Lecture 16 Flashcards
- What two differences in the design of the Point Pleasant Bridge completed in 1929 was the embodiment of risk? What basic requirements were ignored in its design, which meant that, when any one component failed, the bridge would collapse in the catastrophic manner it did on 15 December 1967
It was the first ‘eyebar’ suspension bridge
* the first US bridge to use a recently developed material – high-strength, heat-treated carbon steel. As such, the bridge was the embodiment of risk; it was a new structure, on a new scale, using new materials.
* There was no structural redundancy or robustness in the bridge’s design!
* As a single eyebar resulted in the whole bridge collapse
* The way the structs, pins and eyebars were all so interdependent meant that total collapse was almost a built-in designed feature. Any one component failure meant everything failed.
- As the pieces of Point Pleasant Bridge were recovered from the water and randomly dumped in a 27-acre field nearby, the scale of the forensic engineering investigation would have become obvious. Eliminating the usual suspects was going to be complex, not least because much of the evidence had been interfered with. List the usual suspects and state what good practice the investigating team followed that enabled John Burnett of the US Bureau of Standards to identify eyebar 330 as the source of the fracture that caused the disaster
- The usual suspects are overloading, age, fatigue, an impact, aerodynamic instability and corrosion
- They collected data from the river of the debris found
- Fortunately each piece of the debris had been photographed as it was taken out of the river.
- John Burnett of the US Bureau of Standards (now NIST) soon focused on a photograph of an eyebar that had sheared in half, and not fractured through the pin
- By examination by eye, and later in the laboratory using techniques in the forensic engineering toolkit, what signs did the failed eyebar 330 reveal, and how did they prove it was the critical component in the bridge’s collapse
The sheared eyebar 330 suggested failure was NOT as a result of overloading. Had there been too much weight, the joint would have failed in the pin. This eliminated a major possible cause since traffic ‘live’ loads had increased hugely since the bridge had been designed forty years earlier. As there was only 31 vechiles on the bridge at the time.
* The piece was examined in the laboratory using techniques in the forensic engineering toolkit (see slides 7-22 to 7-25). A tiny area (1.8 mm long and 3.5 mm deep) of deep encrusted rust was observed. Given the slow speed with which high strength steel rusts the crack (defect) went back to the forging of steel in 1926
* It had been a brittle, sudden fracture, the type you get in breaking glass. But the highstrength steel from which the eyebars had been forged had plenty of ductility (slide 3- 12). This was evident from the distorted and fractured eyebar that was opposite to 330 on the other side of the bridge.
* Microscope inspection of the eyebars found a number of minute cracks, as small as pinholes. One had grown inside eyebar 330 and had remained invisible to periodic inspections. Over the years the crack had slowly grown to it critical length.
- Why did the investigators have to consider that the failure of eyebar 330 was due to overloading? And how was this disproved
- The sheared eyebar 330 suggested failure was NOT as a result of overloading. Had there been too much weight, the joint would have failed in the pin. This eliminated a major possible cause since traffic ‘live’ loads had increased hugely since the bridge had been designed forty years earlier. With only 31 vehicles on the bridge at the time collapse overloading was never going to be the root cause.
- As the eyebar was examined and had tiny cracks in its surface which was embed in rust
- What role did the weather condition on 15 December 1967 play in making a small crack, of size 1.8 mm long and 3.5 mm deep, in eyebar 330 be the critical size for fast brittle fracture? For the high-strength steel what could have been the difference between the fracture energy at 0oC and 74oC? And, if the temperature at Point Pleasant was constant at 23oC, how many years might it have taken the same “pinhole” crack to grow to the critical size under the same adverse environmental conditions
- Weather conditions where snow and cold.
- The small gap on the eye bar made it easy for pollunates to get in and cause corrosion
- At 0oC (the temperature at the time of the failure) the fracture energy of the steel was 1/3rd the value for a temperature of 74oC. For an ambient temperature of 23oC it might have taken a millennium of years for the “pinhole” crack to grow to the critical size.
- These flaws were the product of a quenching and tempering process designed to minimise the brittleness of the steel, to make it more ductile, and thus more resistant to stress.
- In the 1920s they just did not know that the relatively slow cooling which occurs in a 51 mm thick piece results in a steel that has quite good strength under static loads but is very sensitive to brittle fracture. Nor did they know that their bridge had poor durability
- What caused a pinhole sized crack from the steel forging to slowly grow to its critical size over forty years? And what feature of the design of the eyebar made matters worse, and why
- A tiny area (1.8 mm long and 3.5 mm deep) of deep encrusted rust was observed. Given the slow speed with which high strength steel rusts the crack (defect) went back to the forging of steel in 1926
- Microscope inspection of the eyebars found a number of minute cracks, as small as pinholes. One had grown inside eyebar 330 and had remained invisible to periodic inspections. Over the years the crack had slowly grown to it critical length.
- Growth was by stress corrosion – the combined effect of the stress and pollutants that can eat away at steel, causing corrosion. There were plenty of local candidates for the pollutants. Corrosion had always been a major problem at Point Pleasant, hence the bridge’s anti-rust silver- coloured preservative.
