Repair work became necessary following discovery of fatigue damage of many of the cross girders and their connections in the Boston Manor viaduct on the M4 motorway in London. This resulted in some hundreds of cracked fillet welds and broken bolts being discovered within the first 10 years of life. The prime cause was racking of the cross girders under traffic loading as a result of relative longitudinal movement between the concrete deck slab and the main girders. TWI was appointed as technical advisor for the repair.
Monitoring confirmed that it was heavy goods vehicles, loaded to their maximum 30-35 tonnes, which contributed most of the damage. Although such vehicles only caused stress fluctuations of 15-25N/mm2 in the main girders, the racking action referred to induced stresses as high as 100-200N/mm2 in the cross girders.
Various options for repair were considered which involved major alteration to the structure. Ways of reducing the stresses by changing the articulation of the structure were considered but were all rejected on grounds of direct cost and traffic disruption. The solution had to take into account not only the immediate cost of repair and of traffic disruption, but also future costs (appropriately discounted) of inspection and further repair.
The simplest solution of cutting out and repairing the fatigue cracks was rejected because it would not cure the problem and repair costs would recur regularly.
The solution finally adopted in consultation with TWI had three main elements:
.Replace web to flange fillet welds by butt welds thus increasing joint strength.
.Cut some shear connectors, also increasing flexibility and thus reducing some of the highest stress.
.Stabilise the more flexible deck sections by providing additional bracing between deck and main girders.
A pilot repair programme involving three plate girders was monitored by TWI using strain and deflection gauges. TWI was then responsible for the design of all structural modifications, monitoring performance and specifying the scope of the repair programme, which was carried out with minimum disruption to traffic over an 18 month period.
The consequential savings were estimated to be approximately £2M.
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