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.
- Add strands in the order shown on the Prestressed Concrete U-Beam (Design Data) (UBND) standard drawing.
- 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). If the strand size is larger than 0.6” diameter, base section increments on 5.9.4.3.3.
- 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.
- 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.
- See Section 4, Pretensioned I-Girders for other design criteria.