Assignment – 1 Bridge EngineeringMackay Bridge was opened to public in 1970 connecting two cities (Halifax and Dartmouth) in Nova Scotia. As per Halifax Harbour Bridges (HHB) website, about 54000 vehicles crosses this bridge daily and it is the only means to go across the ocean for heavy vehicles. In future, if there is a need to replace this structure following options may be considered,Type of Bridge: Type of bridge generally depends upon site location, length and span of structure and funds available. In this case, where structure spans over Atlantic Ocean for more than 1km, Cable stayed, and Suspension bridges are most suitable. Rigid framed bridge can also be considered but because of weather conditions, it may take a long time to complete the project. Also, Nova Scotia is a windy city, storms and hurricanes are frequent.
The structure should be able to withstand the wind impact. Suspension bridges are best suited for this type of situation.Materials: Amongst bridge materials steel has the favourable strength qualities. High strength steel is most commonly used for long span bridges as it is lighter than concrete slabs and can be carved or cut to any desirable geometry.
Location: Mackay Bridge is part of NS Highway 111, changing the location of new bridge can be troublesome as then there will be needing to reroute and re-plan the traffic system. New construction can take place right next to the previous structure.Sequencing: New bridge needs to be constructed first before replacing as the daily average vehicle crossing Mackay 54000 and is the only option for heavy vehicles. Macdonald alone won’t be able to accommodate that much traffic and it is only limited to vehicles weighing up to 3200kg.Design Consideration: Mackay Bridge is 4 lane highway bridge with longest span of 426m, and clearance of 46.9m at central span. Depending on the traffic demand new bridge can be provided up to 6-8 lanes with span depending upon the length of structure. The shortest structure is not always the cheapest, as embankment and abutment cost can be reduced by increasing the length of structure.
The height and span of structure should be selected accordingly with Halifax Harbour, so there won’t be any hinderance to navigation of ships in ocean. The movement ofocean going vessels is of economic importance to the harbour region. The Halifax Port Authority reports that the port has an annual economic impact of $700 million, with employment impacts of 15,000 direct, indirect and induced jobs.
Thus, for a new structure, the clearance should be higher and wider than existing structure.Alternative: HHB can increase the life of Mackay Bridge by rehabilitating the existing structure as they have done similar thing in case of Macdonald Bridge with Big Lift project. This can be done, if the structure is in or can be maintained in good condition for the coming decades to meet the traffic demands of growing city.