strengthening of the w est gate bridge steel box girder

V. Ponnampalam, H. Madrio and E. Ancich 331 Sustainable Bridges: The Thread of Society AP-G90/11_083© ABC 2011

Strengthening of the West Gate Bridge Steel Box Girder

Scott Taylor1, Robert Percy2 and John Noonan3

1VicRoads

2Flint and Neill

3Sinclair Knight Merz

Abstract The West Gate Bridge is a key transport link to Melbourne’s western suburbs. The 2.6 km long bridge crosses the mouth of the Yarra River and comprises two concrete box girder approach viaducts either side of the 850 m long cable stayed steel box girder main spans. The recent West Gate Bridge Strengthening Project was undertaken to; ensure the long term sustainability of the bridge for current and future traffic demands, reduce congestion by increasing the capacity of the carriageways from 4 to 5 lanes, controlled with a freeway management system, and improve public safety on the bridge. This paper describes the treatments developed to strengthen the steel bridge including props for the cantilevered portions of the deck, additional plates at the base of the towers and on the pier diaphragms, bolt replacement and splice strengthening, and a range of details used to strengthen internal members. In addition, public safety barriers have been added to the bridge. A major influence on the design of strengthening elements was constructability, and during the construction phase strengthening details were continually being reviewed as new information on the existing structure became available. The complexity of the project, however, was not limited to the design of strengthening elements. The project was undertaken without peak hour lane restrictions and over the entry to Australia’s busiest port. In addition, strict controls were required over the staging of the works to ensure the structural integrity of the bridge at all times.

Introduction

The West Gate Bridge is a 2.6 km bridge that spans the Yarra River in Melbourne, Victoria. The bridge is part of the M1 freeway and links Melbourne’s CBD with

332 Scott Taylor, Robert Percy and John Noonan

the western suburbs. The bridge’s five central spans consist of an 850 m long cable-stayed steel box girder structure (figure 1) [1]. The central three spans of the steel bridge are supported by a single inner cable and a pair of outer cables. All cables consist of 16 spiral strands. The cables pass over 45 m high steel square box towers. The steel box girder itself is divided into three cells, with an overall width of 37 m. Each box is 16 m long with an intermediate diaphragm at the junction between each adjacent box. Cross beams and cantilevers, spaced at 3.2 m centres, support a steel orthotropic deck. The steel bridge has had a chequered history. In October1970, during the original construction, the steel span between piers 10 and 11 collapsed, killing 35 workers. Following a Royal Commission into the collapse [2], the bridge was completed in 1978.

Fig. 1. General arrangement of the West Gate Bridge.

In 2008, a Business Case was developed and accepted for funding by the Australian Federal and Victorian State Governments to strengthen the whole bridge. The scope of the strengthening project was to;

1. ensure the long term sustainability of the bridge for current and future traffic demands,

2. reduce congestion by increasing the capacity of the carriageways from 4 to 5 lanes controlled with a freeway management system, and

3. improve public safety on the bridge.

The project was delivered by the West Gate Bridge Strengthening Alliance (WGBSA), consisting of VicRoads (asset owner), John Holland (principal constructor), Flint and Neill (steel bridge designer), and SKM (concrete viaduct designer). The physical works for the strengthening were completed in mid-2011. This paper describes the various strengthening solutions adopted on the steel bridge section, and the design process by which these solutions were arrived at. The paper also addresses the sequencing and controls placed on the work to maintain the structural integrity of the bridge and avoid interference with the 160,000 vehicles crossing the bridge and 15 to 20 ships passing under the bridge

Strengthening of the West Gate Bridge Steel Box Girder 333

each day. A similar paper for the concrete viaducts is presented by Allen, Noonan and Cosic [3].

Initial Investigations and Studies

The WGBSA was unique in that it brought together the project partners early to develop the 2008 Business Case to secure funding for strengthening works. It was clear, that the steel portion of the bridge would drive many of the key decisions regarding the viability of options for the design loading and the strengthening. Development of the Business Case included investigation of various strengthening options from limited strengthening, to duplication of the bridge. However, it became evident that the best value for money would be obtained by maintaining the current width of the bridge deck and strengthening the structure to provide for five lanes in each carriageway (figure 2).

Fig. 2. Cross section of steel bridge with final lane positions.

While the focus for the project was the strengthening of the structure for five lanes, other studies were undertaken, in parallel, into elements such as lane widths, lighting, ship clearance requirements, potential pedestrian and bicycle access, barrier capacities, and security improvements. This work included a significant input from architects to ensure all options achieved a suitable urban design outcome. Key considerations were those qualities that make the bridge special, and the flow between the concrete approach viaducts and the central steel spans. The end result is one that does not detract from the slenderness and simplicity of the original structure (figure 3).


Fig. 3. Architectural images of the bridge before (left) and after (right) strengthening.

As a result of the early investigations and development of the Business Case, the scope of strengthening works and the form strengthening elements would take were reasonably well defined, and as the project progressed, there were minimal changes in the direction of the design.

Bridge Specific Assessment Criteria

As outlined in an earlier paper on the West Gate Bridge Strengthening [4], application of current codified standard loadings was deemed not to be appropriate as they are not intended for long spans nor do they cover all the load effects applicable to cable stayed bridges. Furthermore, current design standards, developed for general application to new structures, can be overly conservative and from necessity a simplification of findings from the more detailed studies that form the basis of the code requirements. Bridge specific assessment criteria were developed that covered both loading and capacity requirements for the particular circumstances of this bridge. The Bridge Specific Assessment Live Loading (BSALL) was derived from a probabilistic analysis of the weight and mix of vehicles actually using the bridge, and presented elsewhere [5]. In addition, specific criteria were developed for the bridge assessment and the strengthening design [5]. The bridge specific capacity criteria also allowed significant strengthening mitigation to be undertaken to minimise the amount of additional strengthening.

Strengthening Mitigation

Before detailed designs began on new strengthening elements, an assessment was undertaken using the specially developed assessment criteria to ensure the full available capacity of the existing structure was utilized. Design work was further


refined by an imperfection survey and the application of the Interim Design and Workmanship Rules (IDWR) of the Merrison Committee [6]. The design rules in the existing standards are based on assumed values for the imperfections. The imperfection survey was an extensive survey of the straightness of stiffeners and plate panels to enable the distribution of imperfections to be quantified. This distribution was used to improve the assessment of the capacity of the stiffeners in the bridge as a whole. For specific areas, acceptance criteria were developed to inform further site measurement and identification of stiffeners that were sufficiently straight not to require strengthening. The second technique used to minimise the extent of strengthening was to refer to the IDWR [6] to determine the capacity of existing members. The current design standards (in this case BS5400 and BD/BA 56 [7] were used to design the strengthening for the bridge) are based on the earlier IDWR, however a level of conservatism has been introduced into the current standards to generalize and simplify them for use in the design of new bridge structures. In particular, areas where the application of IDWR rules provided particular benefit were panels subjected to high transverse loads in addition to compression, and the more slender web stiffeners. Some stiffeners with utilisation factors of up to 2 were reassessed using the IDWR and found to be adequate.

Steel Box Girder Strengthening Solutions

Many strengthening details were implemented throughout the bridge, ranging from bolt replacement at longitudinal and transverse splices, to the inclusion of props to support the cantilevered sections of the bridge deck. The bridge deck and cables did not require strengthening as there was sufficient capacity in both these elements to support the increased traffic loading. The additional reserve capacity in the cables, in particular, can be explained by the fact in the original design the deck was intended to be a concrete composite structure; however as part of the redesign following the 1970 collapse it was changed to a lighter steel orthotropic deck, while the cables had already been sourced.

Bottom Flange and Web Strengthening

By far the most common type of strengthening was additional stiffening applied to existing web and bottom flange stiffeners to increase their buckling capacity, although additional strengthening was required in some areas to increase yield capacity. A panel is defined as the area of flat plate bounded by adjacent longitudinal and transverse stiffeners. In areas that were in compression the


capacity of the panel and adjacent stiffeners was limited by the lower bound buckling capacity of all the contributing members. Therefore, additional capacity of a panel could be realized by strengthening the adjacent stiffener member. An example of this type of strengthening solution was on the bottom flange where commonly the longitudinal stiffeners alternate between angles and bulb flats. The bulb flats were identified as the weakest element in the section and would theoretically buckle first. Therefore, if the buckling capacity of the bulb flats were increased, the whole section capacity could be increased. The method used to strengthen the bulb flats was a novel solution where a series of short ‘bent plates’ was bolted to the bulb flat and adjacent angle. The ‘bent plates’ detail used to strengthen bulb flats, was subject to an extensive parametric investigation by Prof Emad Gad at The University of Melbourne to enable determination of the optimum size and spacing of these plates. The bottom flange was modeled using a finite element program and various configurations of ‘bent plates’ inserted to determine the effect of differing sizes. The final design meant that the bulb flats and angles could be designed for close to full yield, rather than a lower buckling stress. These ‘bent plates’ , in combination with the longitudinal stiffeners, effectively provided a vierendeel girder in the horizontal plane for the lateral restraint of the longitudinal stiffeners.

Fig. 4. FE model of ‘bent plates’ (left). Sketch of final ‘bent plate’ design (right).

An alternative to the strengthening of the web and bottom flange panels, to increase their yield capacity, was the inclusion of post tensioning in the bottom flange in the middle of the eastern end span (see figure 5). In the sagging zone of this 112 m span, the tensile stresses from bending effects in the bottom flange were found to be too large with the 10 lane scenario and strengthening of the stiffeners would have been difficult. Therefore, post tensioning was introduced to create a compression force in the high tensile zone. The tendons were positioned to be at the centroid of the combined bottom flange plate and longitudinal stiffeners and were laid over a length of approximately 64 m. The reduction in the tensile stress in the bottom flange had the added benefit of eliminating the need to strengthen some of the bolted transverse splices between boxes.


Fig. 5. Bottom flange post tensioning. Anchorage blocks (left), tendons in ducts (right).

Pier Diaphragms and Base of Towers

The diaphragms over each of the piers differ from standard intermediate diaphragms throughout the bridge. The pier diaphragms transfer the vertical shear forces from the inner and outer webs to the bearings at the top of the piers. The pier diaphragms are characterized by vertical bearing stiffeners located directly above the bearing, and a significantly thicker plate thickness (between 1” to 1¼”). The increase in stress through the diaphragm plate from the increase in traffic loading required additional 16 mm thick plates to be bolted to the face of the diaphragms (see figure 6) to increase the yield capacity of the diaphragm.

Fig. 6. Strengthened pier diaphragm.

While in itself the pier diaphragm strengthening might appear reasonably straightforward, the design and construction of this element was complicated by the fact that the typical way of installing a doubler plate that relied on frictional transfer of load through the faying surface could not be adopted. Typically, the procedure for adding a doubler plate is to drill all the holes in the existing plate, then clean and/or blast the surface to ensure an adequate friction coefficient value


is obtained and finally fix the doubler plate. However, the process of drilling all the necessary holes meant the temporary capacity of the pier diaphragm would not have been sufficient for potential loading during the installation process. Therefore, the design was modified so that load was transferred into the new doubler plate via the bolts going into shear, rather than through the friction surface. This was achieved by using close tolerance bolts that behaved as a ‘non-slip’ connection. A further complication with the design was that the finite element analysis was required to model both the stress state of the diaphragms itself and the behaviour of the structure as a whole, to have the correct shear forces being transferred through the diaphragm and to the bearings. The base of the towers were strengthened in a similar way, where additional plates were required to increase the yield capacity of the section. However the process of drilling numerous holes would again have temporarily weakened the section. Therefore, the use of close tolerance bolts was adopted on these plates as well.

Longitudinal and Transverse Splices

The original steel box girder plates are connected by a mixture of bolted and welded connections. Typically, the deck plates are welded both longitudinally and transversely except along the centre longitudinal splice where it is bolted. However, both the inner and outer webs and the bottom flange contained bolted longitudinal and transverse splices. More particularly, the steel panels used for the superstructure are 3 m wide. These panels are connected by longitudinal bolted splices at 3 m centres around the box girder cross-section. Ordinarily to increase the capacity of a bolted splice, more bolts would simply be added. However, the original splice plates and bolt spacing were not large enough to accommodate further bolts. Therefore, additional plates were added that were bolted to the existing parent plate and splice plate to create additional faying surfaces. The complication with this approach was that the new splices plates required existing bolts to be removed. A major influence on the design and the construction process was that the number of bolts removed at any one time had to be strictly limited to preserve the integrity of the bridge under traffic loads. A unique construction sequence was developed for every splice that required strengthening, that controlled the order bolts could be removed and new plates and bolts inserted. The other complexity of the splice strengthening was the capacity and failure mode of the original and new bolts. The original bolts used in the construction of


the West Gate Bridge were what are referred to as Gilbert Roberts (GR) bolts. These bolts have a waisted shank and therefore for both serviceability and ultimate loading conditions were designed assuming no-slip conditions (eg at both SLS and ULS loading conditions load transfer is via the friction surface). The new Tension Controlled Bolts (TCBs), like normal HSFG bolts, have a constant diameter shank and therefore could be designed for no-slip at SLS loads and slip at ULS loads (bearing of the bolt shank). The variation of failure modes meant that whole sections of splices had to be strengthened to maintain compatibility in the failure mode for both SLS and ULS loading conditions.

Cantilever Props

The cantilever sections of the bridge deck were originally designed for lighter vehicles than the heaviest vehicle that could be expected now. Therefore, props were introduced to provide additional support to the cantilever deck section (see figure 7). The props extend from the underside of the cantilever down to the bottom corner of the box girder. The props were designed to support approximately 50% of the live loading on the cantilever section, resulting in a sharing of load between the new prop and existing cantilever. The thin stiffened web and flange plates at the lower connection point required internal strengthening to accommodate the concentrated prop load. Unfortunately, because of the checkered history of the bridge, there is a large degree of variation in the existing internal details adjacent to each prop connection, as well as the sensitivity of the area to fatigue loading. The design task to account for all these complexities has meant that while the props appear similar, each location represents a unique combination of details to ensure adequate strength is achieved.

Fig. 7. View of the installed cantilever props.


Public Safety Barriers

New public safety barriers (PSBs) were installed along the full length of the 2.6 km bridge. The final design of the PSBs involved a 4 m high mesh panel inclined at an angle of 10 degrees from the vertical away from the bridge (figure 8). A curved nose rail was installed along the top of the barrier to make it more difficult to climb over the barrier, and posts were installed at 3.2 m centres on the steel bridge which coincided with the spacing of the props and existing cantilevers. The panels are designed as discrete sections that clip onto the posts, so that in the event a single panel is damaged it can be quickly removed and replaced without diminishing the integrity of the whole system.

Fig. 8. Architectural images of the PSB (left) and final installed system (right).

Design and development of the barriers is detailed in a paper by Juno & Percy [8], which describes the functional requirements that formed the basis of its design and the challenges presented by the retro fitting of a barrier system. The final list included elements such as minimum height, gap sizes, and ease of scaling, as well as other factors such as permeability (to allow motorists to see through the mesh), maintainability and aerodynamics.

Staging and Sequencing

The construction staging of the work was complicated by the requirement for the bridge to continue to operate with minimal lane closures. Based on recorded traffic volumes, a matrix of allowable lane closures at various times during the day was developed. During peak times no lane closures were allowed, while at night and on weekends up to 3 of the 4 lanes in each carriageway could be closed.


The original emergency lane, that was ultimately being converted into the additional operating lane, was utilised as a 2.5 m wide construction lane. In order to maximize the width of the construction lane and to enable drivers to adjust to the narrow lane widths early, the final lane positions were introduced before major works began on the strengthening. To provide a safe work environment, temporary concrete barriers were installed along the length of each side of the bridge adjacent to the construction lane. The concrete barriers, with a fence structure attached to the top of each barrier panel, were also introduced to reduce the incidence of suicides. However, these temporary concrete barriers came at a significant cost in terms of the flexibility of the work sequencing; the weight of the concrete barriers was roughly equivalent to the weight of half a lane of traffic (after lane reduction factors had been taken into account). The installation of the props and the extensive bolting work required platforms to provide access to the outside of the bridge. The access structures included the three bolting gantries used for the original bridge construction, a maintenance gantry that was later introduced and four temporary access platforms along each side of the bridge. Further detail on the temporary access provisions is provided elsewhere [9]. The additional weight resulting from these platforms had a major influence on the construction staging and the order in which the bridge strengthening had to be implemented. The West Gate Bridge is situated close to the mouth of the Yarra River and passes over the entrance to the Port of Melbourne, the busiest container and general cargo port in Australia. On average 15 to 20 vessels of various sizes pass under the bridge each day. Close coordination and agreement with the Port of Melbourne Authority was required, to ensure platforms and gantries used for the external strengthening works did not inhibit the movement of ships through the channel.

Accommodation of As-Constructed Details

Finally, the nature of the history of the West Gate Bridge has meant there were a vast number of minor differences between all 54 boxes in the steel girder. Therefore, while the design drawings gave strengthening solutions for generic locations, during the construction phase of the project, close communication between the design and construction teams was required. Frequently specially tailored solutions were developed to ensure adequate strengthening was installed.


Conclusions

Strengthening of the cable stayed steel box girder bridge to allow an increased traffic flow over the West Gate Bridge, has been achieved through novel and innovative design solutions. The initial phase of the design process covered a broad range of topics, not only design concepts, but optimum lane widths, architectural considerations, lighting, ship clearances etc, that ensured when the time came for detailed design the scope of works was well defined. The first stage of the detailed design involved reducing the need for strengthening by using measured imperfection values and the Interim Design and Workmanship Rules (IDWR) of the Merrison Committee. The solutions for those areas that required additional strengthening included props for the cantilevered sections of the bridge deck, additional plates at the base of the towers and for the pier diaphragms, bolt replacement and additional plates and bolts for longitudinal and transverse splices, and a range of plates and angles for strengthening internal members. Public safety barriers were also introduced to improve public safety. Sequencing of the works was carefully controlled to ensure that at no point was the structural integrity of the bridge compromised. Close coordination between the design and construction teams was also necessary as the generic designs developed required modification to suit as built details.


References

[1] Balfe, P et al., edited by Toakley, A. (1986) Redesign of West Gate Bridge, Victoria Road Construction Authority.

[2] Government of the State of Victoria, Australia (1971), Report of the royal Commission of Inquiry into the Failure of West Gate Bridge.

[3] Allen, C., Noonan, J., & Cosic, N. (2011) Strengthening of the West Gate Bridge Approach Spans, Proceedings of the 8th Austroads Bridge Conference, Sydney, Australia.

[4] Taylor, S., Percy, R., & Allen, C. (2009) West Gate Bridge Strengthening, Proceedings of the 7th Austroads Bridge Conference, Auckland, New Zealand.

[5] Cooper, D. (2009), Highway Traffic Load Model for Bridge Design and Assessment, Proceedings of the 7th Austroads Bridge Conference, Auckland, New Zealand.

[6] Department of the Environment (Merrison Committee of Inquiry) (1973). Appendix 1: Interim Design and Workmanship Rules, Inquiry into the Basis of Design and Method of Erection of Steel Box Girder Bridges, London, HMSO.

[7] BD/BA 56 (1996) The Assessment of Steel Highway Bridges and Structures, Design Manual for Roads and Bridges, London, UK, HMSO.

[8] Juno, W., & Percy, R. (2011) West Gate Bridge Public Safety Barriers, Proceedings of the 35th International Symposium on Bridge and Structural Engineering, London, UK.

[9] Dauth, J & Taylor, S, (2011) Access systems used in strengthening the West Gate Bridge, Proceedings of the 8th Austroads Bridge Conference, Sydney, Australia.

strengthening of the w est gate bridge steel box girder

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