Milford Haven Bridge Collapse
June 2, 1970
Group 9
The Milford Haven bridge Collapse
Introduction
This report analyzes and discusses the construction and possible
failures of a box-girder supported bridge. In particular the report
focuses on the Milford Haven Bridge in Wales which collapsed during
construction resulting in four deaths in June 1970.
Box-Sections
The typical box girder has two webs and two flanges (illustration
#1.) However, in some cases there are more than two webs, creating
a multiple chamber box girder.
(Illustration #1)
The added second web in a box girder adds stability and
increases resistance to twisting forces. As all the material in a BoxSection is at the extremes of its cross-section the second moment of
area is much greater than that of other section types. Extra stiffening
along the length to prevent buckling can be achieved by the addition of
flanges and diaphragms. Bamboo stems use this same principle as can
be seen in illustration #2.
(Illustration#2)
The forces experienced by box girder bridges are inherently
complex to calculate using traditional methods as it is a three
dimensional problem comprised of narrow members joined by hinges.
Finite element analysis methods using modern computers can now
compute the forces with relative ease.
Bridge construction using Box-Sections
By making a beam out of short sections with wide flanges or closed walls at
the ends, a bridge can be assembled by bolting or welding such sections end
to end. Such a bridge can be cantilevered out from the piers. When the ends
meet, and are joined, with suitable adjustments of the stresses, the result
can be a very strong and rigid beam.
The problem with this Idea is in construction when the beams are
cantilevered out from the piers of the bridge. The stresses on the bridge here
are very different and often much larger than experienced after completion.
This has led to several collapses in the construction stage of such bridges
including the collapse of the Milford Haven Bridge.
The Milford Haven Bridge
The superstructure of the bridge consists of a deep continuous trapezoidal
welded steel girder. The original design consisted of a single continuous box
girder of welded steel with no substantial stiffening of the section other than
the six pier supports and abutments. This design does not take advantage of
the stiffing effects of flanges and diaphragms and so is subject to buckling
due to bending moment especially in a cantilevered state. The construction of
the bridge was carried out using the technique described in the previous
section. However there where no wide flanges or closed walls at the ends of
the individual sections. The spans measured from the south 77m, 77m, 77m,
149m, 213m, 149m and 77m.
The Collapse
The collapse occurred during
launching a pre-assembled deck
section towards the second pier
from one end. The span that
collapsed was the second 77m
span on the south side, the first
having been erected with the aid of
a temporary support. The collapse
occurred when the last section of box for the second span was being
moved out along the cantilever. When the collapse occurred this
section slid forward down the cantilever killing four men.
Failure was initiated by
buckling of the support
diaphragm at the root of the
cantilever being erected
(Figure 3). The diaphragm
was torn away from the
sloping webs near the
bottom of the box, allowing
buckling of the lower web
and bottom flange to take
place. As the diaphragm
buckled, it shortened,
reducing the overall depth of
the box girder; the tendency of the bottom flange to buckle was
inevitably increased by this reduction of the distance between flanges
which increased the force needed in each flange to carry the moment
with a reduced lever arm.
The support diaphragm was in fact a transverse plate girder, which
carried heavy loads from the webs at its extreme ends, and was
supported by the bearings as shown in Figure 3, some distance from
its ends.
It was therefore subjected to:
1) A hogging bending moment
2) A large vertical shear force
The diaphragm plate near the outer bottom corners was subject to a
complex combination of actions:
1) 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.
2) And an out-of-plane bending effects caused by bearing
eccentricity.
The load sustained by the diaphragm just before failure was reported
to be nearly 9700kN, which agrees tolerably well with independent
calculations of strength made after the accident. The calculated design
resistance, using design rules that were drafted subsequently and
making allowance for likely values of distortion and residual stresses
would be considerably less, possibly as low as 5000kN.
Result of the Collapse
A Committee of Inquiry into the Design and Erection of Steel Box
Girder Bridges (1) concluded that the cause of the collapse of Milford
Haven Bridge was the inadequacy of the design of a pier support
diaphragm. It was of the opinion that BS153 (the only British steel
bridge design and construction Code of Practice current in the 1960's)
was inadequate for such application and that there was need to
consider wider implications for the design of such bridges.
The Merrison Committee prepared Interim Design and Workmanship
Rules (IDWR). These contained comprehensive rules for the stress
analysis for box girders and for the design of steel plated components
in complex stress fields, with and without stiffening, and for their
connections.
The bridge was eventually rebuilt as a cantilever and suspended span
in the main span.
Further failures of similar bridges around the world occurred. These
bridges are, in chronological order:




The Fourth Danube Bridge in Vienna (Austria, 6 November 1969)
The Milford Haven Bridge (United Kingdom, 2 June 1970)
The West Gate Bridge in Melbourne (Australia, 15 October 1970)
The Rhine Bridge in Koblenz (West Germany, 10 November
1971).
And although no two of the failures where alike a new British Standard
was introduced, based largely on the Merrison Rules. The Merrison
Rules laid the foundations for the production of a new British Standard,
BS5400, Parts 3, 6 and 10 of which, with the additional benefit of
further research, now contain comprehensive rules for steel box
girders. The implementation of the recommendations led to wideranging changes in contractual procedures and in checking designs and
construction procedures.
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.