Suspension Bridges - Tacoma Narrows Bridge
The over confidence in design, and lack of proper awareness of earlier problems eventually resulted in the classic failure of the Tacoma Narrows Bridge in the USA in 1940.
The first modern major suspension bridges was Thomas Telford's Menai Strait Suspension Bridge completed in 1825. Telford used a heavy suspension chain of flat wrought iron bars to support the 580 foot (178m) font between the masonry towers. He undertook a number of trials to test these chains and noting the yield and failure loads. So that in his design, he allowed a factor of safety of 1.5 against yield and 3 against ultimate failure - quite satisfactory design criteria.
However, he neglected to pay sufficient attention to how the light and flexible roadway might behave in strong winds. The first roadway was blown away during a storm in 1839. It was then reconstructed in a stronger form, and has gone through several reconstructions since. The stonework approaches and towers still exist supporting the modern suspension bridge which presently graces the crossing.
Such failures of the roadways of suspension bridges in severe storms was not uncommon, so suspension bridges were not considered reliable, and certainly not suitable for the heavier rail loads.
However, this did not discourage the American bridge designer, John Roebling, from using the suspension bridge. He considered the form the most economic for long fonts, and was confident he could safely design them for rail loads.
John Roebling studied hydraulics and bridge construction amongst other subjects at the Polytechnic Institution in Berlin in the late 1820's. He moved to USA in 1831, and his first "construction" experience was as a wire rope manufacturer. This experience in making wire ropes gave him additional confidence to design and build long font suspension bridges.
For some years he had been involved in the design and construction of suspension bridges for canals. However, when he proposed an 800 foot (244m) suspension bridge to carry trains over the Niagara Gorge, a 1000 foot (305m) bridge over the Ohio River, or a 1500 foot (457m) bridge over the East River between Brooklyn and New York, he was faced (in the 1840's) with the view that suspension bridges were considered very undependable, if not unsafe.
Roebling carried out considerable research on both successful bridges, and those that failed - in particular the behaviour of bridges in high winds. He concluded that by using stiffer decks, with more suspension cables and anchoring stays, he could overcome the problems of vibration and instability. Roebling's success was not by using more sophisticated theory or more careful calculations, but by concentrating his design judgement on how his bridges might fail - and how to prevent that. Along with this, went attention to the quality of materials and construction.
His Niagara Gorge Suspension Bridge was completed in the mid 1850's, the Ohio Bridge shortly afterwards. His bridge career culminated in the completion of his famous Brooklyn Bridge in 1883. This bridge is still in use.
Confidence in Suspension Bridges continued and then soared with the completion in the 1930's of the Golden Gate, the Bronx-Whitestone and the George Washington Bridges. Suspension bridges became progressively lighter and more slender and graceful, so that even longer, slenderer and lighter suspension bridges were being conceived and constructed.
This design evolution was to culminate in the failure of the Tacoma Narrows Bridge. The Bronx-Whitestone (NY) and the Golden Gate Bridges which preceded the Tacoma Narrows Bridge, both exhibited wind induced oscillations and required modifications to bring them to acceptable and safe loads.
The Tacoma Narrows Bridge, designed by Leon Moisseiff, with a centre font of 1800 feet (853m) and side fonts of 1100 feet (335m) was on completion in mid - 1940 the third largest suspension bridge in the world. Its deck had a slenderness ratio of 1:350, nearly three times that the George Washington and Golden gate Bridges. It has been nicknamed "Galloping Gertie" even before it had opened by people who had experienced its oscillations, even in relatively light winds.
Before and after its completion a number of remedial measures were applied - tie down cables, which snapped soon after installation, inclined stay cables and dynamic dampers, none of which produced any significant improvements. Then in relatively mild winds of 58 to 67 km/hour it developed undamped oscillations which built up to an amplitude of 7.5m eventually tearing the bridge apart.
The bridges undulations and ultimate failure became one of history's most documented disasters.
It was wind vortex shedding that caused tensional oscillations in the bridge deck.
The failure of the bridge resulted in the development of the engineering discipline of aerodynamics, to investigate in greater depth, the effects of vortex shedding and flutter.
To engineers, the Tacoma Narrows were more of an economic challenge than an technological challenge. A bridge here would cut at least 40 miles off the trip between Tacoma and Bremerton. But traffic estimates were low. In 1932, the federal government refused to give financial support for a bridge for that reason. It took major lobbying to get the money. Even then, though, it was only $3 million. So the state scrapped its first design - $11 million - and turned to Leon Moisseiff. Moisseiff, a well-regarded designer and researcher, calculated that a considerably lighter deck -stiffened only by a thin plate girder, not a deep truss - would do, thus cutting the cost to only $6.4 million. Everyone knows what happened: less than a year into its life, the bridge began to twist violently in a 40 mph wind, and soon collapsed. The disaster - which took no human lives - shocked the engineering community and did a lot to shape American bridge building. The bridge was replaced with a very conservative design in 1950.