-
Asked by anon-74964 to Rory on 2 Jul 2020.
-
Rory Shanks answered on 2 Jul 2020:
My favourite bridge has to be the Brooklyn Bridge in New York – not for its looks but for what it helped people achieve. Back when it was built, New York was a small place, restricted because it was separated on different islands. The Brooklyn Bridge helped to connect the islands together and put it on the path to the great city it is today! That’s what I love most about bridges – how helpful they are to the people that use them.
-
Comments
Juan Carlos commented on :
The 3 Forth Bridges in in Scotland are stunning. It is one of my favorite spots in there. One bridge per century…
David commented on :
There are some truly iconic bridges around the world. In this country my favourites are the Forth Rail Bridge, designed by Sir John Fowler and Sir Benjamin Baker, a cantilever design, and the Royal Albert Bridge between Plymouth and Saltash, designed by I K Brunel. This is a clever design where the main spans are lenticular trusses with the top chord of each truss, a heavy tubular arch, in compression. The bottom chord comprises a pair of chains in tension. This arrangement means that the end piers see only vertical loads and no horizontal thrust.
Bridges are a good place to see how the evolution of engineering ideas has developed. The designer is always trying to solve a set of problems:
• How long is the largest single span of the bridge?
• How high must my bridge be?
• What load must my bridge carry?
• What ground will by bridge stand on?
• What conditions must my bridge withstand, e.g.:
o Earthquake
o Wind
o Rain
o Snow
o Ice
o River flood water
o Sea water
o Wave height
• What materials are available for use?
• How many years must my bridge remain in service?
Sometimes, quite spectacularly they go wrong. See the Tacoma Narrows Bridge, ‘Gallopin Gertie’ ( https://www.youtube.com/watch?v=j-zczJXSxnw ) where the influence of winds creating resonant oscillations was not properly understood.
Often in engineering we will talk about Factors of Safety, e.g. a bridge may have a Safe Maximum Load of 10 tonnes but has been designed that it could accept a maximum load of 20 tonnes, a Factor of Safety of 2 before failure may be expected. Thus at 10 tonne load the bridge is a long way from its failure point. Part of the reason for the Factors of Safety is that we cannot always be absolutely certain that the materials we are using, or the ground conditions we are working in, will behave exactly like the theory and calculations we are using.
I often think that some of the old bridges have survived so well because the Engineers designing them knew that they did not have all the answers and so allowed massive Factors of Safety. As Engineering knowledge and calculating power have improved, we can become complacent and believe the equations absolutely and so be tempted to reduce the Factor of Safety. We must always be alert to what we may not know for certain. Factors of safety still have their place.