The existing Forth Road Bridge spans the Firth of Forth in Scotland. The main suspension bridge, with central span of 1006 m, has two multi-span approach viaducts leading up to the main crossing. The deck of the approach viaducts comprises a pair of longitudinal steel box girders supporting a series of transversely spanning steel girders, both acting compositely with a reinforced concrete deck. The steel girders of the approach viaducts are supported on steel roller and rocker bearings on concrete portal piers which vary in height between 11 m and 40 m. An initial study of the bearings identified that the rollers had locked up due to corrosion and distortion, and the concrete beneath the bearings and elsewhere on the pier tops had deteriorated due to chloride contamination. Assessment showed that structural deficiencies in the pier were exacerbated by both the concrete deterioration and change in articulation.These factors led to the decision to replace all the bearings on the viaducts. This paper outlines the design of the strengthening and modifications to the bridge to facilitate bearing replacement, together with a detailed description of the design of the temporary works needed to maintain the bridge's articulation during jacking. IntroductionThe Forth Road Bridge (Figure 1) spans the Firth of Forth and was completed in 1964. The main structure is a three-span suspension bridge. At each end of the bridge, there are two multispan approach viaducts comprising a pair of longitudinal steel box girders with cross-girders supporting a concrete deck slab as shown in Figure 2. The approach viaducts carry two carriageways, each with two lanes, and extend from the abutments to the side towers, which are shared with the main suspension bridge. The total width of the structure is 36 m.The box girders rest on steel roller and rocker bearings on reinforced concrete portal piers, varying between 11 m and 40 m high, founded on rock. The articulation of the two viaducts is shown in Figure 3 During inspections and displacement monitoring, the existing roller bearings were found to exhibit little or no movement and varying amounts of corrosion. At the north side tower, the only roller bearing on the north viaduct, the roller was found to be nearing the limit of its movement range. Figure 4(a) shows a typical roller bearing, and Figure 4(b) shows the roller at the north side tower. Structural assessment of the rollers bearings to BS 5400-9-1:1983 (BSI, 1983) and BS EN 1337-4 (BSI, 2004a showed that the original bearings did not meet modern geometrical limits and were significantly overstressed to the codes.The rocker bearings were generally found to be in a better condition than the rollers, although some corrosion was present. A typical rocker bearing is seen in Figure 5. A structural assessment was also performed on the rocker bearings, which generally found that the bearings complied with the requirements set out in BS EN 1337 -6:2004 (BSI, 2004b).An inspection of the pier tops showed concrete delamination occurring at ma...

The new structural Eurocodes offer increased economy in design over most existing codes of practice. However, the assessment of existing structures is specifically outside the scope of the Eurocodes, so many of the benefits these new codes bring cannot be directly applied when reviewing old structures. This paper investigates the areas where the steel and concrete Eurocodes give increased resistances compared to existing codes and makes recommendations for sections that could be directly applied to the assessment of existing structures. Areas where the Eurocodes cannot be applied directly to existing structures are also identified, together with the reasons why, such as reliance on modern material and execution specifications. Recommendations are then made for how the Eurocode design rules could be modified for assessment situations, including the use of measured strengths and imperfections in calculation. Actions and the format for combining actions are also investigated and recommendations are made for modifications to these aspects for use in assessing bridges. Finally, an overall assessment of the scope of work required to produce an assessment suite of Eurocodes is made.

The bridge discussed in this paper was completed in 1985 and is part of a vital sea crossing. The main cable-stayed bridge has a span of 225 m with side spans of 107?5 m. The bridge carries two three-lane carriageways. The bridge cables comprise lengths of coupled high-yield bars within a grouted steel tube. An assessment of the structure in 1996-1999, using modern design standards, identified large overstress in the shortest cables anchored close to the pylons. Other cables were also found to be overstressed, but to a lesser degree. Bearings were thus installed at the piers to relieve load from the shortest cables and the second shortest sets of cables were replaced. An acoustic monitoring system was also installed on all cables to detect any problems. Since then, two bar breaks were detected in other cables, which have been replaced. Inspection and testing showed fatigue of the couplers to be responsible for the breaks. Following the first bar break detected in December 2004, an additional assessment was completed by Atkins. This paper describes the assessment of the bridge cables, examines the consequences of cable failure and discusses the reasons why the decision to replace all the cable stays was made. It also describes the specification for the new replacement stay system and its advantages over the old system. BACKGROUNDThe bridge considered in this paper was completed in 1985 and is part of a vital sea crossing. The overall length of the viaduct is approximately 13?5 km, mostly consisting of 40 m span posttensioned beams. The main cable-stayed bridge has a span of 225 m with side spans of 107?5 m. The bridge carries two carriageways, each with three lanes. The bridge is shown in Figure 1 and the numbering system for the cables is shown in Figure 2.The bridge cables comprise lengths of coupled high-yield bars within a grouted steel tube. Using modern design standards, the bridge was assessed in 1996-1999. Large overstress was identified in the shortest cables (M1 and E1), anchored close to the pylons, and other cables were also found to be overstressed, but less so. As a result, bearings were installed at the piers in order to relieve load from the shortest cables and the second shortest sets of cables were replaced. At that time, an acoustic monitoring system was also installed on all cables to detect any further problems. Two bar breaks were subsequently detected in other cables, which led to their replacement. When the first of these cables was removed, an additional three bars were found to be broken at couplers. Testing of the couplers showed that some were showing signs of fatigue damage, and fatigue was found to be the cause of the three broken couplers.Following the first bar coupler break detected in December 2004, an additional assessment was carried out by Atkins. This identified that many of the current cables were significantly overstressed in accordance with current standards. The predicted overstress in the cables, combined with the history of coupler breaks, led to the decision to replace ...

The Highways England A14 Cambridge to Huntingdon Improvement Scheme consists of both online improvements to existing sections of the A14 and a new offline section constructed on a green field site bypassing Huntingdon, UK. The scheme includes the design and construction of 22 integral bridges in varying forms typically found across the wider highway network. This paper outlines the different structural forms used on the A14 including ‘bankseat’ abutments on discrete columns, half-height abutments on a single row of piles and full-height abutments. The design challenges encountered for each type of structure are discussed, including the methods of modelling adopted for the soil–structure interaction. Primarily, it will highlight areas where the current codes of practice and design guidance (such as ‘PD 6694-1:2011: Recommendations for the design of structures subject to traffic loading to BS EN 1997 1:2004’) could be developed to reflect the specific requirements of modelling these types of structures, now commonly in use, and how the requirements varied depending on the type of abutment/foundation considered.

The Heathrow expansion project proposals comprise major works around Heathrow airport to allow the construction of new terminal buildings and the third runway. Owing to the critical nature of uninterrupted operation of both runway and the M25 motorway, the preliminary design had to be developed to minimise maintenance operations. At over 140 m in the total length, the adoption of an integral bridge of this length is in excess of most integral bridges in the UK, particularly where full-height abutments are being utilised. The maximum predicted expansion length also lies outside the limit equilibrium method set out in PD6694-1. As such, it was agreed that a full soil–structure interaction study should be carried out to assess the impacts of the structural behaviour of the bridge and its abutments. This paper looks at the calibration work for that study, the derivation of the earth pressures behind the walls, the behaviour of the abutment and how this compares with the predictions set out in PD 6694-1.