Two tragic co lapses of sophisticated designs of trapezoidal-box-girder bridges, resulting in the death of 4 men and 35 men, respectively, occurred during erection in 1970. The first of these was the Milford Haven Bridge. Pembrokeshire, in which a 196-foot (60-metre) steel cantilever collapsed in June 1970-owing towing failure of the vertical steel diaphragm over the pier from which it was being cantilevered. The second was the collapse of a 3(i7-foot (112-metre) steel span of the West Gate high level bridge over the Lower Yarra River in Melbourne, in October 1970. Failing at the middle, the span plunged to the water. bringing down the river pier in its collapse. Failures have also occurred on concrete bridges, one of the most recent, in which two men were killed, in August 1967, being the collapse of two completed sections of the Older Bridge, which was being built to carry the M. 1, motorway over the River Calder near Wakefield. Yorkshire. Here the collapse was caused by the failure of temporary supports due to mild-steel joists being used by mistake instead of joists of high-tensile steel.
In order to combat risk of brittle fracture, high-tensile steel has now been greatly improved in strength at low temperatures. Another most complex problem under research is that of metal fatigue, to which both steel and aluminum are prone, caused by undergoing continual variations of stress that reduce ultimate strength and cause cracks or failure. Susceptibility to fatigue is increased by welding; particularly, by welds across tension members; or by an abrupt change of shape or thickness. All possible data are being obtained as to the number and nature of the stress cycles to which bridges are subject and to types of construction and the quality of steel that can withstand them.
As a result of the tragic collapse of the box-girder bridges at Milford Haven and Melbourne, the government in Great Britain has appointed a team of experts to enquire into the safety of this type of bridge, which has been described as pioneer work pushed toward the limits of the engineer's knowledge. and to examine the design and construction methods.
Accidents caused by failure of plant are too varied to detail here. They can be largely avoided by ensuring that all plant and equipment is kept in good repair and used only within its capacity, with all moving parts adequately guarded and with fail-safe devices provided as necessary. The causes of accidents to personnel are also diverse. The first necessity is to provide safe means of access and safe place of work, by means of ladders, gangways, and working platforms with guardrails, toeboards,and cradles, as necessary. Accidents to men working aloft can be prevented by means of safety harness and safety nets; safety helmets should be worn by all men on site