Comparative analysis o n simply supported pre- stressed box beams in Sri Lankan highway bridges

Abstract

The National Road network of Sri Lanka consists of 4326 bridges. There are 365 bridges which Have the length more than 30m .Only 800 bridges are made of prestressed concrete superstructures and all others are with reinforced concrete, steel and arches. The most popular types of prestressed beams used in Sri Lanka are inverted T, M, I, and the box beams. The inverted T and M beams are widely used. Further, for 30m span simply supported bridges, space rectangular box beams and spaced trapezoidal box beams were used. For the continuous bridges big spine beams also have been used with post tension pre-stressing system in recent bridge constructions. For longer span bridges, box beams are highly suitable. Generally box beam has higher torsional capacity because of its closed geometry. The enhanced torsional stiffness of the box beam sections improves the load distribution properties for the superstructure. It has higher bending carrying capacity and requires reduced beam height compared to other beam section for a particular span. Hollow spaces in box beams can be used for services and it is also aesthetic. In Sri Lanka 19% of the existing bridges are with prestressed concrete and presently many highway projects are under construction. Therefore, the usage of box beams will improve the effect on the time of construction, cost, construction easiness, aesthetic considerations and utility services. There are different types of box beams available that can be used for this simply supported span range. They are standard box beam, standard U beam and spaced box beams. The rectangular spaced box beam has been used for a two lane bridge in a 30m simply supported span and the trapezoidal spaced box beam has been used for a four lane elevated flyover in Sri Lanka. Comparative analysis and design on all these box beams are useful for future bridge constructions. This research is concentrated on the design of 30m simply supported four lane bridge super structures using the above different types of prestressed box beams separately. The results of analysis and design and the properties of the beams are compared. The total width of the designed bridges is 17.4m. It has a central reserve of 1.2m. There are four lanes; each lane is 3.5m width. There are two pedestrian walk ways of 1.1m width. All the bridge decks were modeled in SAP 2000 for the grillage analysis. Loading was done according to BS 5400: Part 2, 1978, and bending moments, shear forces and torsional moments were found for critical load combination. Prestressing designs were carried out for all beams and the final results are compared. Cost for each deck also compared. The different launching methods adapted for these Bridges are also compared. Conclusions and recommendations are laid down based on these compared results.