The Lions Gate Bridge Suspended Span Replacement - Case Study Example

The Lions’ Gate Bridge Suspended Span Replacement The Lions Gate Bridge is used to connect the downtown Vancouver and the points on the east and south from West Vancouver. This article discusses how the bridge replacement was done to regain it durability for the safety of its users. It also puts across recommendations that the reconstruction of the bridge may use today. Introduction The three lane Lions’ Gate Suspension is a landmark structure in Vancouver, British Columbia, Canada. The ridge has a center span of 472 meters and this is the entry to a very busy harbor in the west coast of North America. The bridge was constructed in 1938, and its construction was financed privately, and in such cases, the durability and strength of the bridge was at stake because there had to be savings on the initial capital. Buckland & Taylor LTD. was retained to design the replacement of the bridge structure starting from the deck, sidewalks, stiffening trusses and the suspension hangers needed to be replaced. The design of the replacement was done in 1997. The main factors that affected the parts of the design are that the bridge had an approximate of 70,000 vehicles that used it per day, so there was need for thorough maintenance of the bridge. The replacement design was completed by December 1998, after the completion, the bridge was tendered. The tendering process was done and a Contractor, American Bridge/ Surespan A Joint Venture, was chosen in April 1999. The Contractor was supposed to execute an $8.5 million contract. The cost of the suspension bridge portion of the project was approximately $66 million. The replacement of the Lions’ Gate Bridge was done between September 2000 and September 2001. The replacement was normally done during 10-hour night time closures with other hours being done over the weekends. During the other times of the day the traffic was allowed to use the bridge, but the final paving of the replacement was done during the summer of 2002. The replacement of the Lions’ Gate Bridge is said to be the first suspended structure was replaced while the traffic continued to use it. The outcome of the replacement of the bridge is that today the bridge is wider, safer, and more durable that fits the aesthetics of the other parts of the bridge. The Original Bridge First the bridge was opened while it was still having two lanes, and a concession that the then developers made to the future of the bridge was that a bridge deck will be designed so that it can accommodate other three narrow lanes that were to be fixed between the curbs. The source of revenue that was used to fund for the bridge was provided by Tolls. The debt was paid by 1952, and the bridge was sold to the British Columbia Government. In 1954, there were slight changes that were made to the line markings and they were redone so that it could be able to fit 2.9m lanes, which left the middle lane free. The Lions’ Gate Project By 1990’s, the bridge had 70,000 vehicles each day passing over it. It is because of this traffic and the economy of the initial construction, the deck to the suspension bridge had been affected till the cost that was used to its maintenance was about $3 million annually. During that time, the government made a move to rehabilitate the crossing and other more options were also thought of. They then decided too to replace the suspended structure while three lanes were being maintained at the same time. The project that was to be done had the following major components that needed to be attended to: The Lions’ Gate suspended Bridge was to be replaced with a wider span that could fit the three 3,6m traffic lanes and also 2m width sidewalks. The bridge was improved seismically The road that passes through Stanley Park, to the south of the bridge was also expanded The North approach viaduct was also improved seismically The sidewalks that are on the north approach viaduct were expanded There was a replacement of the electrical, lighting and lane control systems The Design Team The design process of the replacement of the bridge was declared in 1997 by the Government. Buckland & Taylor Ltd. Of Vancouver BC, the Owner’s Bridge Engineer, were assigned to make the final plans and specifications for the bridge deck replacement and suspension bridge seismic retrofit. The Owners Engineer that was assigned to make the final plans and specifications for the causeway and the contract documents for the general project was the N.D. Lea Consultants LTD. As a sub-consultant to N.D. Lea, PBA Engineering LTD. Of Victoria BC made a final plan and specifications for the electrical and lane control components of the project. Suspended Span Replacement Design The replacement structure that was to be constructed needed to be designed so that it had a minimal effect to the ongoing traffic. The closures of the traffic had to be specifically at night as from 20:00 to 06:00, and some weekend closures were also to be done from Friday 22:00 to Monday 06:00. The closures that were to be done at night needed to have a planning for the magnitude that was to be more important than for a day construction project. The design had a lot of focus on the 47 new deck sections, that were to be 20m in length and were to weigh 10 tones, that were to be constructed from north to south of the bridge. The field splices were installed so that the deck section could be raised so that it could not be interfered with the existing deck section. The splicing of the stiffening trusses and deck troughs were joined using bolts to give space for alignment and make the completion of the splice to be fast. Dead Load The weight of the new deck maintained the weight of the existing deck. This was necessary because the towers, main cables, and their anchorages were not going to be improved so that they could be able to bear an extra load. Maintaining the same weight that was designed before was also critical to allow the replacement of the deck in sequence without inducing deformation and stress in the existing or the new structural components during construction. The span of the three traffic lanes was increased from 2.9 to 3.6m and also had traffic barriers that prevented the pedestrians and cyclists to move to traffic lanes, a component that the initial bridge did not have. The span of the sidewalk was also increased from 1.3m to 2.7m The longitudinal stiffening trusses that are to be used in the replacement are composite with the orthotropic deck, and they will also be constructed under the deck. Seismic Loads The major role of the bridge and the way it is located in the West Coast of North America, its reaction to earthquake was key to the design process. B&T Contractors thought of two types of seismic event: a moderate magnitude that was close to the bridge and a major one that was some distance like 200km away from the bridge. The replaced structure has factors that will improve its behavior in the design earthquake. The dead load of the suspended structure was maintained to be low and its key dynamic periods are also extended. These factors help keep low the impact of an earthquake on the structure, the outcomes were that the wind loading controlled the transverse structural elements and some retrofit was needed for the towers and cables (Mazzolani and Tremblay, p. 215). Construction The American Bridge/Surespan A Joint Venture was given the contract of the bridge replacement that was worth $86.5 million by the British Columbia Government in April 1999. The reconstruction cost was estimated to cost $66 million. The viaduct seismic upgrade that was on the north was incorporated as a design-build portion of the project; it was designed by Klohn-Crippen Consultants Ltd. Of Vancouver BC and then it was independently checked by Parsons Brinkerhoff Quade & Douglas Inc. of Sacramento CA, both were working for the contractor (Mazzolani and Tremblay, p. 215). The Government requested the contractor to prepare the design, and check the construction procedures of the replacement because the bridge was open for traffic as the construction continued. Parsons Transportation Group Inc. of Newyork carried out the analysis of the bridge deck replacement, and Parsons Brinkerhoff Quade & Douglas Inc. of Newyork checked the erection analysis and they had been both assigned to do this by the contractor. Construction Equipment The construction used a jacking traveler that was a steel frame and it weighed 55 tones and it was the supporting component for lifting and lowering deck sections. It was designed to get support from the existing hangers when lowering and lifting sections. It was to be supported on the deck when it was moved. Equipment was the continuity link, which weighed about 20 tones, and it was necessary when the traffic was using the bridge. It was also used to connect the old and the new sections together at the construction front and hence it maintained a stability of the roadway surface. It was disconnected at night and moved on the maintenance traveler rails, to another location. How the Lions’ Gate Bridge would be built today The construction of bridges has been made easy by the new technologies and methods that have been discovered over time. Some of the methods that can be used today in the construction of a suspended bridge are: Differential Construction Method This method uses the materials maximally and the structural components of the bridge would be in dependence on their functions. It also has constraint free motions in joints. Due to internal and external effects the materials and the structural components are put to deformation. Such deformation is accommodated by resilience of structural members or through constraints. If the limits are exceeded, cracks may occur, or damages that will be caused by fatigue. Evolution will be able to increase the characteristics that are acting positively and decrease those that are negative. In general, evolution tends to reduce the constraints. This is also applicable in bridge construction: the expansion joints and the bearings facilitate movements and ensure that the structure is functioning as designed. Differential bridges are characterized by defined interfaces to their environment and within the bridge. This aspect leads to many advantages to the structure which are: It can be used in all types of structures and construction methods There is random combination of materials The developers have the freedom of design Modular and precast construction methods are also possible There is the separation of geotechnics and structural design Free from uncertainties in foundations and the soil Reduction of the risk of the subsoil and the system for the contractor Manageable engineering services that are free from the traffic category The safety of the structures is defined and its statistical systems Compensation of the dependence on time of the components of the material and the structural members It has major advantages in recycling The cracks in the structure can be controlled by revolution, this makes the long term damages to be irreparable (Das and Frangopol, p.325). Bridge Bearings There is use of Chloroprene which ensures that the bridge is environmentally compatible. When the mating of steel-steel are used instead of a sliding couple, the fixation elements and the guide bearings are prone to corrosion and wear. Suspended Bridges The cables of the bridge can be hooked to the roadway in different ways. In radial pattern, cables move to other points on the road to a point at the top of the tower. In a parallel pattern, cables are hooked at different levels along the tower, and they are parallel to one another. Precast or Insitu concrete or steel box segments may be used for the deck, which is supported by cables then hooked to a tower. There are four primary configurations that are used which are: radiating, harp, fan and star systems. These arrangements are made to provide compression in the deck by making use of their self-weight. In this manner the deck can be made up of single segments and then designed to be like a prestressed beam (Das, Parag, and Frangopol, Dan, 319). Cables can be spread and spaced so that the horizontal component of the load cancels out any tensile force in the top or bottom flange, individual segments can be collected each and then they can be left disconnected. When a concrete deck has been used, the shear forces would be worked on by the shear keys or epoxy resin glued joints while in the case of steel, it will always be welded. The cables are arranged in a manner that they will be able to prevent sideways and vertical movements of the tower and the deck when subjected to asymmetrical live loading. Stability can be maintained in the suspension bridges by choosing the correct foundation types and the connection of the cables and the girder. These will assist the structure to resist the horizontal and the vertical forces that will be formed as the structure starts its operation. The tower may be made of steel plate or precast concrete elements or at times in situ concrete. Different designs may be selected depending on the aesthetic effect that the client would want. The contractor can select from single tower, twin tower, A frame tower or diamond tower (Mazzolani and Tremblay, p. 215). The deck can be put together in precast concrete elements, steel plate or girders, or made in situ concrete. The most common form that is used is the box section which gives a better torsional restraint. Plate girders can be fixed using double plane system of hangers, where the installation procedures will need joining in small light elements. Trusses may also be used but the expenses are high and the maintenance costs are high to work against corrosion and poor aerodynamic factors. The materials of the cable that are used are same as those that are used in the prestressing work and may have multi-strand cable made of cold drawn wires or a single strand cable that consists of parallel wires may be employed. Protection against corrosion may be done by galvanizing the wires, but the best way is to cover the cable in steel or plastic ducting and the putting cement grout after they have been fixed in place. The cable can be joined to the pylon with pin-type joints or placed in the grove or guide tube of a saddle depending on the requirements of the design. The cables that have been joined with the pin-type joint have swaged or filled sockets. Swaging consist of squeezing a socket into the wire in a hydraulic press and is then used with strand having a diameter ranging between 10-40mm filled sockets are more recommended to the larger diameters that are parallel wire type cable with the socket containing the whole bundle of wires. The deck-to-cable connection is usually of the free type so that it can take the adjustment (Harvey and Niroumand, no page). Conclusion From the discussion it is evident that the methods that were used in construction before have been developed so that they can meet the various requirements of the building codes today. The methods that will be selected to construct a suspension bridge majorly depend on the conditions of the site, and the type of materials that are available and the length of the bridge. Another issue that affects the method that is selected in the construction of a suspension bridge is the economy of the place that the bridge is to be constructed and how economical it can be made but still maintain its ability to function efficiently. Other factors that are being put into consideration today are the aesthetics of abridge. The dead and the live load of the bridge are to be considered extremely in the design of the bridge and when deciding on the method of construction that will be used. Works Cited Das, Parag, and Frangopol, Dan, Current and Future Trends in Building, Design, Construction and Maintenance: Safety, Economy, Sustainability, Aesthetics, Vol. 2, Nowak, Institution of Civil Engineers; Thomas Telford. 2001. Print. pp. 319-325. Harvey, David and Niroumannd, Saeed, J., Fatigue Damage Assessment of The Lions Gate Bridge North Viaduct Deck, Proceedings of the 3rd Orthotropic Bridge Conference, American Society of Civil Engineers, Sacramento Section, U.S.A, California; Sacramento. June 26-28, 2013. Mazzolani, Federic and Tremblay, Robert, STESSA 2000: Behaviour of Steel Structures in Seismic Areas: Proceedings of the Third International Conference STESSA 2000, Montreal, Canada, 21-24 August 2000, Cambridge, CRC Press. 2000. Print. pp. 215-225. Read More