Unit
- Beam bridges: Simple spans where the deck acts as the primary load-carrying member, supported at each end by abutments or piers. They are economical for short spans up to about 80 feet and can be made of steel, concrete, or timber.
- Arch bridges: Use a curved structure that transfers horizontal thrust into the abutments or foundation, allowing longer spans than beam bridges. The arch can be placed above or below the deck.
- Suspension bridges: Employ cables draped between towers to support the deck via vertical suspenders. Ideal for very long spans exceeding 2,000 feet, as the main cables carry the load in tension.
- Cable-stayed bridges: Similar to suspension but with cables directly connecting the towers to the deck at intervals, providing stiffness and stability for medium to long spans.
- Truss bridges: Composed of interconnected triangles that distribute loads efficiently. Common in railway bridges and for spans up to 300 feet.
- Superstructure components: Include the deck, girders, floor beams, and bracing. The deck carries traffic loads; girders transfer these loads to the bearings.
- Substructure components: Consist of abutments (at ends), piers (intermediate supports), and foundations (spread footings or piles). They transfer loads from the superstructure to the ground.
- Bearings: Devices that allow controlled movement between superstructure and substructure due to thermal expansion, creep, and seismic effects. Common types include elastomeric pads and pot bearings.
- Live load: Includes the weight of vehicles, pedestrians, and other transient loads. For highway bridges, standard design trucks such as the HL-93 are used per AASHTO specifications. Live load effects include impact factors, lane loading distributions, and multiple presence factors to account for simultaneous traffic. The design lane load combines a uniformly distributed load with a single concentrated load to envelope maximum shear and moment.
- Dead load: Consists of the permanent weight of the bridge structure itself, including the deck, girders, wearing surface, utilities, barriers, and any fixed equipment. Accurate estimation is critical because dead load often governs the design of shorter spans and influences the required prestress or reinforcement. Additional dead loads from future overlays or utilities must be accounted for in the design.
- Design codes: Bridge design in the United States typically follows the AASHTO LRFD Bridge Design Specifications, which provide load factors, resistance factors, and design procedures for steel, concrete, timber, and composite structures. Internationally, codes such as Eurocode and Canada's CSA S6 serve similar roles. These codes ensure consistent safety, durability, and serviceability while allowing for regional materials and practices.
System Architecture
Pattern: Bridge Load Path
Components:
- Superstructure: Deck, girders, cross-frames, lateral bracing. Transfers vertical loads to bearings.
- Bearings: Allow rotation and translation; transfer vertical and horizontal forces to substructure.
- Substructure: Abutments, piers, foundations. Resists gravity, lateral, and uplift loads.
- Foundation: Spread footings or piles. Distributes loads to competent soil or rock.
Code Example
Python
def span_to_depth_ratio(span_ft, beam_type):
ratios = {'steel_plate_girder': 25,
'prestressed_concrete': 22,
'reinforced_concrete': 18}
if beam_type in ratios:
depth_ft = span_ft / ratios[beam_type]
return round(depth_ft, 1)
else:
return NoneThis function estimates initial girder depth based on span length and typical depth-to-span ratios for common beam types. The ratios are approximate and should be refined using detailed design.
Quick Revision
- Bridge types include beam, arch, suspension, cable-stayed, and truss, each suited to different span ranges.
- Components are divided into superstructure (deck, girders) and substructure (abutments, piers, foundations).
- Bearings accommodate movements and transfer forces between superstructure and substructure.
- Preliminary sizing can be done using span-to-depth ratios as a starting point.