Section 7: Pretensioned Concrete Spread Slab Beams
Materials
Use Class H concrete with a minimum
= 4.0 ksi and
= 5.0 ksi and a maximum
= 6.0 ksi and
= 8.5 ksi. Any exceptions to these limits must be approved in writing by the TxDOT Bridge Division.
= 4.0 ksi and
= 5.0 ksi and a maximum
= 6.0 ksi and
= 8.5 ksi. Any exceptions to these limits must be approved in writing by the TxDOT Bridge Division.
Use prestressing strand with a specified tensile strength,
of 270 ksi.
of 270 ksi.
Geometric Constraints
The maximum skew angle for spread slab beam bridges is 30° without modification to standard drawings.
Target slab overhangs at 0 ft. past beam edge. Minimal overhangs to accommodate roadway curvature are acceptable.
Structural Analysis
Beams designs must meet the following requirements:
- Distribute the weight of one railing to no more than three beams, applied to the composite cross section
- Use section properties given on the Prestressed Slab Beams standard drawings. For the composite section, use gross section properties.
- Composite section properties may be calculated assuming the beam and composite slab have the same modulus of elasticity (for beams with
< 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 for shear and moment are available from the Bridge Division.
- 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 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.
- When prestressed concrete deck panels or stay-in-place metal forms are allowed, design the beam using the basic slab thickness.
Design Criteria
Standard beam designs must meet the following requirements:
- Add and debond strands in the order shown on the PSBND 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 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).
- 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.
- Calculate required stirrup spacing for #4 Grade 60 bars according to the Article 5.7. Change stirrup spacing as shown on relevant standard drawings, only if analysis indicates the inadequacy of the standard design.
- TxDOT standard slab beams satisfy Article 5.7.4 and Article 5.9.4.4.
- Compute deflections due to slab weight and composite dead loads assuming the beam and slab to have the same modulus of elasticity. Assume
= 5,000 ksi for beams with
< 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. - See Section 4, Pretensioned Concrete I-Girders for other design criteria.
Detailing
Modify standard drawings for slab beams by extending composite steel (Bars H) above the top of the beams to reflect use of 8.5 in. thick decks. The standard drawings base their composite steel extension on use of 5 in. thick decks required for adjacently framed slab beams.