Scotland Documentary – Firth of Forth Episode
On a stormy night of the 28 December 1879, the Tay River railway bridge in Scotland collapsed and many people were killed as the train in which they were riding fell into the river. The bridge was built from cast iron that was not as strong as it should have been, and the design of the bridge was not good enough for strong winds. Thomas Bouch, the designer of the bridge was ready to begin a different bridge, a suspension bridge, over another river mouth called the Firth of Forth.
But following the disaster, the government demanded different design standards for all bridges. It was around this same time that Henry Bessemers’ invention for melting iron, making it stronger by blowing oxygen through and burning out impurities, had been improved by the Siemens open hearth. In this way, iron became steel that was much stronger and more predictable.
So the engineers who designed buildings and bridges for the railways, Benjamin Baker and John Fowler, submitted a design for a bridge
over the Firth of Forth using the new steel. But their new design (for a long bridge) was not just to be the first all steel bridge, and instead of being a trussed beam bridge or a suspension bridge, it was an ingenious design called a cantilever bridge, and the Firth of Forth bridge became one of the wonders of the industrial world as the longest bridge in the world.
122 years later it is still the longest steel cantilever, second only to the Pont de Quebec that passes over the St. Lawrence River. Bridges:
diagram 1 beam bridge showing 2 support posts. The beam sags in the middle with compression and crumpling on the top surface while tension and splitting happens on the underside.
diagram 2 Trusses that make the beam stronger by distributing the force around triangles. Note the trusses in the disastrous Tay bridge were so numerous and top heavy, they acted as sails and collected strong wind that finally “pushed the bridge over”
diagram 3 cantilever design uses the ‘posts’ or towers as ‘fulcrums’ and the trussed beams stick out and act as ‘levers’ pushing the heavy middle up and prevent the sagging. imbed movie? The towers used for the Firth of Forth bridge were built using truss design so they could be made stronger without being too heavy, and balanced by heavy construction on either side of the river to support the ongoing construction far out over the river without collapse.
diagram 4 Components of the trussed towers – compression on the underside, and tension on the tops counter acting the sag forces shown in diag 1 The construction details of the proposed engineering feat was published in Scientific American Journal supplement # 354 Oct 14th 1882 as follows:
The builder was William Arrol. The office and bunkhouse were at Inchgarvie Island Castle seen in the photo underneath the bridge and across the river. photo construction began in 1883 and the piers were in place by 1885. Two years work.
< http://architectural-engineering.blogspot.com/2007/06/firth-of-forth-railway-bridge.html>
Some of the pier foundations had to be excavated from inside diving bells deep under water, and the workers such as James Henry Bellis went down into the sea through a series air-lock chambers that were each locked with compressed air. The same ‘pressure lock’ chambers are typically used to day by astronauts in space.
The bridge is over a mile and a half, using 54 thousand tons of steel, 6 and one half million rivets. The spans are 1710 feet long allowing a height of 361 feet for ships to pass under. More than 4000 men worked on the construction and 57 were killed.
The steel bridge needs to be painted to prevent rust, and the painting has been carried on day after day for the last 120 years continuously.
http://www.forthbridges.org.uk/railbridgemain.htm
Sources :
_____________________________________________________
http://architectural-engineering.blogspot.com/2007/06/firth-of-forth-railway-bridge.html
(People and events)
_____________________________________________________
http://www.pbs.org/wgbh/buildingbig/wonder/structure/firth_of_forth.html
(wonders of the world)
_____________________________________________________
http://access.teachersdomain.org/resources/phy03/sci/phys/mfw/bbcantilever/index.html
(video of Benjamin Baker design)
_____________________________________________________
http://en.wikipedia.org/wiki/Tay_Rail_Bridge
(Tay Bridge disaster and Sir Thomas Bouch)
_____________________________________________________
http://en.wikipedia.org/wiki/Cantilever_bridge
(Principle of Cantilever)
_____________________________________________________
http://en.wikipedia.org/wiki/Inchgarvie
(Castle on Island seen in distance under the bridge – original foundation for the Thomas Bouch bridge)
_____________________________________________________
http://en.structurae.de/photos/index.cfm?JS=158412
Photo
http://en.structurae.de/structures/data/photos.cfm?id=s0000024&min=20
Lots more photos
_____________________________________________________
Video of Baker/Fowler demonstration
______________________________________________________
http://www.sciencetech.technomuses.ca/english/schoolzone/Info_structures.cfm
(good school structure site)
_______________________________________________________
http://www.forthbridges.org.uk/railbridgemain.htm
_______________________________________________________