Beam bridges: A beam or "girder" bridge is the simplest kind of bridge. In the past, they may have taken the form of a log a...

The Four Main Types of Bridges

Beam bridges:
A beam or "girder" bridge is the simplest kind of bridge. In the past, they may have taken the form of a log across a stream but today they are more familiar to us large box steel girder bridges. There are lots of different types of beam bridges.

A beam bridge needs to be stiff. It needs to resist twisting and bending under load.
In its most basic form, a beam bridge consists of a horizontal beam that is supported at each end by piers. The weight of the beam pushes straight down on the piers.

Under load, the beam's top surface is pushed down or compressed while the bottom edge is stretched or placed under tension. If we imagine that there is an imaginary line running down the center of the beam this line remains at its original length while the material above is compressed and the material below is stretched. This line is referred to as the neutral axis.

compression and tension in beam bridges 
compression and tension in beam bridges

The farther apart its supports, the weaker a beam bridge gets. As a result, beam bridges rarely span more than 250 feet. This doesn't mean beam bridges aren't used to cross great distances it only means that there may be a series of beam bridges joined together, creating what's known as a "continuous span."

Characteristics of Beam Bridges - Types of Beam Bridges:

Beam bridges basically consist of beam that is laid across the piers or supports. The beam should possess the strength to bear the loads that are expected to be placed on it. These loads are borne by the bridge piers. The loads cause the beam top edge to be compressed, while the lower edge is being stretched and is under tension.

Existing beam bridges are formed by girders, normally box girders, trusses or I-beams, that are supported on strong piers.
  • Box girders are stretched, box-shaped elements that are more suitable to bear the twisting loads.

  • Trusses consist of one or more triangular units connected at joints or nodes.

  • I-beams are economical and simple to fabricate. They are simply beams with an I-shaped or H-shaped cross-section. The horizontal elements of the "I" design are flanges and the vertical is the web of the construction.
Other beam bridges may be fabricated from concrete beams that are pre-stressed. These materials possess the steel characteristics to endure loads in tension, and concrete strength to bear the compressive loads.

Truss Bridge:

Truss bridge is a type of bridge whose main element is a truss which is a structure of connected elements that form triangular units. Truss is used because it is a very rigid structure and it transfers the load from a single point to a much wider area. Truss bridges appeared very early in the history of modern bridges and are economic to construct because they use materials efficiently.

Truss Bridge

From the first truss bridge, engineers experimented with different forms of truss bridges trying to find better shape and the one that will suit them for the particular problems. Because of that we have today many forms of truss bridges. Truss bridge can have deck (roadbed) on top (deck truss), in the middle (through truss), or at the bottom of the truss. If the sides of the truss extend above the roadbed but are not connected, it is called a pony truss or half-through truss.

Here are some more common variants of truss design for bridges:
  • Allan truss: a pony truss based on Howe truss. The first Allan truss was finished on 13 August 1894.

  • Bailey truss: made for military to be easily combined in various configurations.

  • Baltimore truss: made like Pratt truss but it has additional bracing in the lower section of the truss which prevents buckling in the compression members.

  • Bollman truss: an all-metal truss with many independent tension elements which makes for a strong bridge that is easy to assemble.

  • Burr arch truss: a combination of an arch and truss which gives a strong and rigid bridge.

  • Howe truss: has vertical elements and diagonals that slope up towards the center of the bridge.

  • K truss: has one vertical member and two oblique members in each panel (which form a letter “K”).

  • Lenticular truss: uses a lens-shape truss which has an upper and lower curve and diagonal elements between them. If the curves are above and below the roadbed it is a “lenticular pony truss”.

  • Long truss: a variant of Howe truss but made of wood and used for covered bridges.

  • Parker truss: a variant of Pratt truss that has a polygonal upper chord. If chord has exactly five segments it is called camelback.

  • Pegram truss: has chords that are wider at the bottom but of the same length as each other at the top.

  • Pratt truss: has vertical members and diagonals that slope downward to the center. It is a variant commonly used for railroad bridges.

  • Vierendeel truss: has members that are not triangular but rectangular. Rare are bridges made in this variant of truss because it is not cheap.

  • Warren truss: has longitudinal members joined only by angled cross-members. They form equilateral triangles. It is relatively light but strong and economical truss.

Arch bridge:

An arch bridge is a bridge with abutments at each end shaped as a curved arch. Arch bridges work by transferring the weight of the bridge and its loads partially into a horizontal thrust restrained by the abutments at either side. A viaduct (a long bridge) may be made from a series of arches, although other more economical structures are typically used today.
Types of arch bridge:
1-Corbel arch bridge:The corbel arch bridge is a masonry, or stone, bridge where each successively higher course (layer) cantilevers slightly more than the previous course.The steps of the masonry may be trimmed to make the arch have a rounded shape.The corbel arch does not produce thrust, or outward pressure at the bottom of the arch, and is not considered a true arch. It is more stable than a true arch because it does not have this thrust. The disadvantage is that this type of arch is not suitable for large spans.

2-Aqueducts and canal viaducts:In some locations it is necessary to span a wide gap at a relatively high elevation, such as when a canal or water supply must span a valley. Rather than building extremely large arches, or very tall supporting columns (difficult using stone), a series of arched structures are built one atop another, with wider structures at the base.Roman civil engineers developed the design and constructed highly refined structures using only simple materials, equipment, and mathematics. This type is still used in canal viaducts and roadways as it has a pleasing shape, particularly when spanning water, as the reflections of the arches form a visual impression of circles or ellipses.

Corbel arch bridge

3-Deck arch bridge:This type of bridge comprises an arch where the deck is completely above the arch. The area between the arch and the deck is known as the spandrel. If the spandrel is solid, usually the case in a masonry or stone arch bridge, the bridge is called a closed-spandrel deck arch bridge. If the deck is supported by a number of vertical columns rising from the arch, the bridge is known as an open-spandrel deck arch bridge.

4-Through arch bridge:This type of bridge has an arch whose base is at or below the deck, but whose top rises above it, so the deck passes through the arch. The central part of the deck is supported by the arch via suspension cables or tie bars, as with a tied-arch bridge. The ends of the bridge may be supported from below, as with a deck arch bridge. Any part supported from arch below may have spandrels that are closed or open.

Through arch bridge
Use of modern materials:

Most modern arch bridges are made from reinforced concrete. This type of bridge is suitable where a temporary centring may be erected to support the forms, reinforcing steel, and uncured concrete. When the concrete is sufficiently set the forms and falseworks are then removed. It is also possible to construct a reinforced concrete arch from precast concrete, where the arch is built in two halves which are then leaned against each other.

Many modern bridges, made of steel or reinforced concrete, often bear some of their load by tension within their structure. This reduces or eliminates the horizontal thrust against the abutments and allows their construction on weaker ground. Structurally and analytically they are not true arches but rather a beam with the shape of an arch. See truss arch bridge for more on this type.

A modern evolution of the arch bridge is the long-span through arch bridge. This has been made possible by the use of light materials that are strong in tension such as steel and prestressed concrete.

suspension bridge:

suspension bridge is a type of bridge in which the deck (the load-bearing portion) is hung below suspension cables on vertical suspenders. The first modern examples of this type of bridge were built in the early 19th century. Simple suspension bridges, which lack vertical suspenders, have a long history in many mountainous parts of the world.

This type of bridge has cables suspended between towers, plus vertical suspender cables that carry the weight of the deck below, upon which traffic crosses. This arrangement allows the deck to be level or to arc upward for additional clearance. Like other suspension bridge types, this type often is constructed without falsework.

The suspension cables must be anchored at each end of the bridge, since any load applied to the bridge is transformed into a tension in these main cables. The main cables continue beyond the pillars to deck-level supports, and further continue to connections with anchors in the ground. The roadway is supported by vertical suspender cables or rods, called hangers. In some circumstances, the towers may sit on a bluff or canyon edge where the road may proceed directly to the main span, otherwise the bridge will usually have two smaller spans, running between either pair of pillars and the highway, which may be supported by suspender cables or may use a truss bridge to make this connection. In the latter case there will be very little arc in the outboard main cables.

suspension bridge

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