Modern structures are designed not to fail suddenly, but to distort extensively, showing great distress before they collapse.
It is unlikely, therefore, that a bridge will totally collapse under a single over-loading. Repeated over-loading causes cracking and gradual disintegration and reduces the life of the bridge.
By inspecting bridges at regular intervals, early warning signs can be picked up and a detailed investigation carried out.
Some bridges show no signs of disintegration, but calculations show them to be inadequate. Weaknesses do not always manifest themselves in the early stages of failure, particularly if the critical material is brittle. This does not mean that a serious situation does not exist, as these bridges are often at the greatest risk.
It is unlikely that funds will ever be available to repair and strengthen all inadequate bridges. Therefore, some form of prioritisation is required, which takes into account the risk presented to the stability of the structure, the likelihood of overloading and the effect that this would have on the public. Risk assessment does this. Risk is equal to the product of the likelihood of an unwanted event multiplied by the consequences of such an event. We can see that if no vehicles use the bridge, no risk exists and that risk increases as the number of vehicles increases. It can also be seen that if the consequence of an event is insignificant then the risk is negligible. By using risk assessments in this way the funds are targeted where they are most needed.
Contract Partnering
In order to minimise the inconvenience and disruption to the general public, maintenance and strengthening schemes often involve night-time or restricted hours working.
In addition, bridgework is not always specific, as the extent of the problems cannot be fully identified until works actually commence. Contractors tend to view such maintenance work as high risk.
As an alternative to the normal tendering procedure, partnering agreements or contracts may be used for more complex work. The advantages of partnering include risk being taken away from the contractor, reduced consultancy fees for preparation of documents, appraisals and site supervision. Recent partnership arrangements were used on the major maintenance work on the historic Huntingdon Town Bridge (Ancient Monument).
Parish Partnering
Many minor non-structural repairs attract a high overhead cost. The Parish Partnering scheme avoids this by encouraging parish and town councils to undertake this work. Several parishes have been interested in taking part in a pilot schemes. Work done in this way will be restricted to minor works, usually on footbridges, and a full advice service will be available.