New Bridge for the St. Lawrence - Review of the concept proposed by Mr. René Therrien to replace the Champlain Bridge
|Date||January 4, 2013|
|Subject||Review of the concept proposed by Mr. René Therrien to replace the Champlain Bridge|
This memo provides an assessment of the alternative option for the New Bridge for the St. Lawrence (NBSL) as proposed by Mr. René Therrien.
The following memo reviews the proposal in more detail, reviews it objectives and assesses it against the overall context.
Mr. René Therrien's "Champlain Bridge Concept"
As an alternative to the replacement of the existing Champlain bridge structure, Mr. René Therrien has proposed a staged concept that attempts to maximize the use of the varying residual lives of the existing structural elements. This approach defers and balances out the total construction expenditures over a longer period while utilizing the inherent value of the previous investments of the existing infrastructure. The overall approach is to address immediate and long term needs while essentially widening and then reconstructing the original superstructure in a phased approach.
The NBSL options proposed in the pre-feasibility study, as well as additional options in the course of development by Arup, are based on constructing an entirely new bridge, separated from the existing to the north of the existing structure, diverting all traffic and transit to the new structure and then demolishing the existing structure. The NBSL replacement structure requires a 125 year design life and will primarily be selected to minimize schedule at the least whole life cycle cost while providing an aesthetically suitable concept. Additionally, a dedicated transit right of way with flexibility for Bus Rapid Transit (BRT) or Guided Rail Transit (GRT) transit systems is required. The rationale for total replacement is based on the need to address the rapidly deteriorating limited life span of the existing structure with the high costs and disruptions of the associated maintenance requirements.
The existing bridge is primarily composed of prestressed concrete girder approach spans and steel truss main spans over and adjacent to the St. Lawrence Seaway. The concrete girder spans are known to be in an advanced state of deterioration due to the original poor design detailing and construction quality, in conjunction with a prolonged period of unfavorable deck drainage conditions within a severe chloride environment. These approach spans have undergone significant remedial works including straight external post-tensioning and supplementary queen-post strengthening of selective spans and external post-tension with deviators of all the exterior girders of each span. The steel trusses of the main spans have also undergone significant rehabilitation including a lightweight steel orthotropic deck replacement and various structural modifications required to address capacity deficiencies. Furthermore, the eight major concrete piers of the main span are not capable of resisting earthquake loading, and there appears to be some issues in regards to longitudinal wind loading. Furthermore, the concrete in the core of the piers is known to be of deficient strength and quality. A thorough review of the existing condition of the Champlain Bridge was undertaken by Delcan and summarized in their report, "Assessment of the Champlain Bridge", dated March 2011. In consideration of potential rehabilitation schemes, Delcan produced a supplementary report, "The Future of the Champlain Bridge", also dated March 2011. While all the significant deterioration and costly work required is noted, the supplementary Delcan report also noted that repairs to the steel trusses could be carried out cost-effectively in the absence of traffic and result in a remaining life span of 50 years.
Due to the potential relative usefulness of the steel trusses versus the irreversible rapidly deteriorating condition of the approach spans, the Delcan report suggested an option to maintain the existing truss spans for approximately 50 years in conjunction with new approach spans. Mr. René Therrien developed the following proposed staged replacement plan, compatible with Delcan's suggestion.
In the first stage of construction, all traffic is maintained on the existing bridge while new independent spans are constructed adjacent to both sides of the existing bridge. New piers are constructed adjacent to the existing piers on each side to match the existing span lengths. Each new deck superstructure utilizes three trapezoidal steel box girders and accommodates three traffic lanes. New exterior support beams are added to the existing approach spans and the new piers and deck sections are tied together to provide additional capacity and residual life to the existing spans. Similarly, the main steel truss spans and piers are widened and strengthened to provide additional capacity. The completed Stage 1 cross-section for the approach spans is shown in Figure 1 below.
The proposal considers a two year construction period to complete all the works required to construct the first stage section while maintaining traffic on the existing spans.
Figure 1: Stage 1 of Mr. René Therrien's "Champlain Bridge Concept"
The second stage involves the relocation of all traffic to the new exterior spans completed in Stage 1, carrying out rehabilitation and strengthening works on the existing approach and truss spans and then returning two lanes in each direction of truck-restricted traffic to the existing structure. A cross-section of the completed Stage 2 approach spans is shown in Figure 2.
Figure 2: Stage 2 of Mr. René Therrien's "Champlain Bridge Concept"
The proposal forecasts a two year construction period following the completion of Stage 1 works to the completion of the final Stage 2 section above.
The Stage 2 cross-section and traffic patterns would be maintained until approximately 2030 at which time traffic on the existing portion of the Stage 2 cross-section would be removed for the demolition and reconstruction of the existing bridge. The reconstructed central portion would be reserved for the proposed Agence métropolitaine de transport (AMT) GRT transit system as well as one lane in each direction of truck-restricted traffic linked to Highway 15 only. The approach spans would be reconstructed with trapezoidal steel box girders to match the previously constructed widened portions and would be supported on steel beams spanning between the piers constructed for the widened portions. A cross-section of the completed Stage 3 approach spans is shown in Figure 3.
The demolition and reconstruction of the central "existing" portion of the bridge is forecast to take four years to complete, with an anticipated completion of the ultimate cross-section with GRT completed by 2034.
Figure 3: Stage 3 of Mr. René Therrien's "Champlain Bridge Concept"
The Mr. René Therrien "Champlain Bridge Concept" involves multiple stages of construction to essentially reconstruct the existing span arrangements while transferring various traffic scenarios between existing and new constructed portions. The final cross-section results in significant excess deck width than what is required while at the same time, requiring a loss of traffic and transit capacity (compared to what is offered for several years except for Stage 2) during the four-year Stage 3 construction period.
Analysis of the Mr. René Therrien Proposal
As noted by Mr. René Therrien, the "Champlain Bridge Concept" was developed with the intentions of providing an alternative that:
- Considers use of existing portions of the bridge as part of the solution
- Provides lowest cost alternative with costs spread out over longer term
- Meets immediate needs with quickest schedule
- Meets long terms needs at appropriate budget timing
- Meets overall user needs and safety for users
- Sensible use of infrastructure
The development of a concept that maximizes the residual lifespan and financial value of the existing truss spans, where it may be cost effective to do so, is a noble exercise and certainly warrants consideration as provided below. This concept is herein reviewed both for its own merits independent of total replacement schemes and as part of the overall screening process for all options.
In consideration of the proposal for its own merits, there are some major shortcomings. While the Delcan report noted that repairs to provide a 50 year residual life could be cost-effective if the bridge were taken out of service during repairs, the process required to achieve this condition is not practical. The staging and methodology required to re-use the main spans is reasonable but is not a sound proposal in terms of the overall project needs. Instead, it would appear that the truss preservation is the driving force for the Mr. René Therrien proposal rather than being a useful component of a viable bridge replacement scheme. The major shortcomings of the proposal are summarized in the sections below.
There is no doubt that the prestressed concrete girder approach spans are in urgent need of replacement. The continued and unstoppable decay and corrosion of the prestressing strands at exponential rates, including evidence of already sectioned cables in both the girders and highly integrated deck, necessitates a solution that replaces these elements at the earliest opportunity. Plans to extend the useful life of these spans to suit the overall staging plan carries great risk and is fraught with technical problems and is thus not considered a reasonable approach. The fabrication of 100 additional girders for interim support is also a significant investment for temporary measures. The installation of new exterior edge girders to compensate for the rapidly deteriorating existing exterior girders will also be difficult to integrate considering both existing structural conditions and traffic. The live load is only about a quarter of the total load with the remaining loads locked-in to the existing structural system. Developing a detail and methodology to integrate the new beams for some partial load transfer will be difficult particularly considering the external post-tensioning, stirrups and queen post system that exists at the exterior girders and the transverse post-tensioning of the existing girder system. Even if an effective low risk design is developed, the added girders will only have marginal benefit to the overall integrity of the rapidly deteriorating superstructure in general. The approach spans will require significant, complex and high risk work to maintain them in service until 2030. Maintaining the exact same span arrangements while widening to both sides of the existing alignment creates a conflict with Highway 132 (on the south shore) and with René-Lévesque Boulevard on Nuns' Island where solutions and staging are once again limited and complex.
While the existing steel trusses are known to be in relatively better condition than the prestressed concrete girder approach spans, they are not without issues. Due to the limited load carrying capacity of the existing truss system, a lightweight steel orthotropic deck replacement was carried out in 1992. The structural capacity condition has restricted desired roadway capacity enhancements in the past. Furthermore, the existing piers are not earthquake resistant and are not considered structurally sound for the desired lifespan of the new project. The structural limitations of the truss systems will certainly prevent any widening that is tied in to the existing structural system.
The proposed concept also requires the conservation of the eight existing main span piers for the next 135 years by strengthening them for their noted structural deficiencies. Unfortunately, the existing piers are deficient for both earthquake and may be deficient for longitudinal wind loading. As such, the strengthening work in this regard will require an overbuilding of the existing piers with substantial amount of in-water foundation work to achieve the required complimentary foundation capacity. The poor strength and questionable integrity of the concrete core of the existing piers cannot be relied upon for any significant additional service length. The extensive pier and foundation strengthening work combined with an unreliable existing concrete core would also lead us to to consider this aspect of the plan to be unadvisable.
Based on available information, the steel truss spans are at a reasonable level of integrity in consideration of the age and replacement plans for the bridge corridor; however, there is still a lack of capacity and functionality to warrant additional investment.
In general, the Mr. René Therrien proposal involves numerous stages of construction, interfacing with the existing structure with significant investment into remedial works and ultimately resulting in a final cross-section well in excess of what is required. The span lengths would be the most inefficient of any replacement scheme being considered, resulting in the greatest number of piers, bearings and joints than what should reasonably be constructed considering current medium span bridge technology. The numerous construction stages will result in prolonged construction administration and traffic staging costs and limit the potential for preferred procurement methods. In general, the actual extent of interim and strengthening works is unknown and the contractor risk too high for Arup to confidently independently carry out a cost analysis. There are also some additional costs that have not been considered, most notably and significant being the works for the replacement of the deck on the main spans within an approximate 30 year timeframe. Also, the risk of development of fatigue cracks in the bolted and riveted assemblies (due to high cyclic loads), in the welds of numerous secondary bracing components, and the critical pins of the cantilever spans, would require serious considerations.
While capital costs are spread out over a greater period of time, there is an overall inefficiency inherent to this approach and ultimately, only the net present value of total life cycle costs should ever be considered in assessing alternatives. Even though significant funds have been invested to bring the steel truss spans to their current state of integrity, the numerous and extensive additional investments to extend the benefits of these investments is not sound. It is our opinion that in light of the above discussion, the Mr. René Therrien concept for the Champlain Bridge could not be the most cost effective alternative and over the life of the structure would be a poor investment strategy.
User Needs and Safety
Ultimately, users require a new bridge of appropriate cross-section (at this time, the preferred configuration is 6 lanes + transit + bikeway) to be constructed as soon as possible at the lowest total life cycle cost with minimal impact to existing traffic.
There is no reasonable basis to consider that even the first stage of the Mr. René Therrien proposal could be constructed faster than most other completely separated new bridge options. It is difficult to imagine how the work involved at the interface with the existing structure can be carried out without some disruption to traffic in terms of both capacity reductions and traffic safety risk. Significant deflections occur of the edge girder with the passage of trucks, which will make the connection to the existing edge girders difficult without measures to temporarily reduce live loads. Since the existing cross-section has already been maximized for capacity and has a poor level of service at peak times, there will be stages where the required removals at the existing deck edge will cause another notable increase in congestion on the bridge. The staged demolition of components in proximity to traffic also adds increased complexity and safety risks for this alternative.
The staging also provides varying levels of service including removal of added lanes (compared to 2013 conditions) for a four year period between stages 2 and 3 (10 lanes to 6 lanes). The initial excess of lanes will lead to inefficient route planning and associated third party development leading to significant traffic and public acceptance issues once the additional capacity is removed. The final cross-section also results in an excess of lanes and total deck width. The additional deck width is not only costly (both in terms of initial costs and maintenance costs) but detrimental to overall traffic planning optimization and transit sustainability. Furthermore, a consistent traffic-free transit system, whether BRT or GRT, is not realized until 2034. This delay, coupled with an excess of traffic lanes for cars will be detrimental to encouraging the use of public transportation. Considering that the pre-feasibility study identified the need for a modern transit system as a key requirement from all stakeholders, it would be socially unacceptable to both defer and deter the success of such a transit plan.
The Mr. René Therrien proposal does not offer any real schedule enhancements but adds complexity, risk and significant user impacts in contrast to any of the separate replacement options.
Assessment As Part of Overall Options Screening
The various options under consideration were assessed in terms of the following aspects:
- Construction — duration, costs, complexity and interface with the Seaway
- Operation and Maintenance – costs, level of inspection & maintenance and BRT/GRT operations
- Impact on Users, Wildlife and Community – aesthetics, maintenance disruptions and environmental impact
A discussion of the assessment of Mr. René Therrien's proposed concept in the context of all the options for the above categories is provided in the sections below.
The construction of repetitive girder spans is straightforward and is generally considered the most easily constructed and cost effective. Mr. René Therrien's concept would be considered favourable in this context; however, as discussed previously, the multiple stages of construction with complex interfaces with the existing deteriorated structure and live traffic contributes significant risk to the program. Additionally, the shorter, less efficient span lengths of the Mr. René Therrien concept increases the volume of the repetitive activities in contrast to the other similar options.
There will be some added level of complexity to the longer, more prominent spans over the Seaway and Mr. René Therrien's concept allows some flexibility in structure type. However, the need to construct and deconstruct within close proximity or interfacing with existing structures and traffic will add complexity and risk to its implementation under the Mr. René Therrien concept staging. Certainly, this added constraint and duration of construction program will result in the most significant impact to or limitations imposed by the Seaway operations.
As previously discussed, this option is simply not a lowest cost alternative. Furthermore, the inefficient apportioning of capital costs to construction staging and interim strengthening relative to the final as-constructed infrastructure is at cross purposes to the project requirements and the original intent of this alternative.
Thus, in terms of construction issues, this option cannot be considered as favourable as many of the options and potentially carries the greatest construction risk.
Operation and Maintenance
As operation and maintenance costs tend to be a direct relation to initial capital cost, the completed Mr. René Therrien concept would not be the least costly, in comparison to other girder type options. The more numerous less efficient shorter spans will result in more joints, bearings and piers to be maintained as well. More importantly, the staged replacement approach will involve a significant amount of interim maintenance until 2034 and involve the most interruption to traffic up to this milestone.
The Mr. René Therrien proposed concept does not address the transit planning requirements for this project. The requirements dictate that the NBSL shall be flexible to allow for a dedicated allowance for a BRT or GRT. While a dedicated BRT right of way could be provided in the interim Stage 2 cross-section, transit would have to revert back to a single counter flow lane during the four year period of construction between Stage 2 and Stage 3. The impact on transit will be significant and likely unacceptable at that time. Furthermore, in order to benefit from the deferral of construction expenditures, there is no opportunity for installing a GRT system until 2034 which is completely noncompliant with the project requirements.
Impact on Users, Wildlife and Community
In terms of an overall environmental assessment, all options are essentially similar in that a new bridge of set cross-section with transit initiatives will be provided and the existing structures are completely recoverable and recyclable. The Mr. René Therrien proposal, though, has the added disadvantage of providing excess highway capacity to the detriment of transit demand as well as deferring a fully integrated proper transit solution to 2034.
In consideration of all the alternatives, the primary environmental impact relates to the construction activity in the water and its impact on fish habitat. Considering further that all options will involve foundations on bedrock using cofferdam or caisson construction, the extent of the environmental impact will really rely upon the overall footprint of piers in the river. Mr. René Therrien's proposal involves the most significant number of and overall footprint of foundation construction in comparison to all other alternatives.
Accordingly, the Mr. René Therrien proposed concept has the greatest environmental impact within the St. Lawrence River and is uniquely inefficient and damaging in its final excess cross-section and limitations imposed upon transit initiatives.
Moreover, for the main spans, it may be more difficult to develop aesthetically pleasing and cost effective solutions while needing to develop around the existing structural steel form.
In closing, the Mr. René Therrien "Champlain Bridge Concept" does not meet the overall NBSL project objectives and is not a viable solution for the NBSL. Arup recommends that this option is not considered further.
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