4A6(1) Disaster Report –
Milford Haven Bridge
Group 7:
Stephen Cantrell
Chris Hodge
Chris McKeown
Peter Weldon
4A6(1) Disaster Report – Milford Haven Bridge
Introduction:
Our Group’s case study was the Milford Haven Bridge, which collapsed in South
Wales in 1970. The bridge was of a steel box girder design, and collapsed due to errors with
the box girder. Originally conceived to be completed in 1971, work was halted for over two
years due to the collapse and the bridge finally opened in 1975, when it was renamed the
Cleddau Bridge.
This type of bridge was most prominent in the years subsequent to WWII, as military
powers realized that they needed to cross much larger spans, with bridges they could
quickly construct. This meant that they needed to come up with a design which had a much
lower depth (and hence weight) than the current setup they were using, which was basically
a grouping of I-beams fixed together side-by-side.
The box girder they
developed can effectively be
considered to be an I-beam with
two webs. The beauty of this is
that it allows a much wider span
on the flanges, and also thicker
flanges can be used. The closed
‘cell’, as it is known, gives this
cross section much better
torsional stiffness and strength
than its competitors. The
cantilevers on the top flange were
only introduced with the advent of higher grade steels. The bottom tie (or flange) manages
to alleviate the induced stress from these additions. Whilst this cross section is sufficient for
spans, at the pier supports, it requires further bracing. This can be in two main forms, either
thickening of the diaphragms, or addition of vertical stiffeners above the supports.
Why Did the Milford Haven Bridge Collapse?:
The collapse of the Milford Haven Bridge occurred on the 2nd of June 1970. This boxgirder bridge was constructed from sections of steel box girders that were prefabricated and
moved over the previously built sections, aligned in place and welded. Thus this bridge was
erected by cantilevering. With this method of construction the cross frame above a pier
support suffers extra loading due to the cantilevering part.
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4A6(1) Disaster Report – Milford Haven Bridge
The collapse occurred as one of the final sections
of the second span was being moved out along the
cantilever. This section slid forward down the cantilever,
buckled at the support and collapsed into the river
killing four men, including the site-engineer.
Therefore it was the extra loading due to the
cantilevering method used that caused the failure. This
type of loading should not cause any problems,
provided that the diaphragm of the girder has been
designed to carry this extra load. Unfortunately this was
not the case in this situation.
There was no substantial stiffening of the section
other than the six pier supports and abutments, and an
investigation of the failure showed that the collapse was due to the buckling of the
diaphragm at the support of the second span. The diaphragm was torn away from the
sloping web near the bottom. This caused reduction in the lever arm between flanges
resisting negative bending moment at the support. The reduction of this distance increased
the tendency of the bottom flange to buckle, as this increased the force needed in each
flange to carry the moment with the reduced lever arm.
The total load transmitted by the diaphragm to the bearings just before collapse was
computed as 9700kN. This load would not have caused any problem provided the
diaphragm was designed to carry it. Allowing for likely values of distortion and residual
stress, the calculated design strength was found to be as low as 5000kN. Thus, the failure
was essentially due to design inadequacy of a pier support diaphragm.
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4A6(1) Disaster Report – Milford Haven Bridge
The Outcomes of the Milford Haven Bridge Collapse:
Following the collapse of the Milford Haven Bridge, the Merrison Committee of
Inquiry into the Design and Erection of Steel Box Girder Bridges was set up and tasked with
investigating the cause of the disaster. The committee found in 1973 that the cause of the
failure was, indeed, inadequate design of the pier support diaphragm, but also considered
"the failure of site organisation between the parties as of more general significance".
The committee’s opinion was that BS153 (the only British steel bridge design and
construction Code of Practice current in the 1960's) was inadequate for applications like the
Milford Haven bridge, and thought that there was a need to consider wider implications for
the construction of steel box girder bridges. Because of this, the Merrison Committee
prepared Interim Design and Workmanship Rules. These dealt with the design of steel
plated components and their connections, and the effects of welding residual stresses and
geometric imperfections linked to fabrication tolerances.
These rules, along with further research, formed the basis of a new British Standard
BS5400, parts 3, 6, and 10. This British Standard contains comprehensive rules for steel box
girders, and the implementation of the recommendations contained in the report led to
wide-ranging changes in contractual procedures and in checking designs and construction
procedures.
Work was halted after the accident while the Merrison Committee investigated, and
was only restarted in October of 1972, and opened to traffic in March 1975, with the new
name the Cleddau Bridge. The final cost was £11.83million. The original contract was
awarded in 1968 for £2.1 million. £7 million of the overspend was attributed to changes in
the design to comply with the Merrison recommendations. £3 million of this came from an
out of court settlement with between the County Council and the consulting engineers. This
serves as a prime example of how errors in design and insufficient communication can be
very costly for engineers.
The collapse is regarded as the last major bridge disaster in the United Kingdom.
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4A6(1) Disaster Report – Milford Haven Bridge
References:
- Department of the Environment (Merrison Committee of Inquiry) (1973) - Inquiry into the Basis of
Design and Method of Erection of Steel Box Girder Bridges.
- Bridle, Ron; Porter, John (2002) - The Motorway Achievement: Frontiers of Knowledge and Practice.
Thomas Telford Ltd. pp. 346–348. ISBN 978-0727731975.
- http://www.istructe.org/knowledge/topic_areas/learning-from-structural-failure/Pages/steel-boxgirder-bridges.aspx, referencing: Merrison Committee of Inquiry (1973) - Inquiry into the Basis of
Design and Method of Erection of Steel Box Girder Bridges.
Picture 1: http://www.geograph.org.uk/photo/845071
Picture 2: http://www.newcrossing.gov.bm/photo-library/pages/engineering-options-2.html
Picture 3: http://www.istructe.org/knowledge/topic_areas/learning-from-structuralfailure/Pages/steel-box-girder-bridges.aspx
Picture 4: http://theconstructor.org/structures/box-girder-specifications-advantagesdisadvantages/2166/
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