Severn Bridge

HC Deb 16 June 1977 vol 933 cc259-61W

Mr. Wigley

asked the Secretary of State for Transport what is the percentage of total traffic represented by lorries that use the Severn Bridge; and how this compares with the lorry percentage assumed at the time of the design of the bridge.

Mr. Horam

Heavy goods vehicles using the Severn Bridge average about 20 per cent. of the total traffic, rising to about 27 per cent. at peak periods. This compares with about 15 per cent. on highways generally. The Severn Bridge was designed for a standard loading which is derived from the effects of a mix of 22, 10 and 5-ton vehicles.

Mr. Wigley

asked the Secretary of State for Transport what was the average weight of lorries assumed at the time of the design of the Severn Bridge; and what is the present estimated averaged weight of lorries using the bridge.

Mr. Horam

The Severn Bridge was designed for standard loading derived from the effect of closely spaced vehicles weighing 22, 10 and 5 tons. There is no statistical data predating the design of the Severn Bridge from which an average vehicle weight can be derived. From a recent typical sample, the average weight of all commercial vehicles was 9½ tonnes and that of four-axle articulated vehicles was 24 tonnes.

Mr. Wigley

asked the Secretary of State for Transport what was the assumed total weight of static vehicles in a jam-situation on the Severn Bridge at the time of its design assuming the jam holds up traffic in one direction only; what is the actual weight of static vehicles on present traffic patterns; and what percentage difference this makes to the torque on the rockers of the bridge caused by eccentric loading.

Mr. Horam

Two lanes of standard loading assumed in the design of the Severn Bridge would weigh 651 tons (600 tonnes) for a 1,000-foot (305 metres) long jam in one direction. A recent estimate of the weight of a typical 1,000 foot (305 metres) long traffic jam, with lorries only in the slow lane and cars only in the fast lane, is 523 tons (531 tonnes). The rocker loads on Severn Bridge do not relate solely to the total weight of the vehicles but result from a complicated interaction of the amount and eccentricity of the loading, the stiffness of the box deck and the stiffness of the hangers, so that the resultant load on the rocker in the second case exceeds the first by approximately 50 per cent.

Mr. Wigley

asked the Secretary of State for Transport what surveys have been undertaken by his Department on the effects on the rockers on the Severn Bridge caused by eccentric loading of heavy traffic in the slow lane.

Mr. Horam

The deficiencies in the rocker supports were revealed during the recent appraisal of the bridge against the Merrison criteria. No signs of distress have been observed in the rockers themselves.

Mr. Wigley

asked the Secretary of State for Transport what studies have been undertaken by his Department into metal fatigue in the Severn Bridge; what have been the results of such studies; and what action has been taken as a result.

Mr. Horam

The Department has been aware of the problems of metal fatigue on Severn Bridge since its inception and has studies in hand to determine the most economical and effective method of repairing fatigue cracks as they occur.

Continuing observation has recently revealed a very small number of cracks in the deck welds and arrangements for repair are in hand.

Mr. Wigley

asked the Secretary of State for Transport what was the projected life-span of the Severn Bridge at the time of its design; and whether there has been any revision to this assumption subsequently.

Mr. Horam

The projected lifespan of the Severn Bridge is 120 years, in common with all other bridges owned by the Department. It is intended to maintain the bridge to give this life.

Mr. Wigley

asked the Secretary of State for Transport what were the traffic load assumptions for traffic crossing the Severn Bridge at the time of its design; and what is the actual traffic volume at peak time in comparison with the assumed volume.

Mr. Horam

The bridge was designed for standard loading derived from the effect of closely spaced vehicles weighing 22, 10 and 5 tons. This is unrelated to traffic flow figures.

The design traffic flow for a two-lane highway at the time of construction of the Severn Bridge was 33,000 vehicles per day. The maximum daily flow so far recorded is 55,000 vehicles per day.