ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/296997
10 | ASPIRE , Winter 2013 P E R S P E C T I V E The AASHTO LRFD Bridge Design Specifications (AASHTO LRFD) provides b r i d g e e n g i n e e r s w i t h m i n i m u m design requirements for safe highway bridges. However, many bridge owners have adopted more stringent policies for the design of precast, prestressed concrete girder bridges. These policies specify design requirements for section properties, allowable tensile stress, and continuity. Bridges designed using more stringent policies will be more robust and more costly when compared to bridges d e s i g n e d o n l y t o t h e m i n i m u m re q u i re m e n t s . T h e m o s t c o m m o n differences include a reduction in span length, a reduction in girder spacing, or an increase in prestressing levels. This article attempts to quantify the sensitivity of common policies on the design of precast, prestressed concrete bridge girders. Span capability, girder spacing, and prestressing requirements are computed based on the minimum requirements set forth in the AASHTO LRFD. Each of the more stringent policies is then evaluated individually to understand its effect on the design. The combined effect of all the design policies is 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 minimum requirements set forth in the AASHTO LRFD. Bridge owners were asked the following questions: • What type of section properties policy does your state use for the design of precast, prestressed concrete girder bridges (gross, transformed)? • What allowable tensile stress at service III policy does your state have for the design of precast, prestressed concrete girder bridges (AASHTO LRFD Table 188.8.131.52.2-1, zero tension at service limit state, other)? • W h a t c o n t i n u i t y p o l i c y d o e s your state have for the design of continuous precast, prestressed concrete girder bridges (AASHTO LRFD Section 184.108.40.206 "Bridges Composed of Simple Span Precast Girders Made Continuous," simple span moments for superimposed dead loads and live load, other)? The responses of the 38 state DOTs that completed the survey are summarized in Figs. 1 through 3. Sensitivity Study The bridge sections used in this study are slab-on-girder systems composed of a cast-in-place concrete deck on precast concrete wide flange (WSDOT WF) I-girders. Interior girders are analyzed for various bridge configurations consisting of six WF girders. The bridge deck, with haunch build-up of 3 in., is assumed to be 7.5 and 9.5 in. thick for girder spacings of 6 and 12 ft, respectively. The maximum strength of girder concrete is assumed to be 7 ksi at transfer of prestress and 9 ksi at service limit state. The 0.6-in.-diameter strands are jacked to 75% of the tensile strength. The baseline designs use the most liberal provisions allowed by AASHTO LRFD; transformed section properties of strand and cast-in-place deck concrete, allowable tension in accordance with AASHTO LRFD Table 220.127.116.11.2-1, and full continuity for superimposed dead and live loads in accordance with AASHTO LRFD Section 18.104.22.168. Each of these policies is varied to investigate their sensitivity individually. The results are summarized in Table 1. The ranges indicated reflect girder depths from 36 to 100 in. The results indicate that the design policy with the least impact on girder by Richard Brice and Bijan Khaleghi, Washington State Department of Transportation, and Stephen J. Seguirant, Concrete Technology Corp. Evaluation of Common Design Policies for Precast, Prestressed Concrete I-Girder Bridges More-stringent owner requirements affect span capability, girder spacing, prestressing requirements Table 1–Effects of more stringent design policies on span capability, girder spacing, and required prestressing force Conservative design policy Reduction in span capability (%) Reduction in girder spacing (%) Increase in re- quired prestressing force (%) 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 tension 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 2–Effects of combined stringent design policies on span capability, girder spacing, and required prestressing force Reduction in span capability (%) Reduction in girder spacing (%) Increase in required prestressing force (%) 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 46.2–52.2 29.6–33.6 28.6–34.0 28.6–30.9 AspireBook_Win13.indb 10 12/28/12 11:39 AM