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Half measures


It was distressing to read of the Montreal road bridge collapse and the fatalities associated with the use of half-joints (NCE 5 October).

In the UK early half-jointed suspended spans on motorway bridges were often adopted to create an articulated statically determinate structure on pad footings accommodating differential settlement. These could be readily analysed with the aid of a slide-rule.

Codes of practice at the time did not contain specific rules for the design of such details. In-house rules were not developed at the firm where I worked from photoelastic models. Some years later the opportunity arose to load test to failure a half-scale model of one of our designs, which demonstrated the robust 'belt-and-braces' nature of the design that contained horizontal, vertical and inclined reinforcement.

However, despite this reassurance, neither the detrimental efforts of future chloride attack nor the inherent difficulties in inspecting halfjoints were fully recognised until much later.

Ken Weir (M ret), 40 Brookside Glen, Chesterfield, Derbyshire S40 3PF

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