Overview

Bridges are typically classified using a combination of four key characteristics: the span arrangement, construction material, the position of the deck relative to the structure, and the overall structural form. Understanding these principles helps clients, designers and stakeholders identify suitable, buildable and cost-effective bridge solutions at an early project stage.

Bridges are commonly described by how their spans are arranged between supports.

A simple span bridge is supported at each end and behaves independently between bearings. This arrangement is straightforward to design and construct and is widely used for shorter spans and modular construction.

A continuous span bridge extends over more than two supports, allowing loads to be distributed across multiple spans. Continuous arrangements can offer improved structural efficiency and reduced mid‑span deflections but require greater consideration of support movements and construction sequencing.

Both simple and continuous span arrangements are used across beam, girder and truss bridge forms, depending on site constraints, loading, and durability requirements.

Steel Beam Bridges

Beam & Girder Bridges

Beam bridges are the simplest and most widely used form of bridge construction. They consist of horizontal steel members supporting a deck, bearing onto abutments and, where required, intermediate piers.

BEAM BRIDGE

They are commonly used for short to medium spans and form the basis of many highway, access, rail and industrial bridges. Depending on span arrangement and site constraints, beam bridges may be configured as single spans or as continuous spans over multiple supports.

From a delivery and construction perspective, beam bridges are often attractive due to their relative simplicity, repeatability and suitability for off‑site fabrication and efficient site installation.

Design & Buildability Notes (Informative)

  • Beam bridges are typically economical for shorter spans, with practical applications extending into medium‑span ranges depending on loading, deck type and overall arrangement.
  • Structural depth is generally proportionate to span, with shallow construction possible where constraints demand, subject to associated trade‑offs.
  • Continuous arrangements can reduce the number of bearings and expansion joints compared with multiple simple spans, benefiting long‑term maintenance.
  • For multi‑girder configurations, pairing or bracing girders can improve stability during transport and erection, supporting safer and more predictable installation.
  • Girder spacing is influenced by deck form, transport constraints and erection methodology, and is typically rationalised to suit fabrication and site handling.
Plate Girder Bridges

Plate girder bridges use fabricated steel girders, formed from welded webs and flanges, to achieve greater spans and load‑carrying capacity than is practical with rolled sections. They are widely used for highway and rail applications, particularly where structural depth, span length or loading requirements exceed the limits of standard beams.

PLATE GIRDER BRIDGE

The fabricated nature of plate girders allows the cross‑section to be tailored to project needs, while still supporting efficient off‑site manufacture and predictable site installation. Arrangements can be developed to suit transport, erection and maintenance constraints, making plate girder bridges a common choice for complex or constrained sites.

From a delivery perspective, plate girder bridges are well suited to staged construction, modular fabrication and a wide range of erection methodologies.

Design & Buildability Notes (Informative)

  • Plate girders are often preferred where rolled sections become inefficient, with applications extending into longer spans depending on overall configuration and constraints.
  • Stability during transport and erection is a key consideration, particularly before decks or other permanent restraints are in place. Temporary or permanent restraint may be required to manage compression flanges during installation.
  • Girder proportions and detailing are typically rationalised to balance structural efficiency with fabrication complexity, transportation limits and erection methodology.
  • Excessive stiffening can increase fabrication time and cost; practical solutions balance web thickness, stiffener arrangement and buildability.
  • Plate girder bridges lend themselves to phased erection, making them suitable for rail possessions, live highways and restricted access environments.
Box Girder Bridges

Box girder bridges use hollow, box‑shaped steel sections, formed as either closed or open‑topped boxes, to provide high torsional stiffness and efficient load distribution. They are well suited to situations where curved alignments, skewed geometry, shallow construction depth or visual appearance drive the overall form of the bridge.

BOX GIRDER BRIDGE

The inherent torsional rigidity of box girders makes them particularly effective for complex geometries and longer spans, while their clean soffit can be advantageous where the structure is visually prominent or located in sensitive environments.

From a fabrication and construction perspective, box girder bridges allow tailored solutions that integrate structural performance with transport, erection and long‑term maintenance considerations.

Design & Buildability Notes (Informative)

  • Open‑topped box girders can simplify fabrication and inspection and avoid confined‑space working, provided adequate permanent bracing is incorporated.
  • Closed box girders provide high structural efficiency but require careful planning for access, inspection and health & safety during fabrication and erection.
  • For longer spans or shallow depth constraints, twin box arrangements with cross‑girders and cantilevers are commonly adopted to balance stiffness, buildability and transport limits.
  • Box girders are well suited to curved or skewed layouts, where torsional effects would otherwise dictate more complex framing solutions.
  • Erection methodology and temporary conditions are often key drivers in determining box proportions, segmentation and sequencing.

Deck Construction: Fabricated and Orthotropic Decks

In steel bridge construction, the deck may be formed as part of the steel superstructure rather than as a separate concrete slab.
A fabricated steel deck is constructed from steel plate and stiffening members and is typically prefabricated off‑site. Fabricated decks are commonly used where construction depth, weight or installation constraints influence the overall bridge arrangement.

An orthotropic steel deck is a specific form of fabricated deck in which the deck plate is stiffened in both the longitudinal and transverse directions. This allows the deck to act efficiently as a load‑carrying structural element and is often used in plate girder and box girder bridges where reduced weight and structural efficiency are required.

The choice of deck system depends on span, loading, durability requirements, maintenance considerations and construction methodology.

Steel Truss Bridges

Truss Bridge (General)

Truss bridges use a triangulated framework of interconnected steel members working in tension and compression to distribute loads efficiently. They are typically selected for longer spans or where overall construction depth must be minimised, making them well suited to constrained sites and crossings.

Truss bridges may be configured in several forms, including through and half‑through arrangements, depending on clearance requirements and deck position. They are commonly found in rail and pedestrian applications, where possession constraints and lightweight superstructures are advantageous, and are used more selectively on highway schemes.

From a construction perspective, truss bridges offer efficient load paths and can be well suited to modular fabrication and staged erection.

Design & Buildability Notes (Informative)

  • Through and half‑through trusses are often adopted where construction depth is limited or where services, highways or railways must pass through the structure.
  • Truss bridges typically involve a higher number of nodes and connections, which influences fabrication effort and erection planning.
  • Connection detailing and fatigue performance are important considerations, particularly for rail applications with high repetition of loading.
  • Erection is frequently possession‑driven on rail schemes, making advance planning, modularisation and efficient sequencing critical to successful delivery.
  • Truss bridges can be erected in sections or as near‑complete units, depending on site access, lifting capacity and programme constraints.
Deck, Pony and Through Truss Bridges

Truss bridges are further described by the position of the deck in relation to the superstructure. In a deck truss, traffic runs on top of the truss. In a pony truss, traffic passes between trusses that are not connected overhead. In a through truss, traffic passes through a fully braced truss structure above and below the deck.

DECK TRUSS BRIDGE
PONY TRUSS BRIDGE
THROUGH TRUSS BRIDGE
Pratt Truss

The Pratt truss is one of the most common modern truss forms and is identified by diagonal members sloping towards the center of the span. It is highly efficient and widely used for road, rail and pedestrian bridges.

PRATT TRUSS BRIDGE
Warren Truss

A Warren truss consists of a series of equilateral or near-isosceles triangles. The repetitive geometry provides efficient load distribution and is commonly adopted for road and footbridge structures.

WARREN TRUSS BRIDGE

Bowstring & Arch Bridges

Arch Bridges

Arch bridges carry loads primarily through compression along a curved arch, with forces transferred to the supports. In steel bridge construction, arches are often selected where visual impact, span efficiency or site sensitivity are key considerations, such as pedestrian crossings, waterways and public‑realm projects.

ARCH BRIDGE
Bowstring (Tied-Arch) Bridges

A bowstring (tied‑arch) bridge is a specific form of arch bridge in which the deck (or a dedicated tie) restrains the horizontal thrust of the arch. This allows arch construction to be used without transferring significant horizontal forces into the foundations, making the form well suited to sites where ground conditions or existing structures limit foundation capacity.

BOWSTRING BRIDGE

Steel arch bridges can be configured with the deck above, below or suspended from the arch, depending on clearance requirements and overall arrangement.

From a delivery perspective, arch and bowstring bridges combine strong visual character with efficient structural behaviour, while requiring careful coordination between fabrication, transport and erection activities.

Design & Buildability Notes (Informative)

  • Arch and bowstring bridges are often selected for waterway crossings, public‑facing locations or sensitive settings, where aesthetics are a key driver.
  • Bowstring (tied‑arch) arrangements are particularly useful where foundation conditions make unrestrained horizontal thrust impractical.
  • Fabrication typically involves large, curved steel elements that influence transportation planning, lifting strategy and site sequencing.
  • Erection methodology is a critical consideration and may involve temporary works, staged assembly or controlled load‑transfer as the structural form is completed.
  • Early coordination of interfaces between arch ribs, ties, hangers and deck elements supports predictable installation and risk reduction on site.

Cantilever Bridges

Cantilever bridges are formed from structural members that project outwards from piers, supporting the deck without the need for full‑length falsework. The structure is typically built in balanced stages from the supports, allowing the bridge to extend progressively across an obstacle.

CANTILEVER BRIDGE

This construction approach is particularly beneficial where construction over live roads, railways, waterways or environmentally sensitive areas must be minimised. By avoiding extensive temporary works below the deck, cantilever construction can reduce disruption, safety risk and programme impact.

From a delivery perspective, cantilever bridges enable controlled, sequential erection and are well suited to constrained sites where access beneath the crossing is limited or unavailable.

Design & Buildability Notes (Informative)

  • Cantilever construction allows the bridge to be erected without continuous temporary support, reducing the need for falsework over live or sensitive environments.
  • The method supports staged, balanced erection, with construction loads managed incrementally as the structure advances.
  • Careful coordination of fabrication, delivery and erection sequencing is essential to maintain stability and predictability during construction.
  • Cantilever approaches are commonly associated with rail, highway and major water crossings where uninterrupted operation below the bridge is critical.
  • Early planning of interfaces between permanent works, temporary conditions and erection methodology helps reduce risk and improve programme certainty.

Cable Supported Bridges

Suspension Bridges

Suspension bridges carry the deck on vertical hangers suspended from main cables, which are draped over towers and anchored at each end. This structural form is capable of achieving the longest spans of any bridge type, making suspension bridges uniquely suited to major crossings.

SUSPENSION BRIDGE

They are typically used for large river, estuary and valley crossings, where intermediate supports are impractical or undesirable. The deck is supported independently of the main cables, allowing long clear spans and minimal intervention within the obstacle being crossed.

From a construction and delivery perspective, suspension bridges are complex, highly engineered structures that require careful coordination of fabrication, logistics and erection sequencing over extended programs.

Design & Buildability Notes (Informative)

  • Suspension bridges are typically reserved for major infrastructure schemes where very long spans are required.
  • Construction is usually staged and sequential, with primary cables, hangers and deck elements installed as separate operations.
  • Installation often takes place at significant height and over live environments, requiring robust planning for access, safety and temporary conditions.
  • Fabrication and delivery of large components, including cables, towers and deck sections, must be closely aligned with the erection sequence.
  • Early and ongoing coordination between designers, contractors and specialist suppliers is critical to managing interfaces and reducing construction risk.
Cable-Stayed Bridge

In a cable‑stayed bridge, the deck is directly supported by inclined cables connected to one or more towers. Unlike suspension bridges, the cables connect straight to the deck rather than via main cables, creating a stiff and efficient structural system.

CABLE-STAYED BRIDGE

Cable‑stayed bridges are well suited to medium to long spans and offer a wide range of distinctive structural arrangements, including fan, harp or semi‑fan layouts. They are often selected where span length, aesthetics and structural efficiency need to be balanced, and where intermediate supports within the crossing are undesirable.

From a delivery perspective, cable‑stayed bridges combine architectural presence with efficient use of materials, while requiring careful coordination of fabrication, erection sequencing and interface management.

Design & Buildability Notes (Informative)

  • Cable‑stayed bridges are typically adopted where spans exceed the practical range of beam or girder forms but do not justify full suspension construction.
  • Construction is usually staged, with deck segments erected progressively and supported by sequentially installed stay cables.
  • Accurate control of geometry and sequencing during erection is essential to maintain alignment and manage temporary conditions.
  • Tower construction, cable installation and deck erection are closely interdependent, requiring early planning and coordination between trades.
  • The form allows large clear spans over roads, railways or waterways, reducing the need for temporary works or supports below the deck.

Modular & Temporary Bridges

Modular Bridge

Modular steel bridges are formed from prefabricated, standardised units designed for rapid assembly on site. They are widely used where speed of installation, minimal disruption or constrained access is critical, such as for temporary works, construction access, utility crossings and emergency situations.

MODULAR BRIDGE

While many modular systems are deployed as temporary structures, some can be configured and detailed for permanent applications where appropriate, offering a flexible solution that balances programme, cost and site constraints.

From a delivery perspective, modular bridges prioritise off‑site manufacture, simplified logistics and predictable installation, making them particularly well suited to time‑critical or access‑restricted environments.

Design & Buildability Notes (Informative)

  • Modular bridge systems are designed for rapid assembly, often using repeatable components and standard connection details.
  • Installation can frequently be achieved with minimal temporary works, reducing site activity and interface risk.
  • The use of predefined modules supports efficient transport planning and straightforward erection methodologies.
  • Modular bridges are well suited to programme‑critical works, including rail possessions, live highways and remote locations.
  • Where used as permanent structures, consideration is given to durability, inspection and maintenance requirements in line with their intended service life.

Footbridges & Service Bridges

Footbridge

Footbridges are designed for pedestrians and cyclists and may use beam, truss, arch or cable-supported forms. Design considerations include vibration, accessibility, durability and long-term maintenance.

FOOTBRIDGE

Supporting Practical Bridge Delivery

Understanding bridge forms at an early stage supports better decision-making and more predictable delivery. The Steel Bridge Company works with these bridge types daily, providing practical engineering input to ensure safe, compliant and buildable outcomes.