Department of Civil, Structural and Environmental Engineering
Trinity College Dublin
The Schoharie Creek Thruway
bridge collapse
SS structures project
Date of submission: 27th October 2006
Group 18
Cormac Carroll
Laura Dowdall
Patrick Heck
David Steinboim
The Schoharie Creek Thruway bridge collapse
Introduction
On April 5th, 1987, 30 years after it was constructed, two spans of the New York State
Thruway bridge over the Schoharie Creek collapsed. Five vehicles fell into the flooded
river, killing ten people.
Structural History
The Schoharie Bridge is part of the 1:90 highway connecting New York to Buffalo. It
was constructed during the years 1953 and 1954. It is a 4 pier, 5 span road bridge,
totalling 165 metres long. (Figure 1)
112.5'
27.75'
57'
27.75'
Symmetrical about
C
L
Deck
Stringer at
8'-6" o.c.
Floor Beam at
approx. 20' o.c.
Knee Brace
Main Girder
Cantilever Floor
Beam Ends
Bearing
7'-0" sq Column
5'-0" wide X10'-0" deep
Tie Beam
Column
Plinth Reinforcement
Plinth
Footing
Figure 1 - Pier Section ( after "Collapse," 1987 )
Figure 1. Pier Section
In 1955, a year after completion the bridge successfully survived a 100 year flood. It is
thought that this flood may have had a bearing on the collapse three decades later. In
1957 repairs were carried out on the bridge which included, adding plinth reinforcements
to each of the four piers to solve the problem of vertical cracking. It was also noticed that
the expansion bearings were out of plumb, the approach slabs had settled, the roadway
drainage was poor and the supporting material for west embankment dry stone pavement
was deficient. All the above problems were corrected by the end of 1957.
On the morning of April 5, 1987 during the spring flood the bridge collapsed. Rainfall
totalling 150mm along with snowmelt combined to produce an estimated 50 year flood.
Collapse
The collapse started with the toppling of pier three, which in turn caused the collapse of
spans three and four into the flooded creek. Pier two and span two fell ninety minutes
after span three dropped, and pier one and span one fell two hours later.
The main cause of the collapse was the intense scouring of pier three. Scour is defined as
“the removal of sediment from a streambed caused by erosive action of flowing water”. It
created a scour hole approximately 3 metres deep and 9 metres of the pier was
undermined. The pier fell suddenly into the hole, without any warning signs, taking span
3 and span 4 down.
Sloped
Embankment
NORTH
Riprap
Sloped
Embankment
East
Abutment
West Abutment
Flow
PIER 1
100'
SPAN1
PIER 2
110'
SPAN2
PIER 4
PIER 3
120'
SPAN3
110'
SPAN4
100'
SPAN5
Figure 2 - Schematic plan of bridge ( after "Collapse," 1987 )
Figure 2. Schematic plan of bridge (after “Collapse,” 1987)
Figure 3. Effects of Scour on Bridge Piers
Causes of failure
The collapse of the Schoharie Bridge had many contributing factors, including;
Firstly the pier foundations weren’t backfilled appropriately.
The cofferdams were removed, when they should have been left in place.
The riprap was too light-the specifications called for heavier stones to be used.
The foundation of pier 3 was bearing on erodable soil.
The bridge was also poorly maintained – the riprap was failed to be replaced
when recommended.
There was no redundancy in the structure. The spans were not connected together
and the bridge bearings allowed horizontal and vertical movement, causing the
spans to lift or slide off of the concrete piers.
Later modifications of the course of the river directed water downward at the base
of pier 3.
Also an increase in the hydraulic gradient was caused because the Mohawk River
dam downstream was set for winter conditions and was 3 metres lower than in the
1955 flood.
Finally the plinth reinforcement prevented “hinge” movement, therefore resulting
in sudden failure.
How it could have been avoided
To begin with, clear design objectives should have been set out from the start.
The use of the appropriate size of riprap should have been considered, to help
protect the pier.
The cofferdam should have been left in place.
Thorough maintenance should have been carried out following the 1977 study, in
which it was noted that the riprap had been washed away and needed to be
replaced.
The legislation regarding diver inspections should have been implemented.
The bridge could have been supported on piles, which would have resisted scour.
Finally, redundancy should have been added to the structure.
Conclusion
In conclusion, as a result of the Schoharie Bridge collapse, many lessons have been
learned and any similar future disasters can hopefully be avoided.
References:
Levy and Salvadori, “Why buildings fall down” 2002 Norton. ISBN 0-393-31152-X
www.wikipedia.org/wiki/Schoharie_creek
http://www.eng.uab.edu/cee/faculty/ndelatte/case_studies_project/Schoharie.htm
http://mdl.csa.com/partners/viewrecord.php?requester=gs&collection=TRD&recid=A200
42202158CE&q=Schoharie+bridge+collapse%2Cscour&uid=788423659&setcookie=yes