ABSTRACT
In 2003, the Precast/Prestressed Concrete Institute (PCI) Bridge Design Manual provided preliminary design charts for 28-day concrete compressive strengths of f c ′ = 48 https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/eq154.tif"/> , and 83 MPa with 13 mm and 15 mm diameter strands, respectively, based on the American Association of State Highway and Transportation Officials (AASHTO) Standard Specifications (2002). In 2011, PCI revised the charts for f f c ′ = 55 https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/eq155.tif"/> MPa and 15 mm diameter strands based on the AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications (2010). The LRFD charts are useful but they limit the designer to a single girder concrete strength and strand diameter. Innovation in bridge design requires that different parameters be considered including, concrete girder and deck strengths; girder section and spacing; strand size and prestress losses; and allowable concrete stress limits for tension and compression. Therefore, a simplified design method that is capable of evaluating various combinations of these parameters for simple and continuous spans will provide preliminary design alternatives that allow the designer to achieve a feasible and economical bridge design.
Bending moment equations due to dead and live loads were computed for simple and two-span continuous girders. Using these equations and the design criteria given in Márquez et al. (2014), preliminary design charts for simple and two-span continuous bridges using normal strength concrete (NSC), high performance concrete (HPC), and ultra-high performance concrete (UHPC) with 13 mm, 15 mm, and 18 mm strand diameters, respectively, were developed considering serviceability (concrete tension), strength limit, and release limit states.
Figure 1 shows the impact of continuity based on concrete compressive strength of 137.9 MPa and 18 mm diameter strand for simple and two-span continuous layouts. In the chart, the transition points (where strength ceases to govern and service becomes the controlling limit state) are marked by dark and white circles for simple and two-span continuous layouts, respectively, and show that the maximum span lengths are governed mainly by the service limit state. The longest span length reached 70.2 m for a girder spacing of 1.8 m. Preliminary design chart using <italic>f'<sub>c</sub> </italic> = 137.9 MPa with a strand diameter of 18 mm for simple and two-span continuous layouts. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig263_1.tif"/>
The use of the combination of UHPC with two-span continuous layouts and a larger strand diameter results in more slender girders than current practice which may lead to transportation and erection challenges that may be assessed based on the span lengths given in the new LRFD charts. However, there are UHPC design alternatives other than increasing the span length that can be considered to reduce material cost and avoid transportation limitations by truck such as increasing the girder spacing and/or reducing the girder depth.
