Rational for design choice - New Tram Bridge

The final new Tram Bridge design has been chosen based on the following technical, design and environmental criteria:  

• The bridge is a long way from the nearest road and a key requirement is being able to deliver it to site and get equipment close enough to construct it.  
   
• A bridge with a steel truss frame (a type of open web girder frame), has been chosen as it is strong and light and it can be assembled onsite, then lifted into place with a crane. 
   
• A number of variations of the design were put forward for the shape of the steel truss, as well as different options for the number of piers or supports (the legs of the bridge that extend into the water to the foundations).  
   
• The length of the spans (the distance between each of the piers or supports) over the river are restricted due to the weight that the crane can lift at any one time. This affects the positions of piers needed in the river. 
   
• The existing bridge has nine spans and eight piers, six of which are in the river. The proposed design has four spans and only two piers in the river with a further pier on land. Fewer piers in the river reduces flood risk and the effect of debris impact. It also minimises construction time and provides a more efficient and elegant design.  
   
• The bridge carries a bridleway and will be designed to carry pedestrians, cyclists and horse riders. The minimum height of a bridleway has to be 3.75m high for the safety of the horses and their riders.
   
• The design of the bridge support sustainability factors as well as well as minimising any environmental impacts of the new structure. The bridge will be of a high quality that meets local needs and takes account of the setting. 
   
• The new Tram Road Bridge design references a number of trusses along the River Ribble. The reverse arch with two peaks provides the much necessary strength, and articulation whilst creating architectural interest. 
   
• A linear deck is essential to counter future flood level predictions and provides maximum clearance, supporting local ecology.  
   
• A Lighting design will also respect the ecology and will provide a safe and attractive pedestrian bridge link from the South Ribble to Preston City. 
   
• The painting and final colour of the bridge is yet to be decided but will complement its natural surroundings. Consideration has also been given to the ongoing maintenance costs associated with painting the bridge.   

Design options that were considered and why they were rejected

• A bow string profile was considered in the initial bid and a further straight profile in the central span but these were structurally inefficient for a bridleway bridge. 
   
• An arch design was also considered but the truss frame of the bridge would be below the water line during a major flood event and so therefore it was rejected.  
   
• The suspension bridge profile was deemed to be the most practical and attractive option. The proposal includes a splayed truss to give a more open feel for people using the bridge, enhancing the aesthetic interest of the structure and allows for more clear space for users.