Engineering

Part 6 - Bridges 2

The Chaotianmen Bridge is a road-rail bridge over the Yangtze River in the city of Chongqing, China. The bridge, opened in 2009 is the world's longest through-arch bridge. It has a span of 552 m (1,811 ft) and a total length of 1,741 m (5,712 ft). It carries 6 lanes of traffic with pedestrian walkways and the lower deck has 2 traffic lanes on each side of the Chongqing Metro Loop Line.

Tied-arch bridges have an arch-shaped superstructure, but differ from conventional arch bridges. Instead of transferring part of the weight of the bridge and traffic loads into horizontal thrust forces on the abutments, the ends of the arches are restrained by tension in the bottom chord of the structure. For this reason they are often named bowstring arches.

In modern suspension bridges, the cables hang from towers that are attached to caissons or cofferdams which are implanted deep into the bed of the lake, river or sea. The longest suspension bridge in the world is the 3,909 m (12,825 ft) Akashi Kaikyō Bridge in Japan.

Completed in 1998, it has the longest central span of any suspension bridge in the world, at 1991 metres (6532 ft; 1.237 mi). Cable-stayed bridges are also supported by cables but these are shorter and towers holding the cables are proportionately higher. 

The longest cable-stayed bridge since 2012 is the 1,104 m (3,622 ft) Russky Bridge in Vladivostok, Russia.

Completed in 1997, the Kap Shui Mun Bridge (KSMB) in Hong Kong is one of the longest cable-stayed bridges in the world with the upper deck used for motor vehicles and the lower deck used for both vehicles and metro rail traffic. It has a main span of 430 metres (1,410 ft) and an overall length of 750 metres (2,460 ft). 

Temporary bridges, such as Bailey bridges, are designed to be assembled, taken apart, transported to a different site, and re-used. They are important in military engineering and are also used to carry traffic while an old bridge is being rebuilt. Most bridges have no moving parts but movable bridges are designed to move out of the way of boats or other traffic, which would otherwise be too tall to pass under the bridge.

Mobile bridges are used in most modern armies and as temporary bridges required during maintenance or replacement. 

Bridge Design

Bridges design engineers ensure that a bridge will support its own weight and the weight of moving traffic for 50 years or more with minimal maintenance and will survive the worst anticipated environmental conditions such as high winds and earthquakes as well as dynamic loading or vibration caused by traffic and wind conditions. More standard bridge designs are being used particularly for new road bridges and most are now designed for rapid bridge replacement at the end of their working lives. It is now possible to replace some road bridges during a weekend so minimizing traffic disruption. 

Designs are analyzed to establish bending moment and shear force distributions caused by bridge weight and applied traffic and wind loads. Traditionally, stress and deflection calculations were completed on paper using large factors of safety but modern computers permit the use of finite element analysis (FEM) which provides more detailed and more precise stress and strain (deflection) information.

Most bridges use steel and concrete although other materials may be used in special conditions. Steel is particularly strong in tension whereas concrete is strong in compression so both characteristics are used in prestressed and post-stressed concrete bridge designs. 

Beam bridge spans rarely exceed 250 feet (76 m) long, as flexural stresses increase proportionally to the square of the length and deflection increases proportionally to the 4th power of the length.

Economical to construct and one of the oldest types of modern bridges, a truss bridge has a load-bearing structure composed of triangular elements. This type is commonly used on tower cranes used to construct tall buildings.

Cantilever bridges are horizontal beams supported at only one end. Most cantilever bridges use a pair of continuous spans that extend from opposite sides of the supporting piers to meet at the center. The largest cantilever bridge is the 549-metre (1,801 ft) Quebec Bridge in Quebec, Canada.

Bridge maintenance

The estimated life of bridges varies between 25 to 80 years depending on location and material. However, bridge may exceed 100 years with proper maintenance and rehabilitation. Bridges vibrate under load and this may contributes to the stresses. One of the most famous examples is the Tacoma Narrows Bridge that collapsed shortly after being constructed due to excessive vibration.

More recently, the Millennium Bridge in London vibrated excessively under pedestrian loading until it was retrofitted with a system of vibration dampers.

Many long-span bridges are now routinely monitored with a range of sensors including strain transducers, accelerometers, tilt-meters and the Global Positioning System (GPS). Inertial accelerometers have the advantage that they do not require a reference point to measure from.

Non-contact monitoring uses a laser beam from a Doppler Vibrometer directed at the point of interest, and the vibration amplitude and frequency are obtained from the Doppler shift of the laser beam frequency caused by the motion of the surface. This is faster than an accelerometer. Additionally, this method can measure specific points on a bridge that might be difficult to access.

While larger modern bridges are routinely monitored electronically, smaller bridges are generally visually inspected and, increasingly, with special vehicles equipped with accelerometers, gyrometers and Laser Doppler Vibrometers. Some also can apply a resonant force to the road surface in order to dynamically excite the bridge at its resonant frequency.