Introduction to structural integrity
Introduction to structural integrity

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Introduction to structural integrity

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).

Figure 27
Figure 27 The Silver Bridge

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.

Figure 28
Figure 28 Section of an eye-bar joint in the Silver Bridge

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.

Figure 29
Figure 29 Brighton chain pier after storm damage in 1836

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
Montrose Scotland 1838 132 7.9 Samuel Brown
Menai Straits Wales 1839 177 7.3 Thomas Telford
Angers France 1850 110 8.0 Joseph Chaley
Roche-Bernard Scotland 1852 79 1.2 J. and W. Smith
Wheeling USA 1854 309 7.3 Charles Ellet
Lewiston-Queenston USA 1864 318 5.9 Edward Serrell
Niagara-Clifton USA 1889 386 5.2 Samuel Keefer
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.

Figure 30
Figure 30 Sagging of approach at the Tacoma bridge after fall of centre deck, showing intact main cables

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.

Figure 31
Figure 31 Tower of Silver Bridge

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.

Figure 32
Figure 32 Kinzua Viaduct with reconstruction in progress

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).

Figure 33
Figure 33 Aerial view of the collapse

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.

Figure 34
Figure 34 Cracked base-bolt shroud

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

Part 1

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Transcript: Kinzua's weakest link

Francesca Hunt
The Allegheny National Forest in north-western Pennsylvania is the site of the Kinzua rail viaduct. First built out of wrought iron in 1882, it was at one time the highest bridge in the world. In 1900 it was rebuilt using steel and then remained open to rail traffic for over 100 years.
Trestle 'space frame' bridges like this were exploited to open up the American West. They were fast to build, coming in kit form, with a railroad deck supported by towers which were anchored firmly into the ground.
But on July the 21st 2003 catastrophe befell the viaduct. The centre of this huge metal structure collapsed when it was hit by a tornado – despite having been designed to resist high winds.
Thankfully, nobody was hurt, but other than that saving grace it was an engineer's worst nightmare come true. In fact, the forensic investigation to find out what went on here has also been able to cast more light on the causes of another disaster, over 120 years earlier: the collapse of the central section of the Tay rail bridge in Scotland – with 75 people killed, the worst ever disaster to befall a trestle bridge. A failure in strong winds led to the disintegration of its metal towers but the manner of failure has been a matter of controversy. We'll be returning to this later.
The debris at Kinzua is destined to remain where it fell. But let's take a look at how it came to be built.
The viaduct has been of huge importance to the area for many years. The Director of the vacation bureau in the Allegheny National Forest is Linda Devlin...
Linda Devlin
We promote travel and tourism into the Allegheny National Forest region, and we're located in north-western Pennsylvania, and it's a very rural beautiful part of... of the State of Pennsylvania.
When the bridge was first constructed in 1882 it was done with the idea of commerce, but it immediately became a tourist attraction, so on the weekends you would have excursions coming in from Buffalo and Pittsburgh, because it gave the sensation of flying.
In 1900 the bridge was rebuilt to address the need to carry heavier loads and larger trains. The Big Boy train, which was the largest locomotive ever utilized, was now going to be used on this particular viaduct, so the... the bridge structure was rebuilt out of steel.
It was done in less than 94 days with 125 men. And what they did was they actually started on both sides, replacing the previous wrought iron with steel structures, working towards the middle. And a major decision that was made at that time was not to replace the anchor bolts that anchored into the cement structures on the bottom.
In the new trestle bridge they not only changed the materials from wrought iron to steel, they also changed the original diagonal bracing by replacing it with a much stronger and stiffer form, incorporating multiple elements as additional trussing. But no matter what the structure, the state of any bridge needs to be checked continuously...
Gene Comoss
My name's Gene Comoss; I'm the Chief Engineer for the Pennsylvania Department of Conservation and Natural Resources.
My staff of engineers had a general concern about the condition of the bridge. For a long time, we felt a full inspection was needed to determine its structural stability.
The need to inspect the many hundreds of old metal trestle bridges in the USA is a major problem. Another spectacular example is to be found here in south-west Pennsylvania, at Mingo Creek. The trestle bridge here, like all trestle bridges, needs inspection and maintenance. What's involved in inspecting bridges of this sort? Tom Leech is a bridge engineer...
Tom Leech
One important thing that an engineer does is inspect the bridges on a periodic basis. Here are some of the things we would look at. We look critically at the girders, especially for corrosion, where the girders connect to the towers, that connection, that critical base, that bearing. Corrosion is something's that's very important. As we look to the tower bases, we look at all the joints, we examine every single joint. The most critical joints are those joints which are at the base of the towers.
In 2000, the Kinzua Viaduct was inspected. What inspectors found then was many instances of large corrosion holes in the structure, other deterioration at the joints and they closed the structure for public safety.
It became obvious that a lot of the corrosion damage was due to our inability to keep water and dampness out the structure. For example, in each tower leg, there was a stiffener plate placed at each connection point, and the original designer had the foresight to put drain holes, but obviously, early on, the small drain holes clogged, and once they clogged, these, uh, stiffener plates became places for moisture to collect, and it was at these stiffener plates where the most severe corrosion damage occurred.
The W.M. Brode company specializes in bridge construction; Steve Brode is Vice President...
Steve Brode
We were the contractor hired to do the emergency steel repairs to the Kinzua Viaduct.
Our contract for the refurbishment of the bridge was focused when we started mainly on the steel repairs in the towers, uh, where they had severe deterioration. In particular, at the different joints where everything framed together, there was a lot of deterioration because of some horizontal angles that held the water, and our go... our role was to develop, uh, shop drawings, uh, and create replacement pieces in actually strengthening plates at those locations. Some locations required us to replace pieces in kind. There were some moon-shaped gussets that we had fabricated locally to replace in kind, but most of it, again, was in the towers themselves, nothing really with the girders.
When the structure was designed to withstand the prevailing winds, the design provided for the towers on the west side of the structure to be fixed with anchor bolts into the pedestal bases. On the, uh, east side, uh, the towers were fitted with roller bearings to allow for some movement either from wind load or thermal expansion.
Faint b/g Francesca (as US weather girl)
An intensive system of severe weather is moving into Pennsylvania from Ohio. There are reports that this system is packing tornados … among winds of well over 50 miles per hour, the system is tracking North by North West.
When the tornado struck the bridge repairs were approximately 50% complete. As a result of the tornado striking the bridge, uh, the Department convened a board of inquiry to investigate the cause of the failure.
I led the Board of Inquiry investigation. Our first task was to assemble a professional team. Our task consisted of forensic engineers from my firm, Gannett Fleming. We also engaged the Meteorological Department of Pennsylvania State University and the fracture, uh, engineers of Atlas Lehigh University.
In the course of the investigation, we looked for forensic markers. What we disclosed at the site was four specific forensic markers. The four markers we found were order markers. Order markers are looking for clues of what member is on top of what member. We looked for directional markers; directional markers such as the orientation of the trees that collapsed, the direction of the debris field. We noticed separation markers, clean separation of the superstructure from the substructure. And finally we looked for fracture markers: telltale signs of cracking in members that may have precipitated the collapse.
Finding forensic markers like these are a routine way of investigating large structural failures. There was, however, another piece of evidence that was nearly missed...
One important part of our investigation was making good use of the high-resolution aerial photography. As we were trying to determine precisely the sequence of collapse, I was viewing on my computer screen the high resolution photography and I noticed, quite suddenly, large skid marks. These are the marks where the towers actually had gone airborne, slid down the hill and made large depressions in the ground. These depressions were unnoticed by any of us who had wandered all over the site during our day of our Board of Inquiry investigation. These were crucial in determining the precise sequence of collapse.
The testimony of workers at the bridge site was important in determining the sequence of events as the tornado struck. As they were retreating at the time the tornado hit they could only report on what they heard.
Some employees had no idea anything had happened. This gentleman, here, only talks about raining and thunder and lightning, uh, but Mr Quillin's report, in particular, is … has some more information. Uh, he says he had sent the crew home because of weather and he was leaving the site when the winds picked up, and he had a crew member with him, and they were leaving the site 'when I heard four to five loud booms. I wasn't sure what the booming was...' uh, but at that time he went and determined that part of the bridge had actually fallen down.
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Part 2

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We determined the sequence of events where the collapse in three distinct episodes. Let me describe those episodes. During the first episode, as the tornado touched down and moved in a northerly direction, leading edge winds affected the structure. The first group of towers occurred a separation failure at their base and were displaced a little to the west and were held, momentarily, in an upright, uh, position by the rails that were still attached to the towers. The next group of towers as the tornado moved northward were affected by the leading edge winds, separated at the base and toppled in a westerly direction, followed by the northernmost section of towers which similarly fell in a westerly direction.
Finally, as the inflow winds attacked the structure, the southerly group of towers then collapsed in a northerly direction.
The booms which were heard were almost certainly the sound of each group of towers hitting the ground. But the bridge was designed to resist lateral tornadostrength winds of up to 100 miles per hour, so what caused the towers to collapse?
In the structural design of the towers, the load was transmitted from the desk down through the tower legs to the pedestal bases and the tower legs were fastened to the pedestals by, uh, use of an anchor bolt that was embedded in the pedestals and then bolted to the tower legs.
In the pedestal bases, higher up on the flanks of the gorge, the original base bolts from the 1882 bridge were of sufficient length to bolt the towers down. But this wasn't so down at the bottom of the gorge, where the towers failed, where the bolts had been modified ...
In the towers that failed, anchor bolts similar to this anchor bolt was the principal cause of failure. Uh, the reason for the failure was the method, the designers of the 1900 reconstruction chose to fasten the towers to the, uh, 1882 pedestals. In ... in their fastening, they extended the anchor bolt by approximately six inches and then fastened that six-inch extension to the original anchor bolts by use of shims and a coupling.
So the weakest link in the load path occurred where the bolts were lengthened.
To the original bolt embedded in the base, spacing shims were added and then a wrought-iron collar, or coupling, was attached. Into this the additional length of bolt, required for tying-down the new towers, was screwed into place.
These joints were unprotected; water percolated into the collars leading to corrosion fatigue as the rust expanded – resulting in vertical cracking.
Evidence recorded by the inquiry, shows the degree of the corrosion suffered by the collars.
Cracks like this weren't safety critical because the base of a tower will have been held down by gravity, but they became the weakest link with very high lateral wind loads.
There's an interesting comparison between the collapse of the Kinzua Viaduct and the Tay Bridge collapse. Both bridges exhibited the same four forensic markers.
Another interesting fact observed from the historical photographs are the towers on either side of the collapse. Both towers reveal clear evidence of a high degree of oscillation that occurred during the collapse cycle.
At the Tay Bridge, each of the towers which fell collapsed in winds much lighter than those that hit the Kinzua viaduct. Originally, they looked like this. Being a vertical column, it has considerably less lateral stability than an inclined tower, such as the towers at Kinzua. It's a top-heavy structure, because the wrought iron deck weighed nearly four times as much as the towers.
The use of cast-iron lugs to connect struts and tie bars to the columns was wrong, because they cracked when strained cyclically. The way the lugs must have behaved when the towers oscillated can be contrasted, at Kinzua, by the strength of the riveted joints, which kept each fallen tower pretty much intact.
Let's consider the collapse of one individual tower. As the leading edge winds affected the tower, the tower immediately started to vibrate at its natural frequency. As the tower was attacked by the wind, the wind increased in intensity, the tower vibrated more, and when a sufficient wind speed was generated the towers experienced a separation failure at the base, on the windward side.
The tower then became airborne and collapsed in the direction of the wind, and experienced large-scale fractures and deformation upon impact.
Conversely, at the Tay Bridge, the towers fell down more like a house of cards.
Tie bars, both upwind and downwind, were seen to have failed in the towers still standing. The inference has to be that the tie bars failed as the result of the fracture of the brittle, cast-iron connecting lugs.
This is what's surmised happened in the collapse cycle:
Once a high girder tower started to oscillate, the weakest links in the load path became the lugs;
As the oscillations grew in amplitude, the lugs failed in succession. Struts, also held by lugs, similarly failed... , until the tower separated into two... , which then collapsed together, with some toppling, into the estuary.
The remains of the old bridge, just the piers, were left in place and in fact they are still visible today running alongside the replacement Tay rail bridge.
Voices off-screen
What's actually going to happen at the end of the tower? Well, right now...
Kinzua's not going to be replaced, but there's an ongoing debate about its future...
One of the ideas that had been presented was also using maybe a laser light show to recreate the structure visually at night... is that going to be possible, or...?
Well actually when the idea was initially proposed we thought it was a little far out but since then other...
Whatever plans win the day, in one way or another, the viaduct will be preserved as a tribute to an engineering masterpiece – albeit one that met its match when the forces of nature exposed the weakest link in the viaduct's redesign.
But the forensic investigation hasn't just answered questions about what happened at Kinzua and Tay in terms of the dynamic oscillations that both those bridges experienced. More importantly, the findings from any disaster have to be made known as widely as possible, so that remedial action can be taken to prevent similar catastrophes...
Trestle bridges, such as the Kinzua Viaduct, with such a high height and relatively narrow base, are what we've termed wind-susceptible structures, and these are at risk certainly from high wind events. We have made many public presentations on this topic, and as a result of these presentations we have been contacted by owners of other structures enquiring to the manner of collapse and what these folks need to do with their structures.
So, clearly, trestle bridges have to be continually inspected and maintained throughout their lives – but, on open structures like these, any corrosion is at least visible to inspection and so can be treated.
But solid, monoblock structures, or concrete towers, rely on internal steel reinforcing bars. And yet corrosion in the steel reinforcing bars is the weakest link within concrete structures too. Any corrosion damage which occurs is not apparent at all and may only be seen if a bridge cracks – due to the internal pressure of the expansion of corroding metal – at which point a bridge may be doomed.
At Mingo Creek the new road bridge spans an existing trestle in a dramatic juxtaposition of old and new technology. What lessons have gone into the construction of the new bridge with regard to minimizing corrosion and particularly the problems associated with drainage, further exacerbated here by the need to add highly corrosive salt to the road in winter?
There's a wonderful contrast between this new interstate-class structure and the existing viaduct structure. I was the principal designer for the new structure. My design team took corrosion into account in the following manner.
All the structural elements are weathering steel, in tight patina forms that protects the steel from corrosion. We look carefully at the joints, especially at the bearings.
We take the expansion joints and extend them to the very ends of the structure to prohibit the salt penetration.
The work of the design engineer today is crucial to the integrity of any structure, but especially bridges and other structures exposed to the external environment. It's only by careful consideration of the detailed design of the safety critical joints that the long term integrity of structures, such as these, will be assured for their users.
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