The Mianus River Bridge Collapse Introduction

4A6 Case Study Group 11

The Mianus River Bridge Collapse





Introduction:
The Mianus River Bridge is a multi span structure that crosses the Mianus River on Route 95 in Greenwich, Connecticut. Route 95 links Connecticut with New York. The bridge was constructed in 1957 and is a steel deck bridge of welded construction that has 24 spans, 19 of which are approach spans, and is 810 m long. It consists of two, side by side, three lane bridges of cantilever construction.

On the 28^th of June 1983 at 1.30am, a 30m section of the eastbound span collapsed and fell 21m into the River Mianus. The section collapsed as two tractor-trailers and two automobiles crossed the span. Three people were killed and a further three were seriously injured. The timing of the incident was fortunate and there would have been many more deaths had the accident occurred later in the day, as in 1983, the bridge carried more than 100,000 cars daily.

The section that fell was comprised of two 2.7m deep plate girders and four stringers connected by crossbeams. It weighed 450 Mega-grams, including the 190mm thick concrete deck and 50mm thick asphalt topping


Design:
The highway is six lanes wide across the bridge, but an expansion joint on the centreline that runs the length of the bridge separates the structure into two parallel bridges with three lanes on each that act independently of each other. The five spans over the River Mianus are symmetrically arranged around a 62.5m central main span. On either side of the main span there is a 30m suspended span as well as a 36.6m anchor span near each bank. Both the anchor span and the main span are cantilevered 13.7m beyond their piers and the suspended span is connected between them by using a pin and hanger system. This system was widely used in the 1950’s as it reduced construction costs.


The design of the suspended span consisted of two different pin assemblies. On the western edge, where it joined to the cantilevered arm of the main span, a pillow block cradle pin system without hangers was used. But on the northeast and southeast corners where the suspended span joined to the anchor span a pin and hanger system was used. The spans were joined together by a pin in the 63.5 mm web of each girder, with the upper one attached to the cantilever arm of the anchor span and the lower level arm attached to the suspended span. The two pins were connected on either side of the web by a steel hanger. These hangers allowed expansion and contraction of the hung girder due to temperature differences.

The bridge was structurally determinate as there were no redundant members. This meant that if a member failed, the entire structure would fail. This was an inherent design flaw which was only realised in 1968 in the aftermath of the collapse of the Silver Bridge that spans the Ohio River, resulting in the deaths of 46 people.

Failure:
The bridge failure started to occur when the inside hanger at the southwest corner of the suspended span dislodged from the end of the lower pin, due to the formation of corrosion in the assembly. This meant that all the force on the eastern end of the span was transferred to the cantilever by the outside hanger only, doubling the load it had to take. The outside hanger gradually moved outward on the pin and deformed. This caused fatigue cracks to form at the top of the upper pin. After repeated loading and unloading of the slab from the traffic, the upper pin fractured, causing the suspended span to fail and fall into the river.

Reasons for Failure:
There were a number of major causes for the failure of the bridge:

1. 
   The hanger initially failed due to a high level of corrosion present in the pin. This occurred due to maintenance works which were carried out on the bridge ten years prior to the collapse in which a number of drainage ducts were paved over. This forced the water to drain down over the hangers and thus cause the gradual formation of corrosion. Test data showed that the forces caused by corrosion alone were enough to cause the failure of the pin and hanger.
   
2. 
   Maintenance and inspection of the bridge was very poorly resourced, which meant that obvious signs of failure were either missed or not acted upon. In 1982 the state of Connecticut had only 12 engineers working in pairs to inspect 3,425 bridges. In the last inspection before the collapse, minor corrosion on the bridge was noted, although no action was taken. The engineers could only inspect the underside of the bridge using binoculars while standing on the bank and therefore were unable to spot the corrosion occurring in the pin and hanger.
   
3. 
   The fact that it was a flawed design. There was no level of redundancy employed in the slab connection, so when the first hanger failed, it was only a matter of time until the slab collapsed.
   

Conclusions:

1. 
   The Connecticut Department of Transport was judged to have been responsible for the collapse as a result of deficient inspection. The designers of the bridge were cleared of all responsibility by the Connecticut Supreme Court.
   
2. 
   The interstate was not fully reopened for six months, and then only with a temporary truss. In total, final repair costs were over US$20 million.
   
3. 
   The U.S. Department of Transport made a large number of recommendations from strengthening state inspections to establishing a federal inspection enforcement program. This led to bridges of a similar design being carefully inspected and prioritised for retrofit projects.
   
4. 
   Following the incident, the governor of Connecticut initiated a US$5.5 billion transportation spending package to pay for rehabilitation and replacement of bridges in Connecticut.
   
5. 
   However these measures most likely only had a temporary effect, as in 2003, more than one of every four bridges in the U.S. were designated as structurally deficient or functionally obsolete (USA Today, August 3^rd 2007).
   

References:

• 
  United States of America National Transportation Board Highway Accident Report: Collapse of a Suspended Span of Route 95 Highway Bridge over the Mianus River, Greenwich, Connecticut, June 28^th 1983; adopted on July 19^th 1984.
  
• 
  Construction Failure by Jacob Field & Kenneth L. Carper.
  

Published by John Wiley and Sons, 1997.

• 
  Bridge Engineering by Demetrios E. Tonias & Jim J. Zhao.
  

Published by McGraw-Hill Professional, 2006.

• 
  Connecticut Turnpike Historic Review.
  

http://www.nycroads.com/roads/ct-turnpike/

Retrieved at: 21.57, 20^th October 2008.

• 
  http://blogs.usatoday.com/oped/2007/08/a-bridge-in-ame.html
  

Retrieved at: 17.14, 21^st October 2008.

Keith Kelly Cathal Dalton

Craig Reddin Fearghal Ryan Froylan Cordoza