Richard Brice, Bijan Khaleghi, Washington State DOT, and Stephen J. Seguirant, Concrete Technology Corp, Tacoma, Washington
Introduction
The American Association of State Highway and Transportation Officials (AASHTO) Load Resistance Factor Design (LRFD) Bridge Design Specifications, hereinafter referred to as the AASHTO LRFD Specifications, prescribes the minimum design requirements for safe highway bridges. However, many bridge owners have adopted more stringent policies for the design of precast-prestressed girder bridges. These policies specify design requirements for section properties, allowable tensile stress, and continuity.
The most common design parameters influenced by more stringent requirements include span capability (maximum span length that can be achieved), girder spacing, and prestressing levels. The sensitivity of these design parameters to the owner-adopted design requirements were studied for precast-prestressed concrete bridge girders and this article describes the results of the study. Span capabilityand prestressing requirements were computed based on the minimum requirements set forth by the AASHTO LRFD Specifications for 6 ft and 12 ft girder spacing. Each of the more stringent policies was then evaluated individually to understand its effect on the design. The combined effect of all the design policies was also investigated.
Survey of Design Policies
A survey of state Departments of Transportation (DOTs) was conducted to gauge the extent to which bridge owners deviate from the requirements in the AASHTO LRFD Specifications. Bridge owners were asked their policies in three areas: section properties, allowable tension, and continuity. Of the 38 respondents, 76% of them design with gross section properties in lieu of transformed section properties. With respect to the question on allowable tension, 71% design to allowable tension permitted by the AASHTO LRFD Specifications. With respect to the question on continuity, 50% design precast-prestressed concrete girder bridges as simple spans for all loads.
Sensitivity Study
Slab-on-girder systems composed of a cast-in-place concrete deck on Washington State DOT precast prestressed concrete wide flange (WSDOT WF) I-girders were studied. Girder depths range from 36 to 100 inches. Interior girders were analyzed for various bridge configurations consisting of 6 WSDOT WF girders. The bridge deck, with haunch build-up of 3 inches, was assumed to be 7.5 and 9.5 inches thick for girder spacing of 6 and 12 feet, respectively.
The maximum strength of girder concrete was assumed to be 7.0 ksi at release of prestress and 9.0 ksi in service. The strands are 0.6 in. diameter jacked to 75% of the 270 ksi tensile strength (Grade 270 strands).
The baseline designs use the most liberal provisions allowed by the AASHTO LRFD Specifications; transformed section properties, allowable tension in accordance with AASHTO LRFD Specifications’ Table 5.9.4.2.2-1, and full continuity in accordance with AASHTO LRFD Specifications Section 5.14.1.4. Baseline designs were established by setting the girder spacing to 6 ft and 12 ft and then the span capability was computed for various levels of prestressing. Comparative designs were then carried out using each of the policies. Two design parameters were held constant and the value of the third that satisfies the LRFD design criteria was computed. For example, to determine the sensitivity of the section properties policy on girder spacing, the span length and prestressing where held constant while the girder spacing satisfying the design criteria was computed. These designs were compared to the baseline designs. The results are summarized in Table 1.
| Conservative Design Policy |
Reduction in span capability (%) |
Reduction in girder spacing from baseline girder spacing (%) |
Increase in required prestress (%) | |||
| 6 ft spacing | 12 ft spacing |
6 ft spacing | 12 ft spacing |
6 ft spacing | 12 ft spacing | |
| Gross section properties | 1.8-3.0 | 1.9-3.0 | 9.7-12.6 | 6.5-8.7 | 5.7-8.0 | 5.7-8.0 |
| Zero Allowable | 4.9-5.4 | 4.9-5.4 | 22.5-27.3 | 15.3-17.3 | 11.4-12.7 | 11.4-14.0 |
| Simple span analysis | 2.9-3.2 | 2.0-2.9 | 13.2-17.4 | 6.3-10.0 | 7.7-9.1 | 6.0-8.3 |
Table 1. Effects of more stringent design policies on span capability, girder spacing and required prestress
The results indicate that the design policy with the least impact on girder capability is the use of gross section properties. The allowable tension policy carries significantly more impact.
Combined Design Policies
Table 2 summarizes the results if all three conservative design policies are implemented relative to the baseline design. In the case of the baseline girder spacing of 6 ft spacing, the required girder spacing that results from the conservative design policies becomes narrower than the width of the girder top flange making the design unobtainable. In all cases, the increase in prestress requires concrete release strengths in excess of 7.0 ksi.
| Reduction in span capability (%) | Reduction in girder
spacing from baseline girder spacing (%) |
Increase in required prestress (%) | |||
| 6 ft spacing | 12 ft spacing | 6 ft spacing | 12 ft spacing | 6 ft Spacing | 12 ft spacing |
| 10.2 – 11.1 | 10.0 – 10.6 | 46.2 – 52.5 | 29.6 – 33.6 | 28.6 – 34.0 | 28.6 – 30.9 |
Table 2. Effects of combined stringent design policies on span capability, girder spacing and required prestress
Benefits of Conservative Design Policies
The AASHTO LRFD Specifications recommends a minimum service life of 75 years. Conservative design policies leave a margin of safety for unforeseen demands over the life of the structure. Supporting reasons for the conservative design policies include:
- Historical increase in live load: design live loads have increased over the past few decades from HS-15 in 1944 to HL-93 in 1994.
- Increasing use of overload trucks: Many permitted overload vehicles cross precast-prestressed girder bridges. The reserve capacity due to conservative design practices allows prestressed girder bridges to withstand these vehicles. Commerce would be adversely affected if these overloads could not be safely and conveniently moved.
- Increase in number of traveling lanes: Lane widths on some routes have been reduced from 12 feet to 10 feet to accommodate more traffic lanes. Reserve capacity allows these bridges to accommodate increased traffic demand without strengthening or other modifications.
- Reserve capacity for girders damaged by over-height collisions: Over-height load collisions with prestressed girder bridges often result in broken strands. Prior to repairs, the reserve capacity of the undamaged girders helps to keep the bridge in service.
- Uncracked concrete under service conditions: A zero tension policy ensures that prestressed girders remain uncracked for flexure under service load conditions, resulting in longer service life and reduced life cycle cost.
Conclusion
This study shows the sensitivity of span capability, girder spacing, and prestressing requirements to three common owner adopted design policies. These policies are more stringent than the minimum requirements set forth by the AASHTO LRFD Specifications. Span capability is the least sensitive and girder spacing is the most sensitive to the design policies. Designing based on gross section properties in lieu of transformed section properties has the least overall influence. Reducing the allowable tension stress has the greatest overall influence and has the greatest impact on girder spacing requirements.
References
1. American Association of State Highway and Transportation Officials (AASHTO). 2012. AASHTO LRFD Bridge Design Specifications. 6th Edition, Washington, DC: AASHTO.
2. American Institute of Steel Construction (AISC). 1986. Moments, Shears and Reactions for Continuous Highway Bridges. Second Printing, Chicago, IL: AISC
3. Brice, R., Seguirant. S. J., and Khaleghi, B. 2013. Evaluation of common design policies for precast, prestressed concrete I-girder bridges. PCI Journal, V 58, No. 4 (Fall 2013): pp. 68-94
4. Zuraski, P.D. (1991). “Continuous-Beam Analysis for Highway Bridges.” Journal of Structural Engineering. 117(1), 80-99
5. PCI Bridge Design Manual, 3rd Edition, First Release, Precast/Prestressed Concrete Institute, Chicago, IL, November 2011
6. Collins, M.P., and Mitchell, D. (1991). “Prestressed Concrete Structures.” Prentice-Hall, Inc, Englewood Cliffs, NJ.
7. Barker, R.M., and Puckett, J.A. (1997). “Design of Highway Bridges: Based on the AASHTO LRFD Bridge Design Specifications.” John Wiley and Sons, Inc, New York, NY.