Section 7: Prestressed Concrete Slab Beams
Anchor: #i1353807Materials
Use Class H concrete with a minimum ƒ'ci of 4.0 ksi and ƒ'c of 5.0 ksi.
Design beams for 0.5-in. low-relaxation strands.
Use prestressing strand with a specified tensile strength, ƒpu ,of 270 ksi.
Anchor: #i1353988Geometric Constraints
The maximum skew angle for slab beam bridges is 30 degrees.
The minimum gap between adjacent slab beams is 0.5-in. A preferable gap range is 1 in. to 1.5 in.
A 5-in. minimum thickness composite concrete slab is required.
Anchor: #i1354013Structural Analysis
Beam 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 concrete slab beam standard drawings.
- Composite section properties may be calculated assuming the beam and slab overlay have the same modulus of elasticity (for beams with ƒ'c < 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.
- 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.
Design Criteria
Standard beam designs must meet the following requirements:
- Add strands in the order shown on the PSBNS standard drawing, available at http://www.dot.state.tx.us/insdtdot/orgchart/cmd/ cserve/standard/bridge-e.htm.
- Debond strands in 3-ft. increments at beam ends if necessary to control stresses at release.
- Debond no more than 75% of the strands per row or per section. The maximum debonding length must be the lesser of one-half the span length minus the maximum development length as specified in Article 5.11.4 of the AASHTO LRFD Bridge Design Specifications, 0.2 times the span length, or 15 ft.
- Calculate required stirrup spacing for #4 Grade 60 bars according to the AASHTO LRFD Bridge Design Specifications, Article 5.8. Change stirrup spacing as shown on relevant standard drawings available at http://www.dot.state.tx.us/insdtdot/ orgchart/cmd/cserve/standard/bridge-e.htm only if analysis indicates inadequacy of the standard design.
- TxDOT standard slab beams satisfy Article 5.8.4 and Article 5.10.10 of the AASHTO LRFD Bridge Design Specifications.
- Compute deflections due to slab weight and composite dead loads assuming the beam and slab to have the same modulus of elasticity. Assume Ec= 5,000,000 psi for beams with ƒ'c < 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 5, Prestressed Concrete I Beams and I Girders for other design criteria.