Along the bridge design projects, we can face different types of bridges as well as various elements that compose these. In the following article, we will explain the existing types of bridges, structures, and components.
What is a bridge?
A bridge is a construction, usually artificial, that allows overcoming a geographic accident or any other physical obstacle such as a river, a canyon, a valley, a road, or any other topographic inconvenience.
The design of each bridge varies depending on its function and the nature of the terrain on which the bridge is built. The design and calculation belong to structural engineering, being numerous types of design that have been applied throughout history, influenced by the materials available, the techniques developed, and the economic conditions, among other factors.
Between the materials used in bridge construction, we can find stone, wood, steel, reinforced concrete, prestressed concrete, post-tensioned concrete, and mixed.
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Components of bridge structures
The part of the bridge that supports the superstructure usually consists of:
- Abutments.
- Piers or piers.
- Generally, the portion of the bridge that is above the bridge isolators may include:
- Reinforced concrete slab.
- Beams.
- Trusses.
- Bracing.
On suspension bridges:
- Cables.
- Hooks.
- Towers.
- Bridge deck.
- Elements of a bridge.
Elements of a bridge
Superstructure
It is the part of the bridge where the moving load acts and it is constituted by:
- Deck slab.
- Longitudinal and transverse beams.
- Sidewalks and handrails.
- Roadway layer.
- Other facilities.
Substructure
This is the part of the bridge that is responsible for transmitting the stresses to the foundation soil, and is made up of:
- Abutments.
- Piles.
- Wing walls.
The superstructure of a bridge can be classified in several ways
- Construction material (wood, steel, concrete).
- Type of span (arch, girder).
- Structural form (cantilever, suspension).
- Span length (long, short).
- Load path characteristics (orthotropic, slab bridge).
- Position (scale, suspended, rotating).
- The structural form of a bridge superstructure refers to how gravity and lateral loads are transmitted from the bridge deck to the foundation.
Types of bridge superstructures
Slab girder bridge
Consists of two main superstructure components:
- Concrete deck (deck).
- Longitudinal girders.
- Longitudinal girders are generally made of a variety of materials, including:
- Steel.
- Reinforced/prestressed concrete.
- Glued laminated timber (in this case, the deck is usually laminated timber with an asphalt surface).
In many cases, the concrete deck (for both prestressed beams and structural steel beams) is designed to work in composite with the beams.
The result is a smaller cross-section size for the girders and therefore greater economy through more efficient use of materials.
This system is highly recommended for short-span bridges, with spans in the range of 20 to 125 feet. Beyond these lengths, the economic cost is an important factor in switching to a different type of superstructure.
Truss bridges
Truss bridges tend to be more economical for longer spans (assuming depth requirements are not severe) given that the the truss members are subjected to axial loads without any bending.
Truss bridges can be further classified by the position of the deck concerning the bottom or top chords of the truss.
The designations are as follows.
Deck truss
The deck is supported at the upper points of the truss. The truss is therefore completely below the road surface.
Exposed armor
The deck is supported by the lower chords of the armor. The armor itself can obstruct drivers’ vision and is a threat to collisions. Consequently, this type of truss is not used.
However, many bridges with this configuration have been taken out of use and removed to alternate locations due to their historical significance.
Half-exposed truss
The deck is supported by both bottom and top chords.
This type of truss is usually required for very large-span truss structures. This type of truss is sometimes referred to as a pony truss.
Rigid frame bridges
The behavior of a rigid frame bridge is essentially the behavior of a two-jointed arch. Because their structural form is indeterminate, these bridges are not as easy to analyze as beam or truss bridges.
Historically, rigid frame bridges have been constructed with only two main supporting components so their redundancy and load paths have been questioned.
Arch bridges
Arch bridges are essentially the oldest bridge structures. Each element of the arch structure is subjected to compressive forces, historically it was possible to construct these stone bridges without mechanical anchors. Arch bridges that are 2,000 years old are still in operation.
Truss bridges and arch bridges can also be classified based on the location of the road surface about the supporting structure of the arch:
- Deck arch: The arch support structure is completely below the road surface.
- Exposed arch: The arch is above the road deck. In this case, the surface itself can form a tensioner for the reinforcement forces that develop (hence, a tensioned arch).
- Half-exposed arch: This is a bridge structure where the roadway is between the springing and the crown.
- Arch bridges can also be based on other structural forms (trusses, rigid frames, etc).
Cantilever bridges
Cantilever bridges were the first structural form to conquer fairly large spans that were required for railroad traffic in the 1800s. Cantilever bridge structures consisted of the following components: cantilever bridges, cantilevers, trusses, and frames.
Cantilever bridge structures consisted of the following components:
- Anchor spans: Simply supported spans at each end of the bridge structure. The cantilever portion of these spans supports the central span.
- Cantilever span: Central span (usually shorter than the anchor spans) is usually located in the center of the stream or river.
- The Firth of Forth Bridge is an excellent example of a cantilever bridge. The cantilever bridge structure attempts to equalize/cancel the positive and negative moments of the bridge structure between the supports.
Cable-suspended (Suspension) Bridges
Cable-suspended bridges are well known for their aesthetic qualities (Golden Gate Bridge, Brooklyn Bridge, Washington Bridge, etc.), and for their ability to span long distances.
These bridges are made up of four main components:
- Towers
- Cables
- Anchorages
- Deck
Cable-stayed bridges
Although the use of cables to cable-stay a bridge deck was discovered many years earlier, cable-stayed bridges did not receive much attention and use until after World War II.
Cable-stayed bridges transmit the deck load directly to the towers via cables.
Conclusion
Bridge engineering requires that the problem of the long-term behavior and maintenance of such structures be met, in addition to complete knowledge and understanding of design and construction processes. This calls for a creative, multidisciplinary strategy founded on a strong foundation of knowledge and study.
Bridges serve functional and aesthetic purposes. They function as landmarks in addition to being routes between locations. More than that, they have evolved into geographic and architectural icons that contribute to the cultural and historical definition of who we are as people.
