4A6-1 Disaster Case StudyGroup 9

Milford Haven: Steel Box Girder Collapse

Alan HuntAlan Hunt

Danielle PickettDanielle Pickett

Ed LawlorEd Lawlor

Michael CirilloMichael Cirillo

Milford Haven Bridge• Spans 820m across River Cleddau between Neyland

& Pembroke Dock, Wales.• Joint Consulting Engineers: Sir Alexander Gibb &

Partners and Freeman Cox & Partners• Contractor: A.E. Farr Ltd.• Construction began beginning 1969.• During 1970, steel box girder failed during placement

of final girder piece of 4th span.• 4 workers were killed and 5 others were seriously

injured.• Redesign carried out and was completed in 1975.

Bridge Design

• Primary structural members are steel box section girders.

• Greater 2nd Moment of Area than standard I-Beam ⇒better resistance to bending.

• Particularly important near a costal span (such as Milford Haven) with wind loadings.

• Girder of choice for large spans.

Bridge Construction• Consists of 48 box girder sections welded and bolted

together.• These box girders form a continuous section that behaves like

a beam.• During construction the welded box girders behave as a

cantilever until the pier opposite is reached.• Huge bending moments exist in termination of box section at

pier.

Collapse & Failure

• Collapse occurred due to buckling of the support diaphragm.

• Consequently, the distance between flanges decreased buckling of lower flange⇒

• Design Load = 5kN, Load at Failure = 9.7kN

Results and Effects

• BS153 were inadequate for construction of Steel Box Girders at that time.

• IDWR were created by the Merrison Committee, creating new rules for Steel Box Girder Design and Analysis.

• Foundation for the New British Standards, BS5400