Distribute the weight of one railing to no more than three beams.

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

Composite section properties may be calculated assuming the beam and composite concrete slab overlay have the same modulus of elasticity (for beams with $f_c^{\text{'}}$
< 8.5 ksi). When determining section properties, do not include haunch concrete placed on top of the beam. 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.

Live load distribution factors must conform to Article 4.6.2.2.2 and Article 4.6.2.2.3. Use:

Cross section (f) with bridges having a composite concrete slab
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.

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.

Add and debond strands in the order shown on the Prestressed Concrete Box Beam Non-Standard Design (BBND) standard drawings.

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).
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.

Calculate required stirrup spacing for #4 Grade 60 bars according to Article 5.7. Change stirrup spacing as shown on relevant standard drawings only if analysis indicates the inadequacy of the standard design.

TxDOT standard box beams satisfy Article 5.7.4 and Article 5.9.4.4..

For box beams with a composite concrete slab overlay, compute deflections due to slab weight and composite dead loads assuming the beam and slab to have the same modulus of elasticity. Assume $E_c$
= 5,000 ksi for beams with $f_c^{\text{'}}$
< 8.5 ksi. Show predicted slab deflections on the plans even though field experience indicates actual deflections are generally less than predicted. Use the deflection due to slab weight only times 0.8 for calculating haunch depth.

Use shear keys for all box beam bridges. Do not consider composite action between beams and shear keys in computing live load distribution factors, nor for strength, stress, or deflection calculations.

Transverse post-tensioning is required for box beam bridges topped with an ACP overlay applied directly to the tops of beams. Space tendons at 10 ft. maximum with the first tendons set 10 ft. from bent centerlines. Post-tensioning details are provided on the Box Beam Construction Details with Overlay (BBCDO) standard drawing, available from the Bridge Division on request.

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