Flexible Formwork Accommodates Unique Geometry of Twin Cast-in-Place Segmental Bridges

The Challenge:

Remove and replace approximately 12,000 square feet of highway US-281 in Burnet County, Texas, including the steel-deck truss bridge that crosses the Colorado River.

The Players:

Doka
Archer Western Contractors

The Process:

Built in 1936 and rehabilitated in 1975, the steel-deck truss bridge of highway US-281 that travels over the Colorado River in Marble Falls, Texas, is inefficient and will soon be replaced.

The bridge replacement project consists of the demolition of a steel truss bridge that will be replaced with twin cast-in-place segmental bridges. The bridges will each be three-span variable depth cast-in-place segmental structures. The spans of the bridge will be 274, 410 and 274 feet, with a bridge deck width of 47 feet. The structure will be built using the balanced cantilever construction method with end spans constructed on falsework. The box depth will vary from 23 feet at the interior piers, to 9 feet, 6 inches at the end spans, and have a variable super elevation of up to 5.5 percent.

The distinctive feature on this project is the overall V shape of the bridge. The bottom of the bridge starts with a deep V shape that tapers to a very flat V shape at the apex or center span. This required a form that was flexible in all directions, as no two segments were the same.

Formwork solution

The proposed bridge’s unique geometry required careful advance coordination between Doka and Archer Western Contractors. Doka had to create forms that were flexible from start to finish. The different span lengths at varied box depths required flexible formwork to accommodate the many wide-ranging needs.

Archer Western chose Top 50 formwork panels which were easily customizable to almost any configuration in the preassembly yard or on the jobsite. Top 50 allowed the contractor to use less labor, meet on-site requirements economically and improve safety with walkway brackets.

To meet the complicated geometry of this bridge in cost-efficient manner, Doka began by supporting the pier table with the Staxo high performance shoring system and using Top 50 formwork panels for the lower soffit, walls and upper soffit (or road deck). Staxo provided Archer Western with high shoring capacity, versatility, and quick and easy assembly using integrated couplers.

The pier table was poured in three separate steps. The formwork mostly stacked on itself so that shoring and form removal was kept to a minimum. When the pier table was complete, the traveler was set and the segment formwork was put in place.

Like the pier table, the backspan will be a three-step process in which Doka will supply the formwork and shoring. The backspan will use Staxo shoring and Top 50 formwork as well.

Formwork advantages

The ability of the formwork products to integrate with each other simplified the process when more than one product was needed. Additionally, since this project had innovative elements to the design, ingenuity and inventive thinking were required of the form supplier. For example, all of the forms from the pier table to the traveler had to allow for not only geometry changes but also field changes. This required flexibility that could not have been accomplished with the use of the traditional steel forms.

“The thing that I liked most about the Doka forms on this project is that the formwork hinges when stripped,” said Mike Wood, Archer Western Project superintendent. “With a steel form, we would have had to take it apart each time, but with the Doka forms they always stay together. This makes it fast and easy to strip and reset.”

The outcome will be a beautiful, well-constructed bridge that will serve the community for years to come without the constant maintenance required by the current steel bridge.

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