjacent interior beam spacing, use the live load distribution factor for the interior beam. The deck slab cantilever length is defined as the distance from the center line of the exterior beam to the edge of the deck.
• For exterior beam design with a deck slab cantilever length exceeding one-half of the adjacent interior beam 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 beam must never be less than the live load used to design an interior beam.
• Do not use the special analysis based on conventional approximation for loads on piles per AASHTO LRFD Bridge Design Specifications, Article C4.6.2.2.2d, unless the effectiveness of diaphragms on the lateral distribution of truck loads is investigated.

Section 3.6: Prestressed Concrete U Beams (Types U40 and U54)

Live load distribution factors must conform to AASHTO LRFD Bridge Design Specifications, Article 4.6.2.2.2 for flexural moment and Article 4.6.2.2.3 for shear, except for exterior beam design. For exterior beam design, use a distribution factor for two or more design lanes loaded only. Do not use the distribution factor for one design lane loaded unless the clear roadway width is less than 20.0 ft. Use 1.0 for the multiple presence factor for one lane loaded. For exterior beams, multiply the result of the lever rule by 0.9 to account for continuity. The live load used to design the exterior beam must never be less than the live load used to design an interior beam.

For bridges with less than three girders in the cross section, assume the live load distribution factors for flexural moment and shear are equal to the number of lanes divided by the number of girders. Determine the number of lanes as required by AASHTO LRFD Bridge Design Specifications, Article 3.6.1.1.1.

Section 3.7: Prestressed Concrete Slab Beams

Live load distribution factors for all beams, both moment and shear, must conform to AASHTO LRFD Bridge Design Specifications. Table 4.6.2.2.2b-1, using cross section (g), if the beams are connected only enough to prevent relative vertical displacement at their interfaces. This is called S/D distribution.

Do not apply the skew correction factors for moment as suggested in Article 4.6.2.2.2e nor for shear as suggested in Article 4.6.2.2.3c.

Section 3.8: Prestressed Concrete Double-Tee Beams

Regardless of topping, live load distribution factors for all beams–both moment and shear–must conform to AASHTO LRFD Bridge Design Specifications, Table 4.6.2.2.2b-1, using cross section (i) if beams are connected only enough to prevent relative vertical displacement at their interfaces. Use K = 2.2 when determining the live load distribution factor. Use S/10 as maximum limit on live load distribution.

Section 3.9: Prestressed Concrete Box Beams (Types B20, B28, B34, and B40)

Live load distribution factors must conform to AASHTO LRFD Bridge Design Specifications, Article 4.6.2.2.2 and Article 4.6.2.2.3. Use:

• Cross section (f) with bridges having a composite concrete deck
• Cross section (g) with bridges having ACP applied directly to tops of beams, assuming beams are sufficiently connected to act as a unit.
• Do not apply the skew correction factor for moment as suggested in Article 4.6.2.2.2e.