Introduction

Bridges are constructed in many different ways and are often identified by the materials they are constructed from and the designs constructed using these materials. The following diagrams can be used to help identify bridges you may see in your travels.

A Note On Bridge Forms

At the most simple level of engineering design, bridges come in three basic forms: beam, arch, and suspension. From these three basic forms of bridge arise many different bridge types as described below. Some bridge types are further subdivided into a variety of configurations and superstructure arrangements. A number of bridge types can have mechanical elements added to the design to enable them to move to provide clearance for boats. Movable bridges are categorized according to the mechanical design that enables their movement, and the several movable bridge types also come in a variety of configurations.

Bridge Basics

Bridge Parts

A bridge superstructure rests on the bridge’s substructure with the assistance of a bearing, which is sandwiched between the superstructure and the substructure. Some bearings, known as expansion bearings, are designed to allow small amounts of movement associated with changes in temperature. Other bearings may be fixed bearings, and do not allow movement. Single span bridges have a substructure consisting of two abutments, one at each end. Bridge spans may be divided into multiple spans by including one or more piers between abutments.

Span Types

Simple spans feature a structurally independent superstructure forming each span of the bridge.

Continuous spans feature what has the appearance of a single span superstructure, but is supported by piers at one or more points.

Cantilever spans have spans that extend outward from a substructure element and end at a location unsupported by a substructure element. These outward-extending cantilever arms are balanced by the other half of the span system, which extends in the opposite direction of the pier, forming an anchor arm.

Many cantilever spans have cantilever arms that, rather than extending all the way to meet an opposing cantilever arm, stop short of the center so as to hold a suspended span in place.

Superstructure Arrangements

Bridge arrangements refer to the position of the deck (roadway) relative to the superstructure of the bridge.
Not all bridge types make use of this nomenclature. These terms are commonly used with truss bridges, and two of the terms are commonly used with arch and girder bridges as well.

With a through bridge, the superstructure is beside the roadway. With truss bridges, overhead bracing must also be present for the bridge to be considered a through truss.

With a pony bridge, the superstructure is above and beside the roadway, but no overhead bracing is present. The term “pony” is normally only used when describing truss bridges, although you will find that some girder and arch bridges come in this form as well, even if not typically described as such. What would appear to be a “pony girder” or “pony arch” bridge is normally described as a “through girder” or “through arch.”

With a deck bridge, the superstructure, as well as any bracing, are located below the roadway.

Beam and Girder Bridges

Compared to multi-girder bridges, deck girder bridges have larger and fewer beams, and the roadway rests on transverse floorbeams that span between the girders.

Through girder bridges are similar in design to deck girders, except that the girders rise above the roadway. Transverse floorbeams remain under the deck.

A concrete t-beam is a common type of concrete beam bridge where the beams are cast integrally with the deck, forming a repeating “T” shape in the transverse view.

In a slab, the bridge is supported by a superstructure that acts as a single solid beam over the obstruction. While most commonly associated with reinforced concrete, timber can also be used.

A rare type of bridge (more common in the UK) are simple stone slab bridges, which consist of a single massive slab of stone that is placed between a substructure. Single span stone slab bridges are known as clam bridges.

Multiple span stone slab bridges are known as clapper bridges. Sometimes “clapper bridge” may be used to describe all forms of stone slab bridges, including single span examples.

Truss Bridges

Truss bridges are a form of a beam bridge, where a triangular framework of members and chords form a structure that functions as a large beam. Truss bridges are described according to the arrangement of the trusses: deck trusses (below the roadway), pony trusses (above the roadway, but without overhead bracing), and through trusses (above the roadway, with overhead bracing). Researchers may wish to note that historically, through truss bridges were sometimes called high trusses, and pony trusses were sometimes called low trusses.

Truss Bridge Parts

Metal Truss Bridge Configurations

Metal truss bridges come in a wide variety of arrangements and configurations as shown below. Most of the following truss designs are adapted from those which appeared in J.A.L. Waddell’s 1916 book Bridge Engineering and for the most part they remain relevant today with only minor name and classification changes as noted below.

Pratt: The Pratt truss is one of the most common truss configurations.

Pratt Half-Hip: Some Pratt pony truss bridges lack hip vertical members, and are known as half-hip Pratt truss bridges.

Whipple (Double-Intersection Pratt): This variation on the Pratt was common for longer spans in the 19th Century.

Howe: Diagonals follow the opposite orientation as that of the Pratt. The Pratt was more common on metal truss, while the Howe was more common on timber trusses.

Parker: A Pratt truss with a polygonal top chord. Used for longer spans.

Camelback: A Parker truss with exactly five slopes to the top chord (including end posts).

Baltimore: A Pratt truss with subdivided panels. Two common forms shown above.

Pennsylvania (Petit) Truss: A Parker truss with subdivided panels. Two common forms shown above. Today, Pennsylvania truss bridges with five slopes of the top chord are usually called simply “Pennsylvania” trusses, but could be noted as “Camelback Pennsylvania trusses.” Pennsylvania trusses were used for long spans, including some of the longest simple spans ever constructed.

Warren: While Waddell’s book distinguished between a “Triangular” and “Warren” type of truss, today all varieties are known as Warren truss bridges.

Warren (With Verticals): Most Warren truss bridges display vertical members, although the frequency of the verticals can vary. Warren trusses (with or without verticals) were a common truss type.

Polygonal Warren: Warren trusses can have polygonal top chords, much like Parker truss bridges. These are known as polygonal Warren truss bridges. Today, polygonal Warren truss bridges with five slopes of the top chord are usually called simply “Polygonal Warren” trusses, but could be noted as “Camelback Polygonal Warren trusses.”

Double-Intersection Warren: This design can be thought of as two Warren trusses superimposed and offset on top of each other.

Lattice Truss: Triple-Intersection, Quadruple Intersection, and Quintuple Intersection Warren trusses are commonly called simply “lattice trusses.” Sometimes the Quadruple Intersection Warren is also called a Quadrangular Warren.

K-Truss: Invented during the construction of the Quebec Bridge, the K-Truss can be arranged in several different ways, with two orientations shown above. The top chord does not have to be polygonal.

Bollman: An unusual truss design used on some of the earliest iron railroad bridges.

Fink: An unusual truss design used on some of the earliest iron railroad bridges.

Bowstring (Parabolic): This form of truss typically has a curved top chord and is sometimes called an “arch-truss” as well. Most commonly used in the 1870s as an early metal truss form.

Kingpost: Used for very short spans.

Queenpost: Used for very short spans. May or may not have the “X” pattern of diagonals in the center.

Waddell “A” Truss: This truss, sometimes called an A-Frame truss, was built in small quantities.

Lenticular: Most examples in the United States were built to a patented design by the Berlin Iron Bridge Company of East Berlin, Connecticut.

Kellogg: An archaic and rare truss form.

Post: An archaic and rare truss form.

Pegram: An archaic and rare truss form.

Thacher: An archaic and rare truss form. Exact form may vary slightly, no existing examples exactly follow the design patented by Edwin Thacher.

Stearns: An archaic and rare truss form.

Metal Truss Connections

Metal truss bridges are further classified by the manner in which the members are joined together. Connections may be pinned, riveted, bolted, or welded.

Pinned connections, common from ca. 1870 to 1910 were easy to assemble in the field, consisting of a large threaded pin that passes through all the members, and is fastened on the ends with nuts.

Pinned connections were made obsolete by the rise of riveted connections, which offered a more rigid connection. With riveted connections, truss members were riveted to a gusset plate to connect the members. Later, rivets would be substituted for bolts or welds to fasten the members onto the gusset plates. Details for bolted and welded connections are similar to the riveted connection shown above.

Covered Bridge Truss Configurations

Wooden covered bridges are truss bridges built from timber and covered with a non-structural roof and wall system. Underneath the roof and wall are truss members. Like metal truss bridges, covered bridges have different configurations. While there is some overlap between metal and covered truss configurations, details for truss configurations of the same name may differ between covered and metal, and there are many covered truss configurations that were not used in metal truss construction. Also note that while the predominant material used in covered bridges is timber, iron was sometimes used in limited areas, (iron performed better than timber for tension members) and iron was sometimes used for connecting the timber members.

Arch Bridges

Arch bridges are subdivided in a variety of configurations, depending on the material type used in construction. Stone, concrete, or metal are the three most common materials, although timber may be used as well.

Arch bridges are described in two arrangements: Deck arches, with the arch below the roadway, and through arches, with the arch rising above the roadway for at least a portion of the span.

Arch Bridge Types

Most arch bridges function by thrusting the forces at an angle into the side of the abutments or piers at each end of the spans. As such, they often have angled bearings called skewbacks.

Steel arch bridges and concrete through arch bridges are sometimes designed as “tied” arch bridges, where a large beam connects each end of the arch. In these cases, the bridge can rest on its piers or abutments like a beam bridge, with a horizontal bearing sending forces vertically into the substructure.

Concrete Arch Bridge Configurations

Open Spandrel Arch: A type of deck arch bridge, as shown above, uses vertical columns to connect the deck to the arch.

Closed Spandrel Arch: A type of deck arch bridge which uses solid concrete walls to connect the deck to the arch. Closed Spandrel Arch bridges often contain an earthen fill hidden inside the concrete “box” formed by the deck and the superstructure, consisting of an arch that is as wide as the bridge itself. Alternatively, if the bridge has the appearance of repeating arch-shaped beams under the deck, it is known as a “ribbed” arch bridge.

Through (Rainbow) Arch: For concrete arch bridges, when the arch is above the roadway, it is often known as a rainbow arch.

Stone Arch Bridge Configurations

Stone arch bridges are almost always deck arches, and closed spandrel in configuration. Engineers may categorize stone arch bridges based on the shape of the arch (semicircular, segmental, or elliptical).

A semicircular arch consists of an arch that is in the shape of a half circle.

A segmental arch consists of an arch that is in the shape of less than half of a circle.

An elliptical arch consists of an arch that is in the shape of a half oval.

Steel Arch Bridge Configurations

Steel arch bridges, like concrete arch bridges, can be arranged as deck arch bridges or through arch bridges. Steel arch bridges are further categorized based on the design of the arch rib, the presence of hinges, and any bracing.

If the arch rib is a solid or box beam, it is a “solid ribbed” arch bridge.

If the arch rib is designed from many different members and chords, similar to a truss bridge, the bridge is a “braced ribbed” arch bridge.

Deck arch bridges sometimes include diagonal bracing in addition to the vertical columns. In these cases, the bridge is known as a spandrel-braced arch bridge.

Steel arch bridges can have giant pins called “hinges” in the design. A bridge can have hinges at the skewbacks (bearings) only (a two-hinged arch bridge).

A steel arch bridge can also have a third hinge at the crown (mid-span), making it a three-hinged arch bridge.

If no hinges are present, a steel arch bridge is known as a hingeless arch bridge.

Suspension Bridges

A suspension bridge consists of a deck that is held underneath main cables that are stretched out over the span from tall towers that rise above the deck. The deck is connected to the main cable via suspender cables which “hang” the deck from the main cables. The far ends of the main cable (known as backstays) are held in place by anchorages. Suspension bridges typically consist of three spans that are held in place by the suspension system, consisting of a central span, flanked by “anchor” or “side” spans. The deck itself is kept stiff in the presence of wind and the loads that pass over the bridge by way of a stiffening system. The stiffening system can take the form of various simple bridge types. With historic bridges, the stiffening system typically takes the form of a truss (pony, through or deck) or girder (through or deck).

Suspension Bridge Configurations

A three span suspension bridge, with each of the three spans suspended from a main cable is the most common configuration of a suspension bridge.

A rare design of suspension bridge eliminates the use of large anchorages by connecting the bridge to itself: the main cables are fastened directly to the bridge superstructure at the ends of the bridge, forming a self-anchored suspension bridge.

Some suspension bridges only have a single suspension span. In this case, the main cable becomes an unloaded backstay cable that proceeds directly from the top of the tower down to the anchorage. With single span suspension bridges, simple approach spans may be found supporting the deck where the side/anchor spans would be in a three span suspension bridge. This layout is shown above.

Main Cable Types

The main cable can take the form of a wire cable, or an eyebar chain. Eyebar chain bridges are rare; most suspension bridges use some form of wire rope for the cable.

Cable-Stayed Bridges

One of the common types of the basic suspension bridge form is a the cable-stayed bridge. In a cable-stayed bridge, the individual suspender cables directly connect from the tower to the deck, with no catenary main cable involved. Cable-stayed bridges tend to be more stable than traditional suspension bridges, and thus often do not have large stiffening trusses and girders. Cable-stayed bridges can feature cables configured in a variety of manners.

Cable Configurations

Tower Configurations

Double Vertical Plane (Parallel): If the cables spread out to both sides of the deck from double posts (one post for each side of the deck), it is a double vertical plane cable-stayed bridge.

Double Vertical Plane (Inclined): In a double vertical plane, the posts can be inclined as well.

Single Vertical Plane: If the cables spread out to both sides of the deck from single posts in the center of the deck, it is a single vertical plane cable-stayed bridge.

Movable Bridges

Many of the bridge types described above can be designed alongside mechanical features to allow the bridge to clear a waterway and make way for boats that would otherwise not fit underneath the bridge when closed. These movable bridges are categorized based on the design of the mechanical features that enable the movement of the bridge.

Bascule Bridges

Bascule bridges rotate up to open for boats. They include a counterweight to balance the span and enable relatively small motors to raise the large leaf.

If the span consists of two bascule leaves that raise up, the bridge is a double-leaf bascule. Most highway bascule bridges are double-leaf.

If the span consists of two bascule leaves that raise up, the bridge is a double-leaf bascule. Most highway bascule bridges are double-leaf.

Bascule Bridge Configurations

Among movable bridge types, bascule bridges display the most variety in mechanical operation. The following bascule designs are adapted from those which appeared in J.A.L. Waddell’s 1916 book Bridge Engineering and for the most part they remain relevant today with only minor name and classification changes as noted below.

Fixed-Trunnion (Chicago Type): The counterweight is fixed to the leaf, and the leaf rotates around a fixed trunnion. First used in London’s Tower Bridge, the design was modernized and made popular by the City of Chicago.

Scherzer Rolling Lift: This type of bascule rotates on a track, and has a counterweight fixed to the leaf. Invented in 1893 by Albert Scherzer and popularized by his brother William Scherzer.

Strauss Bascule: Joseph Strauss invented his variety of trunnion bascule bridge that is noted for having a separate trunnion for the counterweight. As such, the counterweight is not fixed to the leaf. This overhead counterweight design was also adaptable to a design where the counterweight was hidden below the roadway.

Heel-Trunnion: This variation on the Strauss bascule is noted for its parallelogram shape and typically takes the form seen above. The counterweight is always above the roadway.

Page: A rare bascule design.

Rall: A rare bascule design.

Swing Bridges

A movable bridge type, swing bridges are typically classified into two major categories based upon how they rest on the swing pier as noted below. Most swing bridges are symmetrical (center pier) but if the pier is not at the center, then the bridge is a bobtail swing bridge and a balancing counterweight may be present at the shorter end.

Swing Bridge  Configurations

Rim Bearing: Span bears upon the circular track around the rim. Typically has a dense series of numerous rollers around the rim.

Center Bearing: Span bears upon a single point at the center of the swing pier. As such, there may be less rollers around the rim as they only serve to guide the truss when moving, and are not load-bearing.

Some swing bridges are designed such that the swing pier is not in the center of the bridge, with one arm offering a swing span over the waterway, and another shorter arm having a counterweight to keep the bridge balanced. These are known as bobtail swing bridges.

Vertical Lift Bridges

A movable bridge type, vertical lift bridges rise directly up to provide clearance for boats. The two most common forms utilize tall towers that house counterweights that move to keep the span in balance as it is lifted by motors. These types of vertical lift bridges are categorized based on where these motors are located.

Vertical Lift Bridge Parts

The above diagram, created by famous vertical lift bridge engineer J. A. L. Waddell shows the many parts of a vertical lift bridge.

Vertical Lift Bridge  Configurations

Many vertical lift bridges house the motors on the lift span itself, with a series of cables leading off of the span and onto the tower to move the counterweight and lifting (uphaul) cables. Called Span Drive Vertical Lift Bridges, these tend to be older vertical lift bridges.

Other vertical lift bridges house the motors on the towers, typically at the top next to the sheaves. These are known as Tower Drive Vertical Lift Bridges, and they tend to be newer vertical lift bridges.