Walk Bridge Replacement Design Alternatives
The Connecticut Department of Transportation (CTDOT) evaluated more than 70 replacement design variations prior to selecting the 240’ Vertical Lift Span Bridge. The alternatives assessed fell within the following categories:
Representative options in each category were evaluated against factors such as local impacts, construction duration, impacts to rail service, and cohesion with the project’s purpose & need.
While each alternative has varying impacts, there is no viable alternative that results in a markedly shorter construction timeframe or significantly fewer construction impacts than the 240” Vertical Lift Span Bridge preferred alternative. The figures for the alternatives outlined below are based on the concepts prepared for the Environmental Assessment and Environmental Impact Study published in 2016.
A “no-build” alternative is not viable due to the systemic nature of the 123-year-old bridge’s deteriorated condition. The existing bridge, while safe for travel, suffers from extensive section loss due to corrosion, widespread fatigue damage, excessive mechanical wear, and obsolete electrical systems leading to higher maintenance costs.
Extending the Walk Bridge’s life span by 100 years through rehabilitation requires the strengthening or replacement of thousands of internal and external components. Systems requiring attention include steel trusses, bridge piers, foundations, abutments and retaining walls; mechanical, electrical and architectural components; track and catenary systems; and the high tower transmission structures. Despite the extensive breadth of work required to strengthen and repair the bridge’s deficiencies, the underlying structure would still be more than 120 years old.
Rehabilitation work is performed directly adjacent to the operating railroad. This proximity to the active rail line results in restrictive limits on construction, extending the project’s duration and inflating costs. A rehabilitation option would require the construction of a temporary two track bridge to the north of the existing bridge. This temporary structure would limit marine traffic to only those vessels that don’t require an opening.
The low-level fixed bridge alternative retains the existing height and grade of the railroad tracks over the Norwalk River, while imposing a fixed limit on the vertical clearance between the bridge and waterway. This alternative fails to maintain the current level of navigation, which fails to meet the project’s purpose & need criteria developed in consultation with federal and state oversight agencies. Additionally, this alternative requires multiple piers to be constructed in the water, resulting in a larger environmental footprint than other alternatives.
During construction, a temporary two track bridge to the north of the existing structure may be required, restricting navigation for the duration of the project. Like rehabilitation, this option presents challenges working near an operating railroad, resulting in construction inefficiencies. The impacts of this design are estimated to be almost twice that of the preferred alternative.
This modified low-level design envisions a fixed bridge constructed on the same horizontal alignment as the preferred movable alternative, with a vertical clearance of 26’ and a channel width of 170’. Unlike the original low-level alternative evaluated in the planning phase, this alternative removes the need for multiple foundations in the water. New foundations would be constructed at similar locations as the movable option, creating the same construction efficiencies as the movable alternative and promoting safer vessel movements with improved channel alignment with the Stroffolino and I-95 bridges.
The modified low-level fixed bridge would be a through truss, like the vertical lift alternative. The elimination of movable bridge elements such as lift span towers, mechanical equipment, electrical equipment and utilities result in an approximately 15% cost savings. However, this alternative is costlier than the original low-level fixed bridge that was evaluated, due to the larger quantities of steel and concrete necessary to accommodate a longer span.
This option would restrict navigability on the Norwalk River for all vessels taller than 26’, and would prevent future dredging of the Norwalk River north of the Bridge.
With an estimated 100-year service life, the preferred alternative provides two side-by-side vertical lift spans carrying two tracks each across the Norwalk River. The 240’ vertical lift span has a vertical clearance of 26’ when closed and 60’ open. Each span possesses independent mechanical and electrical equipment allowing two tracks to remain in service while the others are taken out of service for routine maintenance.
Construction of two new piers, bridge foundations and towers occurs outside of the navigation channel. This allows the existing swing span to remain in operation for most of construction and minimizes environmental impacts. The movable bridge alternative improves the channel alignment with the adjacent Stroffolino and I-95 bridges.
The bridge truss is constructed offsite and the two spans are floated in by barge, one half at a time. The second float-in will occur about a year and half after the first. The first float-in will require a complete rail service outage over an extended weekend with the second float-in taking about a weekend. The construction staging of the movable option minimizes risk and impacts to rail operations by maintaining two-track service while satisfying the project’s purpose & need.