Bridge Types and Structural Systems
Bridge engineering encompasses the design, analysis, construction, and maintenance of structures that carry roadways, railways, or pedestrians over obstacles. The choice of bridge type depends on span length, site conditions, construction constraints, and aesthetic requirements. Beam bridges are the simplest type, consisting of horizontal beams supported at each end by piers or abutments. Girder bridges use steel or concrete girders to achieve longer spans.
Arch bridges transfer loads through compression in the arch rib to the abutments at each end. The arch shape is efficient for spans up to 1,000 feet and is often selected for its aesthetic appeal. Suspension bridges use main cables hung from towers to support the deck through vertical suspender cables. They are the longest-spanning bridge type, with the Akashi Kaikyo Bridge in Japan having a main span of 6,532 feet.
Loads and Design Criteria
Bridges must be designed to resist multiple types of loads simultaneously. Dead loads include the weight of the structure itself, while live loads represent the weight of traffic using the bridge. The AASHTO LRFD Bridge Design Specifications define design live loads including the HL-93 design truck and lane load. Environmental loads include wind, earthquake, temperature, ice, and water forces. pavement thickness design. roundabout intersection design. bridge load testing.
Live load distribution factors determine how loads are shared among multiple girders. Impact factors increase static loads to account for dynamic effects of moving vehicles. Fatigue design considers the cumulative effect of millions of load cycles over the bridge life. Serviceability criteria ensure acceptable deflection, vibration, and crack control under normal use.
Bridge Construction Methods
Construction methods vary significantly based on bridge type, span length, and site conditions. Cast-in-place concrete construction uses formwork supported on falsework for shorter spans. Precast concrete girders are manufactured off-site and lifted into position, reducing construction time and traffic disruption. Segmental construction builds the superstructure in short segments using form travelers or launching gantries.
Steel bridge erection methods include crane lifting, launching, and incremental launching for longer spans. Cantilever construction builds the bridge outward from each pier without temporary supports, suitable for deep valleys and water crossings. Accelerated bridge construction techniques prefabricate major components off-site to minimize on-site construction time and traffic impacts.