Collaboration and Careful Planning Lead to a Successful Two-Week Bridge Replacement

The Narlon rail bridge is perched between ancient sand dunes approximately 1 mile from the Pacific Ocean over San Antonio Creek near Lompoc, California. Winds off the ocean push heavy, moisture-laden air high in chlorides to the bridge site most days. After over 125 years of service, the 720-foot-long and 80-foot-tall iron and steel tower bridge at Narlon reached the end of its functional life due to aggressive and accelerating corrosion.

In 2016, HDR as Union Pacific’s owner’s representative suggested a construction manager/general contractor “light” approach to delivering a new, lower risk, lower cost, more constructible replacement design. This CM/GC approach offered several advantages:

• Collaborative development of the construction plan among owner, contractor, engineer and environmental and the ability to incorporate other stakeholder interests
• Greater cost and schedule certainty
• Clear assignment of project risks to each entity

In addition to our owner’s representative work, HDR’s role also included design of the project and construction oversight.

In August 2021, the bridge was replaced during a two-week curfew, the culmination of extensive planning and collaboration.

Flexible, Long-Lasting Design

Like the existing bridge, the new bridge consists of alternating 30-foot spans on braced towers with 60-foot spans in between. HDR used lidar scanning techniques in addition to conventional survey to assist in design. The complex bridge geometry led to a novel 3D CAD development effort, where the model was developed from individual element 3D cells and 2D sheet views cut from the 3D model.

As-built drawings of timber pile foundations and previous geotechnical investigation allowed for estimating existing pile capacities. HDR analyzed the existing foundations and determined they were adequate for supporting the new bridge. An open deck design was necessary to keep dead loads close to existing conditions.

HDR’s corrosion experts performed wet candle testing, which showed high concentrations of chlorides at the site. Union Pacific needed the bridge to have a 100-year lifespan with very little maintenance.

Uncoated steel structures in this environment may only have a service life of 40 years before significant corrosion would limit the load capacity of the bridge. The HDR team developed a multi-coating system to achieve the desired life span, including hot dip galvanizing of the structural steel, an epoxy intermediate coat and a polyurethane/polysiloxane top coat.

Two-Week Bridge Changeout

Bridge changeout occurred during a two-week complete shutdown of railroad traffic with work proceeding 24/7. Demolition started slowly and cutting the heavy tower columns made up of layers of plates, angles and gussets required a learning curve. Due to the limited space available, activities had to be well planned to maintain a safe work site and avoid interference. New towers were staged on piles in the willow riparian area beneath the bridge to allow for fast installation upon removal of the existing bridge. HDR’s team was on site during the entire complex process.

The success of the entire project relied on active and diligent collaboration that drove progress, solutions, improvements and innovation.

At the end of the two weeks the new bridge was complete and has been in operation since.