Chapter 9-Miaus And Schoharie Bridge Flashcards
Why is the deterioration of bridges of concrete and steel a particularly severe problem in Northern regions of the USA, and what two other substances are required for this durability problem to occur
In the northern region of USA the detoriatioon of steel and concrete is due to the salts used to melt snow slowly destroy it. The chlorides in these salt attack the steel casusing oxiadation rusting through galvanic action of an electric current generated in the presence of water.
So water and oxygen cause this problem
In the construction of the Mianus River Bridge there were two suspended spans of 30 m long, on each side of a central span. Figs. 9-2 to 9.4 show details of the suspended spans and their connections to adjacent cantilever spans. Why were the hanger plates, just like the ropes in a child’s swing, required to provide a swing movement from the resting vertical position
The suspended plates had hanger plates in the vertical position to allow the shortening of the hung girder in cold weather and lenghting in the heat of summer. This was deemed unsafe as a failure of a single pin or bar could cause the span to fail.
What was inherently wrong with the design and detailing of the pin and hanger assembly
The pin and hanger assembly was wrong as a failure of a single pin or bar could cause the span to fail. This was due to lack of reducuncey. Bridge that had this were made safer with a steel yoke or sling. This acted as a second hanger in case the first one failures.
The forensic engineering investigation after the collapse of the Mianus River Bridge on 27 June 1983 brought together several experts. They all focused their attention on the hanger assemblies. As is not unusual, each emphasised different root causes and they all reached different conclusions. What modification was made to the bridge ten years before the collapse, and how did this modification adversely affect the durability of the pin and hanger assemblies
10 years before the collapses the modification of a new draining system. Drains to the road way had been paved over therefore allowing the salt, dirt and water to flow through the joints between the cantilever and suspended spans dropping directly onto the hanger assembly causeing accelerated rusting of the steel. This lowered the durability of the bridge
- Due to vehicle accelerations or braking forces on the bridge, the suspended spans would have experienced imposed sideways forces on the four corner supports. Why were the lateral forces in the Mianus River Bridge almost six times larger than the reactions on a similar rectangular bridge (pg. 141, F)? 1
The lateral forces was almost 6 times more due the bridge being at an angle of 54 degrees with respect to the direction of the river.
- Given the reconstruction of the events leading to collapse of the Mianus River Bridge describe (can be with illustrations) three different design changes that you would propose to make the suspended spans safe
Reconstruction of the events leading up to the bridge collapse
* corrsion built up due to the rain
* Every car that passed over the bridge a skew effect imposed a lateral force on the hangers, so the hanger plate was pushed of the pin
* the skew effect and corrosion caused the pin cap to dish put and pop off, then allowing the hanger plate to slip out.
*therefore different material that was corrosive protective
*the bridge angles at a lower angle
*a different design for the ion that connected the bride together
- What forensic engineering method of investigation proved that scouring could easily have occurred at the speeds the creek flowed
A geotechnical investigation of the ground to bedrock showed good resistance to scour. Testing at a university of Cornwall showed increased scouring at speeds of 8 to 10m/s. Pier 3 had the highest velocity flow rate.
Model testing of the bridge using a 1:50 scale showed the water speed was 11.3ft/s
- At the beginning of April 1987, the Schohaire Creek was very swollen to a depth of over 4 m (15 ft), following heavy rain for a day. A lorry with a “heavy” load of paper was the variable load that caused what was then a very substandard bridge to collapse at pier No. 3. The root cause was scouring under the pier in the creek. What is scouring, and how should it have been prevented, if the original design had been followed
Scour is the wearing away of material by running water and currents.
Scouring is when the sand is washed out from underneath the bridge by the fast undercurrents of the river. Due to scouring the underparts of the bridges feet was under suppoted. The depth under the feet at pier 3 was 9 ft which caused the pier to tip up stream into the scoured hole and the bridge girders slope off their support.
In the original design cofferdams which is a steel box was used to ensure that riff raffles was kept in place and was not moved upstream. This would also proctest the pier against corrsion.
If the girders had been continuous ( one piece for the full length of the bridge) one pier would have sagged giving plenty of warning before collapse.
- Why did the staples fitted to the damaged piers in 1955 make the collapse worse than if this remedial work had not been performed
The staples installed in 1955 made the collapse worse as pier 3 was held together so tightly that it prevented a softening. This is a hinge type failure mechanism which would have allowed the plinth to gradually slip into the growing scour hole . This would have given warning before a sudden failure. There was no risk assement made for long term changes that would occur on the bridge.
Also provided a false sense of security
And the staples acted as a concentration of stress
- Given that this is another case study where lives were lost, in part at least, because of a lack of redundancy what form of bridge construction would have prevented the two spans from disappearing into the fast flowing creek
Having a continues concrete box with 5 spans from one abundament to the other. This redundancy would have allowed a slow failure which have be detected early one in the collapse.
- On knowing that the first of the fourteen arches, to the 1841 stone aqueduct over the Schohaire Creek, had collapsed in 1940 as a result of the undermining of its foundation by the water currents, why do you think the responsibility for maintenance of the Schohaire Bridge was always fragmented
Due to serval reasons :
1. Historical precedents- lack of cohesive management
2. Funding and research
3. Jurisidical complexaty
This all lead to gaps in maintance
