PHASE 3 - Bridge Design

Continental Crossings considered three alternatives during the preliminary design process. The design team analyzed the potential use of a timber truss bridge, a cable suspension bridge, as well as a cable suspended bridge. The design team considered a number of variables specific to the site when deciding upon the most economical and constructible bridge for their design.

Following the decision to utilize steel cable, a comparative analysis was conducted between a suspension and suspended cable bridge. Quantities of cement were the first consideration. One of the main differences between the abutment designs is the size of the piers. The top of pier elevation necessary for a suspension bridge was approximately four meters. Alternatively, since the critical members in a suspension bridge run through the base of the pier rather than atop the pier, the suspended cable bridge required a pier elevation of only 1.5 meters from the top of the abutment. Therefore, it was determined that construction of a suspension bridge would be easier then construction of a suspended bridge and the suspended cable design alternative required a lower quantity of cement. Furthermore, based on preliminary calculations, a suspension bridge would require less steel cable than a suspended bridge for the relatively short span of 15 meters.

The suspended cable bridge was our design team’s final selection for the Peruvian bridge project. Based off the preliminary calculations, the design team modified some of the typical specifications to lessen the dead-load weight of the bridge and to minimize material quantities to better suit the isolated location of the bridge site. The team designed the main load bearing cables to be only the two running across the base of the decking, allowing the top handrail cable to bear the weight of the suspenders and mesh wiring. The mesh wire was chosen as an alternative to the typical chained link fence in order to minimize the load and because the thin wire installation will require less time. The design team also verified the optimum tower height to be 1.5 meters. This relatively short tower height created a greater tension in the cable due to the low sag ratio of 5%, but in the case of this relatively light loading of 50 psf live load and 10 psf dead load, this design alternative was very feasible. We also were able to create a more favorable connection with the abutments because the non load bearing cable railings were secured just past the towers, as can be seen in the drawing S3. The complete plan set can be seen below.