Omission of Web Reinforcement in Double Tees

Code References

ACI 318

Background

Between 1971 and 2004, a number of load tests were conducted on full-size un-topped and topped double tees by at least eight companies or plants. The objective of these tests was to show that web reinforcement can be eliminated without impairing the flexural and shear capacities of prestressed concrete double tees. This object was met, and three of the tests resulted in recommendations adopted by the precast industry (1986, 1989, and 2004).

Recommendations

From the results of the latest test in 2004, it was shown that the shear reinforcement in double tee legs can be omitted, provided the following conditions are applied:

  1. Double tees are simply supported

  2. Design should be Class U or T for bottom tensile stresses

  3. Design should include appropriate end bearing plates and reinforcement

  4. Loads are substantially uniform

  5. For shear, the designer needs to consider the stress reduction at the ends of the double tee due to partial strand debonding

  6. To prevent the effects of loss of bond near the ends of double tees without shear reinforcement, strand bond should be verified or certified to conform to the development length as predicted by ACI 318

  7. Avci should not exceed 0.08 in2/ft

Due to one of the test specimens in the 2004 testing exhibiting reduced ductility and significant shear cracks, it is recommended that whenever the steel ratio for 12DT30 double tees is such that Pi/A is less than approximately 550 psi, the φ-factor for strength reduction in flexure be lowered to 0.85 for spans close to 62 feet and carrying heavy live loads such as 80 psf.  For shorter spans, lighter live loads, or both, the 550 psi limit should be reduced based on engineering judgment.  From the PCI Standard Design Practice Manual for ACI 318, a reduction in the φ-factor for flexure may be warranted when Pi/A levels are low.

References

Aswad, A., et al., 1986, "Elimination of Shear Reinforcement, Test Report No. 2," Stanley Structures, Inc., Denver, Colorado.

Aswad, A., G. Burnley, 1989, “Omission of Web Reinforcement in Prestressed Double Tees,” PCI Journal, V. 34, No. 2 (March-April).

Aswad, A., G. Burnley, N. Cleland, D. Orndorff, and C. Wynings, 2004, “Load Testing of Prestressed Concrete Double-Tees without Web Reinforcement,” PCI Journal, V. 49, No. 2 (March-April).

 

 

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