Section 8: Prestressed Concrete Double-Tee Beams
Anchor: #i1354220Materials
Use Class H concrete with a minimum ƒ'ci of 4.0 ksi and ƒ'c of 5.0 ksi.
Use non-shrink cementitious grout for shear keys.
Design beams for 0.5-in. low-relaxation strands.
Use prestressing strand with a specified tensile strength, ƒpu ,of 270 ksi.
Anchor: #i1356642Geometric Constraints
Double-tee standard drawings do not accommodate skewed bridges.
A 5-in. minimum thickness composite concrete slab or 2-in. minimum thickness asphaltic concrete pavement (ACP) overlay is required.
Six-ft. wide beams are required for fascia positions when beams are not topped with a 5-in. slab.
Anchor: #i1354241Structural Analysis
Beam designs must meet the following requirements:
- Distribute the weight of one railing to no more than three beams.
- Use section properties provided on the prestressed concrete double-tee beam standard drawings.
- Composite section properties may be calculated assuming the beam and composite concrete slab overlay have the same modulus of elasticity (for beams with ƒ'c < 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.
- Regardless of topping, live load distribution factors for all beams, both interior as well as exterior, and 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.
Design Criteria
Standard beam designs must meet the following requirements:
- Strands should be added and depressed in the order shown on the DTBND standard drawings, available at http://www.dot.state.tx.us/insdtdot/orgchart/cmd/cserve/s tandard/bridge-e.htm.
- Use hold-down points shown on the DTBS and DTBO standard drawings, available at http://www.dot.state.tx.us/insdtdot/orgchart/cmd/cserve/standard/bridge-e.htm.
- The end position of depressed strands should be as low as possible so that the position of the strands does not control the release strength. Release strength is occasionally controlled by end conditions when the depressed strands have been raised to their highest possible position.
- Calculate required stirrup spacing for #3 Grade 60 bars according to the AASHTO LRFD Bridge Design Specifications, Article 5.8. Change stirrup spacing as shown on the DTBS and DTBO standard drawings, available at http://www.dot.state.tx.us /insdtdot/orgchart/cmd/cserve/standard/bridge-e.htm, only if analysis indicates the inadequacy of the standard design.
- TxDOT standard double-tee beams satisfy Article 5.8.4 of the AASHTO LRFD Bridge Design Specifications.
- For double-tee 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 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 that actual deflections are generally less than predicted. Use the deflection due to slab weight only times 0.8 for calculating haunch depth.
- TxDOT standard double-tee beams satisfy Article 5.10.10 of the AASHTO LRFD Bridge Design Specifications.
- Connect adjacent beams with lateral connectors, shown on standard drawings DTBS and DTBO, spaced at 5 ft. maximum, with the first lateral connectors set 5 ft. from bent centerlines. See span standard drawings for completion of lateral connection details.
- See Section 5, Prestressed Concrete I Beams and I Girders, for other design criteria.