Group 7:
Stephen Cantrell
Chris Hodge
Chris McKeown
Peter Weldon
Milford Haven
Pembrokeshire
Wales
Cleddau River
 Pembroke Dock and Neyland separated by 1 mile of
water, but a journey of 28 miles was required by road
 Ferries operated up until 1975
Conception
 1960s: Decision to replace Ferry with a Bridge
 Sir Alexander Gibb and Partners and Freeman Fox and
Partners appointed as joint consulting engineers
 Contract awarded to A.E. Farr Limited in September
1968.
 Initial Value: £2.1 million
Bridge
 820 meter span
 Daily traffic: 10905 vehicles
 Original opening time:
March 1971
 Box Girder Design
Origins of the Box Girder
 Aftermath of WWII
 Bailey Bridge insufficient
 Follow on from I-beams
Box Girder Design
 Main beams comprise girders
in the shape of a hollow box
 Comprises of pre-stressed
concrete, structural steel or a
composite of steel and
reinforced concrete
 Either rectangular or
trapezoidal cross-section
 Very good in torsion.
Failure
 June 2nd, 1970
 Constructed by cantilevering.
 Occurred as one of the final
sections was being moved
out.
 70 metre cantilever putting
135 tonne section into place
collapsed.
 The extra loading from the
cantilevering caused the
collapse.
Failure
 The diaphragm of the box girder was
not designed to carry the extra load
and buckled.
 Diaphragm torn away from web.
 Lever arm reduced.
 Bottom flange
then buckled.
Failure
 4 workers killed, 5 injured.
 Could have been much
worse.
 Bridge passed virtually over
a village
 Debris and girder sections
missed houses by feet.
 Construction halted until
October 1972
Merrison Committee
 Committee of Inquiry into the Design and Erection of
Steel Box Girder Bridges
 Findings:
 BS153 was inadequate for applications like Cleddau
 Inadequacy of the design of a pier support diaphragm
 “the failure of site organisation between the parties
[was] of more general significance”
 Interim Design and Workmanship Rules, which
formed basis of BS5400, Parts 3, 6 and 10
Completion
 Final Cost: £11.7 million (more than 5 times initial
estimate)
 £7 million due to design changes
 Opened to traffic March 1975
 885,900 crossings in first year of operation
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-structuralfailure/Pages/steel-box-girder-bridges.aspx, referencing: Merrison Committee of Inquiry
(1973) - Inquiry into the Basis of Design and Method of Erection of Steel Box Girder
Bridges.
 http://www.geograph.org.uk/photo/845071
 http://www.newcrossing.gov.bm/photo-library/pages/engineering-options-2.html
 http://www.istructe.org/knowledge/topic_areas/learning-from-structuralfailure/Pages/steel-box-girder-bridges.aspx
 http://theconstructor.org/structures/box-girder-specifications-advantagesdisadvantages/2166/
Any Questions?
Thanks for listening!