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Inspection and Maintenance of the Orthotropic Deck of Avonmouth Bridge

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Inspection and Maintenance of the Orthotropic Deck of Avonmouth Bridge Life-Cycle and Sustainability of Civil Infrastructure Systems – Strauss, Frangopol & Bergmeister (Eds) © 2013 Taylor & Francis Group, London, ISBN 978-0-415-62126-7 Inspection and maintenance of the orthotropic deck of Avonmouth Bridge B.I. Frey Mott MacDonald, Bristol, UK ABSTRACT: Avonmouth Bridge is a twin steel box girder bridge with 20 spans, an orthotropic deck in the three main spans and a concrete deck which is acting compositely in the approach spans. The bridge has been widened to accommodate four lanes of traffic in each direction resulting in asymmetrical loading. The ortho- tropic deck is suffering from recurring cracks and is inspected and repaired annually. Access is provided by gantries and a mezzanine floor inside the box girders. The repaired welds are improved by carrying out partial penetration welds instead of fillet welds; the deck stiffness has also been improved by a new guss-asphalt sur- facing. 1 DESCRIPTION OF THE STRUCTURE 6.5m to 18.3m long. In the approach spans the deck is a 220mm reinforced concrete deck which acts Avonmouth Bridge carries the M5 motorway over compositely with the steel box girders. In the main the River Avon, A4 Portway, the Severn Beach span (the river span and the two adjacent spans) the railway line and the industrial estates of Avonmouth. deck is an orthotropic steel deck. The bridge is an- The M5 motorway is the major link of the Southwest chored against longitudinal movements at both to the rest of England. The bridge was originally de- abutments. Movement is accommodated by flexure signed by Freeman Fox Ltd. and first opened to traf- of the piers and the expansion joint adjacent to pier fic in 1974. In the 1990s the bridge was strengthened 10 connecting the orthotropic steel deck and the to accommodate an additional fourth lane of traffic composite northern approach span. The piers and on both carriageways. The bridge is approximately abutments are made of reinforced concrete and have 1.4km long and has 20 spans varying in length be- spread footing foundations, except for piers 6, 7 and tween 30m to 174m with a vertical clearance of 30m 8 which are supported on piles. The strengthening over the river (Figure 1). Balfour Beatty Mott Mac- consisted of external post-tensioning above the river Donald are the Highways Agency’s maintaining piers and some areas of the approach spans, im- agents for Area 2 in the Southwest of England. provement of the steel knuckle bearings, and instal- The superstructure consists of two continuous lation of additional stiffeners and web plates in the steel box girders connected by steel cross girders box girders. Access to the bridge is provided by two which in turn support the deck. The box girders it- centre and eight cantilever gantries (Figure 2). self are divided into 83 box sections each between Figure 1. Photo of Avonmouth Bridge, looking North. 365 Figure 2. Cross section of Avonmouth Bridge indicating the alignment of centre and cantilever gantries. 3 THE ORTHOTROPIC BRIDGE DECK Avonmouth Bridge has two box girders instead of 2 MAINTENANCE REQUIREMENTS OF open main girders (Figure 3); this provides more tor- AVONMOUTH BRIDGE sional stiffness but does not change the function of the orthotropic deck. The distance between the webs Due to the high importance of Avonmouth Bridge of each box is 5.942m and 18.436m between the for the UK infrastructure a maintenance team is boxes. The cross girders connect the webs every permanently based at the Avonmouth Bridge Unit 3.660m and continue under the cantilevers. The lon- (ABU) close to the bridge. This enables maintenance gitudinal deck stiffeners are of a V-shape trough tasks to be carried out on a day to day basis and profile at 0.610m centres outside the boxes and greatly simplifies the logistics for scheduled works 0.572m centres inside the boxes. and inspections. The trough stiffeners run continuously through The inspection regime for Avonmouth Bridge is the cross girders with cope holes cut around the bot- risk based depending on the criticality of the particu- tom and the longitudinal welds of the stiffeners. lar element of the structure. Each element is graded The increased loading after the widening lead to an as high, medium or low risk. High risk elements are increased risk of fatigue in the orthotropic deck; inspected every 12 months, medium risk elements several measures have been undertaken to deal with every 24 months and low risk elements every 36 this challenge including a stringent inspection re- months. gime, installation of gussasphalt surfacing, installa- All components of the orthotropic deck are con- tion of fatigue sensors and improving the weld detail sidered to be high risk. This includes the welded for repairs. connections between trough stiffeners and deck plate and cross girder; the trough stiffener splices; the deck plate splices; and the deck plate to web connec- tions. The inspections of the orthotropic deck are “Close Visual Inspections” in accordance with BS EN 970. When a weld defect is suspected non- destructive testing is carried out, usually magnetic particle inspection. If a weld defect is confirmed repairs are promptly carried out. Weld repairs usually take place at night when the traffic volume is lower and lane closures on the bridge are possible. In order to repair a dam- aged weld the existing weld is removed along the length of the crack plus an extra 100mm on either end of the crack. After surface preparation the new weld is carried out with close supervision to ensure good workmanship. Figure 3. Isometric view of an orthotropic deck. 366 3.1 Changes in the course of the strengthening works The strengthening works were carried out in order to accommodate an additional fourth lane in each di- rection. This resulted in the central reserve being moved from the centre line of the bridge by 2.037m towards the west and the closure of the west side cy- cle track. The steel box girders were strengthened to accommodate 40t vehicles which also led to strengthening of the cross girders in the main span and the addition of flat stiffeners on the orthotropic deck inside and between the box girders in the mid- dle of the river span. Figure 4. Crack between stiffener and deck plate. 3.2 Fatigue This type of crack however is not an immediate threat to traffic safety and the integrity of the deck In spite of the strengthening Avonmouth Bridge be- because loads can redistribute after a crack has came more heavily loaded since the widening mak- formed; it is also the most common type of weld de- ing it more vulnerable to fatigue – which by defini- fect on Avonmouth Bridge. tion is progressive and localised structural damage caused by fluctuations of stress. It occurs when a 3.2.2 Cracks in the welded trough splice structure is subjected to cyclic loading; in the case of Splicing of the troughs is carried out after the pre- road bridges the primary cause is traffic loading (De fabricated box sections are assembled. This is done Jong 2004). Structural damage can appear without by fitting short sections of trough profile between the peak stress reaching the ultimate tensile stress the preassembled trough stiffeners with backing limit and may also be below the yield stress limit. strips to allow for welding (Figure 5). Three different types of cracks have been detected in The reasons for this type of crack are bending the orthotropic deck of Avonmouth Bridge. moments in the troughs due to local wheel loads and the associated stress ranges. The quality of the weld 3.2.1 Cracks in the stiffener to deck welded connec- is of crucial importance, for this form of defect (De tion Jong 2004). Cracks of this type can appear in any part of the Provided the crack dimensions are limited, cracks trough to deck connection except at the intersection of this type do not pose a threat to safety of the with a cross girder. The crack initiates at the root of structure as load redistribution is possible. Cracks of the weld and first grows towards the surface of the this type have only seldom occurred on Avonmouth weld and then spreads longitudinally along the fillet Bridge because the splices are located at quarter weld (Figure 4). points. If the deck is viewed in cross section between the box girders, the deck plate functions as a continuous 3.2.3 Cracks in the welded connection between beam with each of the stiffener webs acting as a trough and crossbeam support. Depending on the exact location of a local In this connection cracks originate around the cope wheel load, each section between the stiffeners is holes and spread in either the trough stiffener or the subject to constant upward or downward bending. cross girder (Figure 6). Hence these deformations cause bending moments Cracks in this area are due to poor quality weld- in the deck and stiffener webs. The stress caused by ing. However cracks of limited dimension are not the bending moments in the trough web is the main crucial to traffic safety as loads are sufficiently re- cause for cracks in the fillet welds between trough distributed (De Jong 2004). Again, cracks of this stiffener and deck plate. type have only seldom been recorded on Avonmouth A thick asphalt layer can help to prevent the for- Bridge. mation of this type of crack as thicker surfacing dis- tributes local wheel loads over a larger area. The quality of the weld has a significant influence on the growth rate of the crack. It is assumed the fatigue behaviour can be improved by avoiding a gap be- tween trough profile and deck plate and executing the connection as a full penetration weld instead of a fillet weld (De Jong 2004).
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