Milford Haven Bridge Collapse Page |1
4A6 Disaster Study
Milford Haven-Steel Box Girder Collapse
Group 9
Shane Corr
Jenny Cusack
Ian Kavanagh
Eamonn McManus
Milford Haven Bridge Collapse Page |2
Introduction:
Milford Haven or Cleddau Bridge as it now known was a very notable structural collapse. Victorian
engineers built large tabular bridges of riveted wrought iron plating but this died out for some time.
However, in the 1960’s this form of bridge design was revived, due to the development of high
grades of steel, welding and fabrication techniques. Since the Milford Haven Bridge had to span over
700 metres, a steel box girder design of trapezoidal cross section was seen as ideal. The trapezoidal
top box was 6 m deep, 20 m wide at the top, and 6.67 m wide at the bottom. It was designed to be
continuous over 7 spans from 76.8 to 213.4 m and was claimed to be Europe’s longest bridge
without cable support.
Steel Box Girders:
Steel Box Girders are typically rectangular or trapezoidal in cross section, They are mostly used for
flyovers, usually a form of beam bridge. However, they may also be used on cable stayed bridges.
When compared to I-Beams Box, girders have much higher resistance to torsion (particular benefit if
plan of deck is curved). Larger girders can be constructed because the presence of two or more
webs, increasing the I-value of the beam, which allows wider, and hence, stronger flanges, in turn
allowing longer spans. Some disadvantages are that they are more expensive to fabricate, and
difficult to maintain, as there is a need for access to a confined space inside the box.
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Bridge Construction:
The superstructure of the bridge was constructed by welding and bolting the trapezoidal sections to
each abutment. After this each section was welded to the previous one. The box sections were
therefore, cantilevered across the span until it reached the next support. This construction method
is adequate for the loads after the structure is completed, however during construction very large
bending moments are present at the supports.
Failure:
The first span was erected without any problems, but when the second span on the south side was
being cantilevered from pier 1 to pier 2, the lower flange buckled resulting in the failure of the
cantilevered portion. The steel diaphragm above pier 1 failed in compression causing the lower
flange to buckle, bend, and fall to the ground, killing four workers and injuring five. A committee of
inquiry called the Merrison Committee was set up to investigate the cause of the collapse. It found
that the cause was the inadequacy of the design of the pier support diaphragm. It emerged that a
diaphragm of half the thickness of which was specified was used in the construction of the pier. It
also concluded that BS 153 (the only British steel bridge design and construction Code of Practice
current in the 1960's) was insufficient for such an application; there was a need to consider wider
implications for the design of such bridges. The support diaphragm was subjected to a hogging and
bending moment as well as a very large vertical shear force. A combination of forces affected the
diaphragm in the bottom corners. The shear of the transverse girder, and diffusion of the point load
from the bearings, was compounded with the effects of inclination of the webs of the main bridge
girder, which produced an additional horizontal compression action, and out-of-plane bending
effects caused by bearing eccentricity. It was reported, just before failure that the diaphragm
sustained a load of 9700kN. The design resistances calculated were considerably less, possibly as low
as 5000kN.
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Consequences of failure:
The Merrison Committee found that the rules for steel box girder design (BS 153) were inadequate.
They also introduced the Interim Design and Workmanship Rules (IDWR). These contain a
comprehensive set of rules for the stress analysis for box girders and the design of steel plated
components in complex fields. These rules in turn laid the groundwork for a new British Standard
(BS5400) dealing with box girder design. Along with further research, a comprehensive set of rules
(BS 5400, Parts 3, 6 and 10) dealing with steel box girder design was decided upon.
References:
IStructE - The Institution of Structural Engineers
Cleddau Bridge - Wikipedia