The Milford Haven
Box Girder Disaster
A Report by Group 9
Gary Donohoe
Joseph Gill
Aisling Kearney
James Thompson
Introduction
Construction of The Milford Haven bridge began in June 1968 using steel box girders,
but later collapsed in June 1970.
The Box Girder
A box girder is a very suitable method of bridge construction. The shape of the girder
makes its inherently strong. In bridge construction box girders can be bolted or
welded together to form a section which behaves like a single beam. This presents
solutions to designers as when used appropriately it can created a very strong and stiff
beam.
The Disaster
Many box girder beams are assembled “piece by piece” ie. sections are bolted to one
another, and in this way the beam moves across the span. However while being
assembled a cantilever effect can develop between the piers. At this stage the stresses
are quite different of what might be expected on completion of the bridge. This led to
the failure of the Milford haven bridge as during the cantilever construction stage the
section buckled resulting in four deaths.
Reason for Collapse
Box girder sections made of wrought iron plating were quite popular in Victorian time
but poor quality control in the production of steel meant that designers had to grossly
increase the size of box sections to avoid a structural failure. Much improved steel
producing techniques during the 1960’s and 70’s meant that designers could be more
confident of the strengths of steel being consistent and so section sizes could be
reduced. This led to a renaissance in the use of steel box girder sections in the 70’s as
this type of construction became more economic.
At the time the stresses which arise in this situation were not fully appreciated. The
advent of computers has made our lives easier and this also applies to structural
calculations. As a box girder section is wide and deep the interaction and distribution
of stress in the member is quite complicated. Modern day computing power makes it
quite easy to calculate these values but in the 1970’s these calculations were not
possible as the computing power was quite primitive by today’s standards.
The major problem leading to the structural failure of the Milford Haven was that
there was no substantial stiffening of the box sections other than at the pier support’s
of the bridge. This meant that during construction the box section developed very
large moments causing it to buckle about the pier it was being constructed from.
The Bridge was later completed in 1974
Actions taken as a Result of the Milford Haven Collapse
This and two other box girder bridge disasters led to the formation of the Merrison
Commission which looked at the adequacy of existing codes in relation to box girders.
They set down guidelines for stress analysis and the way in which box girders were to
be connected. These were formalised in BS 5400 and led to the creation of specialised
sections dealing with box girders. Other recommendations included clarifying the
roles of the engineer and contractor, such as vetting of design and construction
methods. It was recommended that the design rules should be used only by suitably
experienced designers.
Conclusion
This failure occurred due to the fact that the engineers were pushing the boundaries of
their knowledge at the time of this incident. The disaster led to a better understanding
of the complexities of steel box girder design. Armed with this knowledge, more
powerful computers, and the updated codes, we are hopeful that a disaster like this
will not happen again.
Key References
1. Inquiry into the Basis of Design and Method of Erection of Steel-Box Girder
Bridges. Report of the Committee. HMSO.1973.
2. Report of the Royal Commission into the Failure of West Gate Bridge. Government
of the State of Victoria. 1971.
3. http://www.istructe.org/technical/db/281.asp