Looking northeast toward downtown Pueblo, the balanced cantilever construction method can be seen in the blue slipform mechanism at the end of the westbound span. The Arkansas River levee is in the foreground. CDOT Photo.

Pueblo’s new Fourth Street Bridge project has hit its halfway point, a nearly four-year, $27.7 million construction job that will give Colorado 96 a graceful twin span crossing of the extensive Pueblo rail yard and the Arkansas River.

The bridge is being built by Flatiron Constructors Intermountain of Longmont with an unusual method designed by FIGG Bridge Engineers, a Tallahassee, Fla., firm with its western regional office in Denver.

The design is a set of three segmental concrete spans built from atop the structure itself via traveling forms that gradually extend out from either side of the tall bridge piers, a method called balanced cantilever construction. There are also two end spans that use cast-in-place construction from the opposite abutments to the first support piers, for a total of five spans.

The concrete approach saved the Colorado Department of Transportation about $5 million in costs over steel. In addition, Flatiron’s low bid was $1 million below CDOT’s engineering estimate.

CDOT’s total cost including design and management is $35.5 million.

Movable form enables cantilevered construction of the concrete span out from the supporting pier without working from the ground level.

The overhead slip forms used in the balanced cantilever method minimize the need for ground equipment and staging underneath the structure. In addition, the concrete design and construction method allowed CDOT to include a main span of 378 feet, the longest concrete span for any bridge in Colorado. That is sufficient width to span the entire Union Pacific portion of the yard – 23 tracks –without having to sink a single pier in the middle of it. The five BNSF tracks will be cleared without a bridge pier as well.

As a result, Flatiron is able to build these artistic spans without requiring any track closures below.

I got a chance to see this construction during a recent trip to Pueblo for a family get-together. While crossing the existing Fourth Street Bridge, built in 1958, it’s easy to see how the new one will improve driver safety. Primarily, it will straighten out a horizontal reverse-curve on the west approach, where Lincoln Street jogs slightly left, then right, then left again to line up with the span.

That’s because the existing four-lane bridge was built to the east of the older one it replaced, resulting in the jogged approach. The new bridge is in the ghost alignment of the pre-1958 structure and lines up more directly with the Lincoln Street approach.

In addition, the east approach curve will be slightly wider and less steep than the current one.

Rendering shows the twin spans clearing the Union Pacific yard, with the BNSF tracks in the foreground.

CDOT investigated rehabbing the existing bridge but found the poor condition of the supporting substructure made that a poor choice. It is rated structurally deficient, with a rating of 26.3 on a scale of 100. It is the lowest rated state highway bridge in all of Pueblo County.

The existing bridge is 1,069 feet long and was built as part of the citywide highway improvements that connected to the Pueblo Freeway project in the 1950s. It consists of seven spans, and five of the piers rise out of the spaces between tracks in the rail yard. Getting rid of them will improve safety in the rail yard as well and meet updated railroad standards for clearances.

UP and BNSF use the large Pueblo yard – formerly part of the Arkansas River floodplain until the levee was built following a disastrous 1921 flood – as a transfer point for many of the coal and freight trains that traverse the Front Range every day.

The project started in December 2007 and the opening is scheduled for June 2011.

As far as staging, FIGG and Flatiron worked out a sequence that will keep vehicle traffic flowing as well.

The first of the two new spans that will be complete is the westbound one, and is being built to the west of the existing bridge. Support piers for the new eastbound span are also being built in between the two. The new bridge’s footprint is almost entirely outside the footprint of the existing structure – except for the easternmost final span of the new eastbound bridge. That means it can’t be connected down to grade on the downtown end until the old bridge is demolished.

As a result, Flatiron will complete and open the westbound bridge, which will temporarily handle all four lanes, allowing for demolition of the current bridge and final completion of the eastbound span.

When completed, the two new bridges will be about six feet apart. They will each have two traffic lanes, with shoulders of six and 10 feet, plus 10-foot multi-use sections for pedestrians and cyclists. They will be wide enough to accommodate three traffic lanes each if needed in the future.

This will be the fourth bridge to cross the Arkansas River here on Fourth Street. On the west side, it connects through to Lincoln Street and then Thatcher Avenue, which takes CO 96 west out of town. To the east, it leads into Fourth Street and then straight out of town toward Avondale.

The first bridge, from the 1880s, was washed out in the 1921 flood. It was replaced with a steel truss bridge in 1924 that then was replaced in the fifties by the current span. The existing bridge cost $1.77 million in 1958, and was the single largest contract among all the roadwork associated with Pueblo Freeway construction.

Post card shows the original Fourth Street Bridge over the Arkansas River.