bridge

Bridge


Siduhe River Bridge – World’s highest bridge 2009

Bridge – a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle. There are many different designs that all serve unique purposes and apply to different situations. Designs of bridges vary depending on the function of the bridge, the nature of the terrain where the bridge is constructed, the material used to make it and the funds available to build it.

There are many types of bridges but only a few basic bridge structures. They include: girder bridge, truss, rigid frame, arch, cable stayed, suspension. This video describes how bridges work.

  • Arch Bridge – utilizes the arch as its main design as this bridge carries its weight outward along the curve of the arch to the supports at each end. The supports, known as the abutments, carry the load. The abutments also keep the ends of the bridge from spreading out by creating two equal and opposite horizontal forces that push inward on the arch. Arch bridges rely primarily on compression because as forces are carried outwards along the curve of the arch, the molecules of the material are compressed. When selecting a building material for an arch bridge, engineers must consider only materials that are strong under compression. These materials include stone, concrete, or steel.
  • Trusses – like a beam bridge because forces are transverse to the axis of the whole truss. A truss is identified by its series of triangles that are connected by a lower and upper chord or flange. Forces are carried along the axis of the assembled members that form the shape of a triangle. The members of the truss carry forces of tension or compression but not both. Bending occurs when both tension and compression are acting on a member. Members which hold the deck for the cars, trains, people, bikes and other objects crossing the bridge are subjected to both tension and compression. Bending in theses members may combine with axial loads in truss members which add some complexity to the analysis of this type of bridge.
  • Suspension bridges – Five centuries ago, the Andes were strung with suspension bridges. By some estimates there were as many as 200 of them, braided from nothing more than twisted mountain grass and other vegetation, with cables sometimes as thick as a human torso. Three hundred years before Europe saw its first suspension bridge, the Incas were spanning longer distances and deeper gorges than anything that the best European engineers, working with stone, were capable of.
  • Active support – To feel the difference between a cable that ends at the tower top and one that goes over it and continues to the ground, grab your head with your right hand and gently pull. If you try to keep your head straight, your neck will feel compressed, but also pulled to the right. Now interlace the fingers on your hands, put them over your head and pull with both arms (see attachment). Your neck and head will feel compressed, but your neck will not feel a pull to the right or left, since the force of your right arm is balanced by that of your left arm. Can you identify which activity supports the idea of tension and which activity supports the idea of compression? Explain your answer.

Getting to the other side

For centuries, bridges have been built to provide a shortcut over rivers and uneven terrain. Simple bridges are just a log or a piece of wood over a stream. Other bridges are amazing structures that required innovation and extreme engineering to design and build. The basic requirement is the same – getting to the other side. Questions correspond to the steps in the [/Engineering%20Design%20Process Engineering Design Process].

  • Ask – Some bridges need to be really long. Others have to be constructed high over the surrounding land. Some bridges must carry heavy loads like trains, cars and trucks. Other bridges support pipelines. How will the bridge be used?
  • Imagine – Some remarkable bridges were built from stone thousands of years ago, long before steel or concrete were available. What materials are available? How can these be used in new ways?
  • Plan, Create – Arches are a classic design component in bridges. Triangles are used to construct bridges from straight pieces of material such as steel. How can the bridge be structured to make it light and strong? Bridge construction is one of the oldest demonstrations of engineering. People have always needed to cross rivers and streams. Where are there examples of amazing bridges?
  • Improve – Sometimes disaster provides an opportunity to make improvements. How were bridges repaired and improved?

That’s engineering

  • strength of materials – The engineer needs to know how strong of each of the materials in the bridge, to ensure that the bridge will support its own weight as well as that of the load – cars, trucks, trains, etc that will be crossing the bridge.
  • fatigue – Some materials will breakdown or fatigue with use and time. Some materials will withstand bending with wind and traffic without much change. Other materials will fail after repeated bending and flexing.
  • deformation – a change in shape that is result of a force that influences the object. It can be a result of tensile (pulling) forces, compressive (pushing) forces, shear, bending or torsion (twisting).
  • solid – one of the four common states of matter. The molecules in solids are closely bound together, they can only vibrate. This means solids have a definite shape that only changes when a force is applied.
  • statics – We usually think about a bridge just standing in place. The forces and loads associated with a stationary object are addressed in "Statics".
  • dynamics – Bridges actually move a lot even if the distances may be very small. Wind, the number of cars, the loads on the trucks crossing the bridge will cause the bridge structure to move just a fraction of an inch. This is considered Dynamics.
  • stress and strain – These are specific kinds of changes that occur during the life of a bridge.

Engineering ideas

  • forces, compressions, tension, torsion, shear, abutment, arch, beam, bridge, girder, keystone, rigid, span, suspension bridge, trestle, truss, Civil Engineering, construction, transportation, load

Do it
Here are some challenges for you to work on…

  • Penny bridge – arch and truss bridges – How many pennies will your bridge hold? – build a bridge with a single file card, explore shapes that provide additional strength and stability, test ability to support weight with file cards, pennies, books for end supports


Testing gumdrop bridge strength

Take your pick – real or virtual bridge building.

  • Build a bridge in the Cargo Bridge game – web-based, free, FLASH, mobile version needs special browser
  • Bridge Constructor Playground (game-based learning, app, free / $1.99) – bridge building across 30 innovative levels over deep valleys, canals or rivers. Bridges are subjected to a stress test to see whether they can support the weight of the cars and/or trucks which drive across them.
  • Bridge Constructor app, $1.99
  • Gundrop Bridge – comprehensive introduction to bridges and instructions for constructing than actual bridge

Learn more…

Bridges, arches

Keywords

  • Abutment: A support for the end of a bridge
  • Arch: A curved structure that rests on its two extremities and spans a space. Constructed to support weight above it.
  • Beam: large, heavy piece of wood, steel, or other material, used in construction
  • Bridge: Any structure built across a waterway, chasm, or other obstacle to afford passage
  • Compression: Act or process of compressing; being compressed
  • Engineer: Person skilled in or practicing any branch of engineering
  • Girder: Large horizontal beams, used to support joist and other beams
  • Keystone: Central, top-most stone of an arch, serving to lock the remaining stones of an arch together
  • Rigid: Not yielding or bending; stiff
  • Span: Distance between two supports
  • Suspension Bridge: Bridge suspended by chains or cables
  • Tension: Act of stretching or making taut
  • Trestle: Short beam or supported by four diverging legs, used as a support
  • Truss: Triangular units, used to span an opening or support heavy weight