Section 6: Prestressed Concrete U Beams (Types U40 and U54)
Anchor: #i1372088Materials
Use Class H concrete with a minimum ƒ'ci = 4.0 ksi and ƒ'c = 5.0 ksi.
Design beams for 0.5-in. low-relaxation strands. You may use 0.6-in. low-relaxation strands for unusual cases but should check its availability with fabricators.
Use prestressing strand with a specified tensile strength ƒpu, of 270 ksi.
You need not increase section properties of the beam to account for the transformed area of strands or mild steel.
Anchor: #i1372515Geometric Constraints
The maximum skew angle for U-beam bridges is 45 degrees.
Anchor: #i1372537Structural Analysis
Beam designs must meet the following requirements:
- Include the overlay at the discretion of the designer or if the bridge will receive the overlay immediately after construction. Recognize that including the overlay in the design of U beams can significantly limit their ability to span longer span lengths.
- 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.
- Composite section properties may be calculated assuming the beam and slab to 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 interior beams must conform to AASHTO LRFD Bridge Design Specifications, 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 AASHTO LRFD Bridge Design Specifications,
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 continuity.
- 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.
-
The live load used to design the exterior beam must never be less than the live load used to design an interior beam.
- 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 AASHTO LRFD Bridge Design Specifications, Article 3.6.1.1.1.
- For interior as well as exterior girders, do
not take the live load distribution factor, for moment or shear,
as less than
Where,
- m = multiple presence factor per AASHTO LRFD 3.6.1.1.2
-
= number of lanes -
= number of beams
or girders
Design Criteria
Standard beam designs must meet the following requirements:
- Stresses at the ends of the beam are controlled with the use of debonding. Draped strands are not permitted in U beams.
- Debonded strands must conform to AASHTO
LRFD Bridge Design Specifications, Article
5.11.43 except as noted below:
- Debond no more than 75 percent of the total number of strands.
- Debond no more than 75 percent of the number of strands in that row.
- The maximum debonding length must be the lesser of one-half the span length minus the maximum development length, 0.2 times the beam length, or 15 feet.
- Not more than 75 percent 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).
- 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.
- SeePrestressed Concrete I Beams and I Girders for other design criteria.
Detailing
Detail span sheets for a cast-in-place slab with precast concrete panels.