US 281 Colorado River Bridge, Marble Falls, Texas

The new U.S. 281 Bridge over the Colorado River replaces a functionally obsolete steel truss bridge with a 958’ long segmental bridge. The bridge design by the Texas Department of Transportation (TxDOT) along with design innovations from the Contractor-Engineer team created a highly aesthetic bridge with a minimal footprint in the water. The bridge was completed on October 9, 2014.

U.S. 281 is a major north-south highway from Wichita Falls to San Antonio and serves as an important evacuation route and emergency services access for the area. Factors leading to selecting a cast-in-place segmental design include:

The nearest river crossing detour is located 30 miles north.
Limited site access and right-of-way due to adjacent businesses and utilities restricting an alignment change.
Active recreational lake traffic in the area that is reliant on tourists.
High local regard for the look of the old truss bridge.

There are 24 concrete segments per cantilever, with 48 segments total per bridge. Each segment measures 14’ to 16’ long and 47’ wide. The variable segments sport a unique tapered boat hull design in the bottom slab, an aesthetic treatment that matches the community’s focus on recreational boat racing. The segments have a box depth that ranges from 23’ at the interior piers to 9’5” at the end spans, with a variable superelevation up to 5.5%. Each segment weighs a maximum of 150 tons.

Innovation of Design and/or Construction

The major changes proposed by the Contractor-Engineer team included revising the pier table design, segment layout and post-tensioning specifications. The design modification called for an unbalanced design (22′ x 14′ from centerline of column). This required less falsework, and only two temporary supports during construction on a reduced schedule.
Temporary shoring for the prop drilled shafts which were incorporated into the pier table falsework.
The transverse and longitudinal post-tensioning was modified to utilize 4 strand tendons at 2’9-1/2″ spacing. This modification saved on duct, heads, grout, caps and allowed for smaller stressing anchors in some areas.
While reducing the length of each segment required more segments, the process of pouring each segment using the form travelers was optimized, and required less labor-intensive falsework to be built.
Cofferdams could not be used due to hard limestone rock at the bottom of the lake, close proximity to the existing bridge, and the need to maintain an open water channel for boating traffic. The design modification called for drilling shafts into the rock riverbed and lowering an on-site precast concrete footing form to accommodate the forms and the work platform.

Rapid Construction

The structures were built with two form travelers using balanced cantilever construction, with end segments constructed on falsework.
The modified pier table length allowed for a significant reduction in falsework. The unbalanced design also eliminated the need for a stability prop on both sides of the pier. This reduced approximately 12 weeks off the construction schedule and maintained the horizontal clearance envelope throughout construction.
Since it was desirable to limit the drilling in the water, the Contractor’s Engineer developed an innovative approach for the pier table falsework and stability prop design. The prop also served as the support for the main pier table falsework beams.
The Contractor’s Engineer used Bridge Information Modeling (BrIM) which made it possible to develop details quickly to meet the demanding schedule.

Aesthetics/Harmony with Environment

The public was very sensitive to the aesthetics of this bridge since the lake is also used for recreation.
The variable segments feature a unique tapered boat hull design in the bottom slab, an aesthetic treatment that matches the community’s focus on recreational boat racing. A flared column design with a seamless transition between the pier and pier table required a custom built form poured in two pieces, with 6′ of column base, 3′ under the normal water level.
To maintain a minimal footprint, traditional footing was not used. Pier table falsework was designed with bolted connections to ease installation and removal. Drilled shafts were designed to stop at the mud line. The props incorporated sand jacks to aid in removal by slowly relieving the 1000 kip per leg load.

Cost Competitiveness

The Contractor secured two form travelers that met the specifications for this project which resulted in cost savings of $750,000 and several month’s reduction to the schedule for design and fabrication of new travelers. These and other design modifications met all the goals of the project providing approximately $2 million savings, 5% below the TxDOT estimate.

Cost per square foot is $206.74

Minimization of Construction Impact

Since the closest river crossing was over 30 miles away, this bridge crossing had to remain open continuously, requiring phased construction.

Reused form travelers from another project resulted in cost and time savings. Highly aesthetic shape in an area with heavy recreational boat traffic. Innovative precast footing box allowed footings to be placed below the waterline without cofferdams. The perfect fit to meet this project’s diverse demands of the traveling public, local environment, and surrounding community. The boat-shaped hull bottom slab, tapered piers, absence of visible foundations, and lighting have combined to produce a beautiful bridge day or night.