3 Case study: The Silver Bridge
3.1 Background information
Stress corrosion cracking can produce devastating damage in large structures, as the examples of swimming-pool ceilings and roofs in Section 2 showed. But even larger structures can also be attacked, as was revealed by the events at the Silver Bridge in 1967.
The Silver Bridge spanned the Ohio river between West Virginia and Ohio. It was a long bridge (680 m), owing to the breadth of the river at Point Pleasant, the small settlement on the east bank where one end was sited. It was nearly 12 metres wide in total, carrying a concrete road two lanes in width as well as a footpath. It had been built in 1928 alongside an older railway bridge, but to a strikingly different design (Figure 27).
It was effectively a suspension bridge, but one that used eye bars and rigid hangers to support the deck, rather than the familiar steel cables of modern suspension bridges. The main chain system was not unlike a bicycle chain, each structural unit being a set of steel links held together laterally on a pin (Figure 28). Such a design had been used previously on the Brighton chain pier, Telford's Menai Straits bridge and Brunel's Clifton bridge in the Victorian period, although in those cases the links were generally made of three or more bars, rather than just two. The Clifton bridge possessed no fewer than six such bars strapped together.
There had previously been problems with suspension bridge roadways that were too flexible, and several bridges had been severely damaged, if not destroyed, by lateral winds: see Box 4: Historic suspension bridge failures .
Box 4: Historic suspension bridge failures
Failures of suspension bridges (see Table 3) have most frequently involved failure of the suspended deck rather than of the support structure. A famous early failure occurred in the Brighton chain pier in 1836, when a storm destroyed the deck (Figure 29). Built in 1822, the pier was 352 m long and 3.9 m wide. Five cast-iron towers, spaced 78 m apart, supported the decks. The pier, which served for embarkation of new ferry services to France after the Napoleonic wars, was damaged many times by storms and then rebuilt, until it was finally demolished in 1896.
Table 3 Suspension bridge failures
|Bridge||Location||Year of failure||Main span/m||Width of deck/m||Designer|
|Dryburgh Abbey||Scotland||1818||79||1.2||J. and W. Smith|
|Nassau||Germany||1834||75||—||Lossen and Wolf|
|Brighton Chain Pier||England||1836||78||3.9||Samuel Brown|
|Menai Straits||Wales||1839||177||7.3||Thomas Telford|
|Roche-Bernard||Scotland||1852||79||1.2||J. and W. Smith|
|Tacoma Narrows||USA||1940||854||11.9||Leon Moisseiff|
Perhaps more famous was the bridge built by Thomas Telford over the Menai Straits in 1826 (the deck of which was also damaged severely on several occasions, especially in 1839). The Angers disaster in France was the most serious failure, since 226 soldiers were killed when the bridge fell. Corrosion of the anchors of the main cables was one cause of the disaster.
The Clifton suspension bridge was built to a design by Isambard Kingdom Brunel, but was not completed until 1864, after his death. Despite having been damaged in the past, the Menai Straits and Clifton bridges still stand today. Both use many eye-bar chains to support the deck, giving high redundancy if one fails. However, many other suspension bridges failed as designers produced longer and longer decks, culminating in the famous collapse of the bridge over the Tacoma Narrows.
The Tacoma bridge was completely wrecked in 1940 (although the towers and main cables remained intact). The failure occurred through dynamic stimulus of the very long deck by a steady wind of approximately 40 mph blowing at right angles to the axis of the deck. But rather than oscillating from side to side, it started rolling up and down as it resonated. The amplitude rose steadily until failure of the deck occurred, and then other parts followed (Figure 30). The accident was the culmination of a sequence of similar incidents in which sub-critical oscillations occurred (which gave the bridge the nickname ‘Galloping Gertie’). The failure highlighted the problem of the effect of winds on very large structures. Large buildings, such as skyscrapers, routinely have giant pistons fitted within to damp movements caused by high winds.
In the Silver Bridge there was a small clearance of about 3 mm between the pins and eye-bar holes, to allow easy fitting of the parts together when on site. The links were made from a high-strength steel that had been developed in the 1920s by the American Bridge Company. The eye bars had been cast to shape and then heat treated to develop the strength of the steel; then the holes were drilled out.
The roadway of the bridge was trussed with steel girders to improve its rigidity. There were two separate towers supporting the main suspension chains, each 40 m high; each tower consisted of four tiers that were cross-braced except for the road gap (Figure 31). The towers were supported on two massive masonry piers anchored in the river bed, and the two sides supported by two further piers, one in the river bed and the other on land on the Ohio side of the river.
The main suspension chains were anchored in concrete troughs on each bank, each trough being pinned by 405 concrete piles to resist the tension exerted by the main chains. Each of the two towers was designed to move with temperature fluctuations in the chains by having curved rocker joints at its base on the masonry piers.
The bridge was inspected in 1954 and remedial work on decaying concrete recommended, together with painting of the metalwork. It was inspected again (after the remedial work had been finished) in 1955, 1961 and 1965.
Describe the types of load carried by:
(a) the towers
(b) the roadway
(c) each eye-bar link
(d) the pin through each joint.
Assuming that there was no traffic on the bridge, and no other external loading, where in the bridge were the eye-bar joints likely to be under the greatest load?
(a) The towers support all the suspension chains, the hangers and the trussed roadway. This load puts the tower structure into compression.
(b) The roadway is supporting its self-weight and any traffic, and it will be in bending.
(c) Each eye bar is under tension, supporting the tension in the main chain at either side, and the downward-pointing tension from the hanger.
(d) The pin is in a state of bending from the successive tension loads from the separate bars in the joint.
The top eye-bar joints should be under greatest tensile load because they support the complete length of chain below. The load diminishes at successively lower eye-bar joints.
The bridge appeared to fulfil its function well, despite the loading on the bridge increasing steadily after it was built with the increase in car population and road traffic. The bridge had been built at a time when the Model T Ford was the most popular automobile on the roads, but by the 1960s cars were significantly larger and heavier, further increasing the load on the bridge.
Corrosion of the steel structure of the Silver Bridge was likely, especially as the acidity of the rain was enhanced locally by industry nearby. So the entire structure was painted with an aluminium-based paint, hence the name ‘silver’ bridge. An example of how corrosion can lead to failure of a bridge structure is described in Box 5: Fall of the Kinzua Viaduct.
Box 5: Fall of the Kinzua Viaduct
Many historic bridges and viaducts show the effects of long-term corrosion, which can often lead to catastrophic failure. Such a failure happened to the Kinzua Viaduct during the passage of a large storm through western Pennsylvania in July 2003. The Kinzua Viaduct had been built from cast and wrought iron by the New York, Lake Erie and Western Railroad Company in 1882 to deliver coal from local coalfields to the Great Lakes. It was reconstructed in steel in 1900 (Figure 32) to allow for the greater loads on the structure.
The viaduct was retired from active service in 1959, but the structure remained as the centrepiece of a national park, taking occasional traffic from sightseeing tours. However, the bridge required extensive renovation owing to rusting and was closed again to traffic. Renovation was started in February 2003, but on 21 July 2003 an unusually severe storm hit the region. During the storm, 11 of the viaduct's 20 towers fell, destroying the landmark (Figure 33).
Forensic investigation of the fall showed that the centre of the structure had oscillated from side to side as two tornados (with wind speeds of about 100 mph) struck the site. It was estimated that about four complete cycles occurred before its collapse.
Final failure had occurred at the rusted base bolts holding the structure to the foundations, which had fatigue cracked over a long period of time (Figure 34), assisted by internal rusting.
The lattice superstructure fell in several parts, the railway line ending up hundreds of feet away. The failure bears some similarity to the classic fall of the Tay Bridge in a severe storm in 1879, the main difference lying in the way the joints in the structure behaved. Most of the cast-iron joints of the Tay Bridge failed either before or during the fall, so the railway line ended up close to the foundations. The steel joints at Kinzua remained mostly intact.
The Kinzua bridge collapse is covered in the ‘Kinzua's weakest link’ video link below.
Kinzua's weakest link
Transcript: Kinzua's weakest link