4A6 (1) Structural Design Project
The Schoharie Bridge Collapse, Scour
By
Group 18:
Brendan Cleary
John Arrigan
Ciaran Reilly
Joe Kelly
4th Year Civil Engineering
Submitted on the 28th of October 2005
1. Introduction
Civil engineering disasters have occurred since engineering itself ‘began’. Several
examples exist in modern history of such disasters and today’s civil engineers aim to use
these previous failures to improve their understanding of civil engineering structures and
hopefully prevent similar disasters in the future.
This paper describes the collapse of the Schoharie Creek Bridge, in 1987, in the state of
New York in the USA. This event triggered significant change in the field of bridge
engineering and some of the lessons learned from the collapse are discussed below. This
paper is not intended to be an in-depth analysis of the bridge’s collapse.
1
2. Structural Details
The Schoharie Creek Bridge was designed to the 1949, American Association of State
Highway Officials, ‘Standard Specification for Highway Bridges’. The final design in
January 1952 specified the bridge at 165m long, with 5 simply supported spans. The
bridge was supported by 4 concrete piers, 2 on the creek banks and 2 on shallow footings
in the Schoharie Creek.
The concrete piers each consisted of 2 columns and tie beams, with the columns fixed
into a lightly reinforced plinth, all positioned on a shallow reinforced spread footing.
2
3. History of the Bridge
The Schoharie Creek Bridge was fully completed just after October 1954. Within a year
it survived a 100 year flood, but it is thought damage suffered by the bridge may have
influenced the eventual collapse in 1987.
The as-built plans for the bridge did not reflect its true condition. The plans indicated that
sheet piling had been left in place to protect the piers. However, it had really been
removed after construction.
There were other problems encountered within the bridge’s first year. The concrete pier
plinths began to form vertical cracks, ranging from 3 to 5 mm wide. These cracks were a
result of high tensile stresses in the concrete plinth as the plinth could not resist the
bending stresses between the two columns.
It was discovered that insufficient reinforcement had been added to the plinth. This
problem was correct in 1957 with the addition of further reinforcement so as to prevent
the vertical cracking.
3
4. Collapse
The Schoharie Creek Bridge collapsed on the morning of the 5th of April 1987 during a
Spring flood. The flood consisted of 150mm rainfall and melted snow to produce an
estimated 50 year flood [1].
The collapse was triggered by the toppling of pier three, which in turn caused the collapse
of spans three and four into the flooded creek as shown in Fig.1 below. Pier two and span
two fell ninety minutes after span three dropped, and pier one and span one shifted two
hours after that . It was suggested that pier two collapsed because the wreckage of pier
three and the two spans partially blocked the river, therefore redirecting the water to pier
two and increasing the stream velocity [1].
Fig.1 Schematic plan of bridge after “Collapse”
4
4. Causes of Collapse
The main cause of collapse of the Schoharie Creek Bridge was due to the extensive scour
under pier three. Scour is the removal of sediment from a streambed caused by the
erosive action of flowing water. This was allowed to occur because of the inadequate
riprap (broken stones to prevent erosion) around the base of the piers and the relatively
shallow foundations which were not deep enough to resist scour.
Other important factors which caused the collapse of the Schoharie Creek Bridge were:

The shallow footings used were bearing on soil which could be undermined.
Therefore the depth of the footings was not sufficient to take the probable limit of
scour.

The foundation of pier 3 was bearing on erodable soil and not riprap therefore
this allowed the floodwaters to penetrate the “bearing stratum”

The as-built plans did not match the true condition of the bridge. Sheet piles that
were to be left in place to protect the piers had been removed after construction.

The plinths were initially under reinforced and the additional reinforcement that
was added was inadequately anchored which contributed to sudden brittle failure.
Other factors which contributed to the collapse of the Schoharie Creek Bridge were
the expansion bearings were out of line, the roadway approach slabs had settled, the
roadway drainage was poor and the supporting material for the west embankment dry
stone pavement was deficient [1].
5
5. Lessons Learnt
Many lessons can be learnt from the collapse of the Schoharie Bridge.
Had the design specifications been strictly followed, the scour which caused the bridge to
collapse would not have occurred to such an extent.
The design called for leaving the sheet piles in place around the piers after construction
was complete with the areas between the piers and the sheet piles to be filled with riprap.
However, the sheet piles were removed when construction was completed leaving the
pier footings vulnerable to scour.
Adequate inspection and maintenance of the bridge was not carried out during its
lifespan. In New York State, bridge inspections were required on at least a biannual basis.
These inspections were performed, however underwater inspections which were required
at least once every five years weren’t carried out once in the bridge’s lifespan (from
1954- 1987 when failure occurred).
The deck of the bridge consisted of simple spans. Had continuous spans been used the
bridge may not have collapsed immediately after the first pier failure, allowing enough
time for people to evacuate the bridge safely before it failed.
The riprap surrounding the bridge piers to protect them from scouring was not monitored
satisfactorily. A report by Thornton-Tomasetti suggested “where riprap is used to prevent
scour, inspection of protective riprap should be performed after every major flood… and
the replacement stones used as riprap should be heavier then those which were observed
to shift [2].
6
Overall the collapse brought several key lessons to light:

The importance of designing footings deep enough to avoid loss of support due to
scour and the provision of adequate protection for the footings.

The need to frequently perform underwater inspection of bridge piers.

The need to anticipate how changes in the river flow (due to the construction of
embankments etc. upstream of the bridge) can affect the hydraulic loading on the
bridge and increase the scour on the piers.

The importance of designing the bridge for rare intense flooding like the 100yr
storm it survived in 1955.

The importance of following the designers specifications (here in the case of the
sheet piles being removed after construction was complete when the designers
specifically stated they were to be left in place for scour protection).
7
6. References
1. Lessons from the Collapse of the Schoharie Creek Bridge. [HTML], San Diego,
California. (October 19th, 2003), accessed on the 22nd of October 2005, available
@
http://www.eng.uab.edu/cee/faculty/ndelatte/case_studies_project/Schoharie.htm
2. Thornton-Tomasetti, P. C. (1987) “Overview Report Investigation of the New
York State Thruway Schoharie Creek Bridge Collapse.” Prepared for: New York
State Disaster Preparedness Commission, December.
8
9