Distribute 2/3 of the rail dead load to the exterior beam and 1/3 of the rail dead load to the adjacent interior beam applied to the composite cross section.

Each U beam has two interior diaphragms at a maximum average thickness of 13 in. They are located as close as 10 ft. from midspan of the beam. Account for each diaphragm as a 2-kip load for U40 beams and as a 3 kip load for U54 beams applied to the non-composite cross section.

Use section properties given on the standard drawings. For the composite section, use gross section properties.

Calculate composite section properties assuming the beam and slab to have the same modulus of elasticity (for beams with $f_c^{\text{'}}$
< 8.5 ksi). Do not include haunch concrete placed on top of the beam when determining section properties. Section properties based on final beam and slab modulus of elasticity may also be used; however, this design assumption must be noted on the plans. Use the deflection due to slab weight only times 0.8 and camber times 0.75 for calculating haunch depth.

Live load distribution factors for interior beams must conform to Article 4.6.2.2.2 for flexural moment and Article 4.6.2.2.3 for shear.

Live load distribution factors for exterior beams must conform to Article 4.6.2.2.2 for flexural moment and Article 4.6.2.2.3 for shear, with the following exceptions:

When using the lever rule, multiply the result of the lever rule by 0.9 to account for better live load distribution arising from the beneficial torsional stiffness of the box girder system.
When the clear roadway width is greater than or equal to 20.0 ft., use a distribution factor for two or more design lanes loaded only. Do not design for one lane loaded.
When the clear roadway width is less than 20.0 ft., design for one lane loaded with a multiple presence factor of 1.0.

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 Article 3.6.1.1.1.

Do not take the live load distribution factor for moment or shear as less than the number of lanes divided by the number of girders, including the multiple presence factor per Article 3.6.1.1.2.

Stresses at the ends of the beam are controlled with the use of debonding. Draped strands are not permitted in U beams.

Add strands in the order shown on the Prestressed Concrete U-Beam (Design Data) (UBND) standard drawing.

Debond strands in 3-ft. increments at beam ends if necessary to control stresses at release. If the strand size is larger than 0.6” diameter, base section increments on Article 5.9.4.3.3.

Debonded strands must conform to Article 5.9.4.3.3 except as noted below:

Debond no more than 50% of the total number of strands.
Debond no more than 50% of the number of strands in that row.
Replace Restriction B with, not more than 50% of the debonded strands, or 10 strands, whichever is greater, shall have the debonding terminated at any section, where section is defined as an increment (e.g., 3 feet, 6 feet, 9 feet). If the strand size is larger than 0.6” diameter, base section increments on 5.9.4.3.3.
Up to 75 percent of debonded strands may be used for the total number, the number of strands per row, and number terminated in a section as long as principal stress at or near the transfer length is designed for per Article 5.9.2.3.3, regardless of the concrete strength.
Do not design for Restriction E.
Replace Restriction G with, the maximum debonding length is the lesser of: (a) one-half the span length minus the maximum development length; (b) 0.2 times the beam length; or (c) 15 ft.
For multi-web sections having bottom flanges, replace Restriction J with:
Uniformly distribute debonded strands.
Bond the outer-most strand in each row.

Grouping of U-beam designs are at the discretion of the designer. However, no exterior U-beam may have less carrying capacity than that of an interior U-beam of equal length. If the designer chooses to group beams, a general rule is to group beams with no more than a four-strand difference.

See Section 4, Pretensioned I-Girders for other design criteria.