A truss is a structure comprised of straight members put together into one or more triangular units. The ends of the straight members are connected at joints detailed such that the straight members are capable of carrying only tension or compression, with no bending whatsoever. A planar truss lies entirely within two-dimensions, whereas a space truss has members and joints in three dimensions. The top and bottom members of the truss are referred to as chords and the remaining members are called diagonals.
Truss bridges are those that utilize truss construction. In the early 1900s in the US, many truss bridges were constructed from timber. Covered bridges also often utilize truss construction, which is often covered with panels and/or a roof.
There are multiple different types of truss configurations, all of which have strengths and weaknesses and are dependent on the orientation of the diagonals (for example, in a Warren Truss, the diagonals are oriented in the shape of a W and in a Pratt Truss, the diagonals are in the opposite direction). Trusses can be deck trusses, where the truss is below the bridge deck but both the top and bottom chords are connected with stiffeners, forming a box truss; through trusses, where the deck is between the trusses – again the top and bottom chords are connected; and a pony truss, in which the roadway is centered within the truss and the top and bottom chords are not connected.
Trusses are used as basic building blocks of other bridge types as well. For example, the deck and towers of suspension bridges are often trusses. Cantilever type bridges often use truss construction for the cantilevers and suspended spans.
A cantilever is a structure that projects horizontally into space and is supported only on one end. A cantilever bridge uses cantilevers to carry the loads. For smaller bridges, the cantilevers may be simple beams; for larger highway or railroad bridges, though, the cantilevers generally use truss structures or prestressed concrete. This type of bridge was a major engineering breakthrough when first utilized, as it can span distances in excess of 1,500 ft and can be constructed using little or no falsework (falsework is the structure constructed to facilitate the construction of the bridge. It is not possible, at times, to utilize falsework due to environmental or other constraints, such as conflicts with another road or a body of water).
A simple cantilever bridge span is created using two cantilever arms extending from opposing sides of the obstacle to be crossed and meeting at the center. Oftentimes, the two cantilever arms do not meet in the center, but rather support a separate truss structure that rests on the ends of the cantilever arms. This separate truss structure can be constructed off site and lifted into place, or it can be constructed in place using traveling supports. This is called a suspended span. The most famous example of a suspended span bridge is the Firth of Forth Rail Bridge in Scotland.
A very common way to construct both steel truss and prestressed concrete cantilever bridge is the balanced cantilever. In this construction method, the each cantilever arm on one side of a support is balanced, during construction, with a cantilever of the same approximate size on the opposite side of the support. Steel truss cantilevers transfer tensile loads to the outermost supports and compressive loads to the central towers. Concrete balanced cantilever bridges are generally built using segmental construction wherein a prestressed segment of the bridge is formed, cast and erected then post-tensioned to adjacent, already erected, segments.
The longest cantilever bridge is the Quebec Bridge in Quebec, Canada. It is 1,800 ft in length, and utilizes steel truss construction.
Movable bridges move to allow the passage of boats and barges. By allowing the bridge to open, the cost of bridge construction is reduced, as the need for long approaches and high piers is eliminated. These bridges are less than advantageous, however, in that traffic on the bridge must be stopped so that the bridge can open. Railroad bridges, which are seldom used, may be left open most of the time to allow for boat passage, and then closed to allow trains to pass. While the overall length of a movable bridge may be quite long, the movable portion is generally limited to a few hundred feet.
There are multiple kinds of movable bridges, for example:
- Drawbridge – the deck is hinged on one end, where the deck rotates upward when open
- Bascule – a drawbridge that utilizes a counterweight to facilitate opening
- Folding – a drawbridge with multiple sections that collapse together horizontally
- Curling – a drawbridge with multiple sections that curl upward
- The majority of movable bridges in the US are drawbridges
- Vertical Lift – the deck is lifted vertically with no rotation. The deck is lifted using counterweighted cables mounted on towers
- Table Lift – Similar to a vertical lift bridge, but the lifting mechanism is below the deck
- Swing Bridge – The deck rotates horizontally around a fixed point
The Tower Bridge in London is one famous example of a bascule bridge.
Man has an innate desire to reach what is supposed to be unreachable or venture into new areas. This led him to build bridges that can span huge distances to help him achieve this desire. Who knows if a log of wood floating on the water may have been the very first inspiration for man to build a bridge for the first time? All of these are attributed to the evolution of engineering and architecture, which made it possible to bridge distances and make places that seem to be unreachable become reachable.