Project Overview
Caltrans has completed a 1,000-foot pedestrian bridge with the Transportation Authority of Marin, connecting Marin County to San Francisco via the Larkspur Ferry Terminal, regional bus service, and local businesses, as part of the North-South Greenway corridor. The project aimed to create a Class I multi-use path over Corte Madera Creek without additional supports and maintain a 5% ADA grade.
The project faced a significant challenge in maintaining 21 feet of vertical clearance beneath the navigation span of the off-ramp, as the U.S. Coast Guard prohibited building it lower that the current off-ramp due to Corte Madera Creek's navigable waterway. This resulted in a nearly 72-foot-long span, despite the structures depth being only two feet.
An environmentally sensitive salt marsh near the bridges northern end limited the use of on-site construction equipment and temporary supports needed to build the bridge's northernmost span. The 52-foot span is small yet nearly reaches the 100-year water surface elevation.
The final project restriction was that the bridge must be open to pedestrians and bicyclists during construction.
The site's complexity and limitations influenced the choice to use precast, prestressed concrete for the pedestrian bridge. Precast concrete provided resiliency, and durability and is well-suited for the heavily used community bridge. The bridge was constructed using four unique precast girder types, including a one-of-a-kind, horizontally curved, and vertically haunched "navigation span" girder.
"The Corte Madera Creek Bridge is an outstanding example of implementing creative ideas in practical ways. The exceptional collaboration between the owner, designer, contractor, and precast producer elevated the success of the project, establishing it as a major regional accomplishment", says Garrett Dekker, PE, Moffatt & Nichol.
Precast Solution
The project team utilized existing supports in Corte Madera Creek to create a Class I multi-use path without installing new or temporary supports. The off-ramp underwent seismic retrofitting in 1998, enabling the use of existing infrastructure to support the new pedestrian bridge. Precast girders were designed to cross the gap between supports, serving as load-bearing members and a work platform for the contractor to build the bridge above the creek without needing temporary or permanent supports in the water.
Supporting corbels were installed on the outrigger caps to set precast girders at specific elevations along the bridge's length, achieving a maximum grade of 5%. Precast girders are ideal for this structural arrangement, as seat elevations can be adjusted to achieve the desired longitudinal grade. Precast concrete spans made it possible to complete this significant structural feat.
The design team developed a pair of custom "L-shaped" precast reinforced concrete girders to maintain 21 feet of vertical clearance over the creek. These girders were constructed with a precise vertical curve, considering the deck profile and girders' long-term deflection. The resulting precast span is an architectural highlight, serving as the bridge's focal point and giving the center span a distinctive appearance.
The design team chose precast voided slab beams with half the dept of I-girders for the relatively short 52-foot span, which nearly touches the 100-year water surface elevation over the salt marsh, near the bridge's north end. The beams appear to float above the marsh, avoiding impacts on the habitat for the endangered salt marsh harvest mouse.
Precast, prestressed concrete was manufactured off-site, transported to the construction site, and quickly erected. This helped overcome the final project obstacle of keeping the bridge open for pedestrian and bicycle traffic. The precast was installed over a period of two moths, with on 40 hours of overnight bridge closures. Precast construction allowed the contractor to use limited nighttime closures to erect the superstructure and limit disruptions to the traveling public.
Precast concrete proved to be the most effective solution for addressing complex site restrictions, achieving project goals, accelerating the building schedule and controlling the project budget.
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