TxDOT’s Triumph in Steel Engineering

The Texas Department of Transportation’s pioneering efforts in the realm of steel bridge design and construction, particularly showcased through innovative projects such as the Brazos River Bridge, recently earned the department the National Steel Bridge Alliance’s Owner of the Year award. (Texas Department of Transportation)

The Brazos River Bridge reflects the Texas Department of Transportation’s award-winning approach to engineering excellence.

The National Steel Bridge Alliance (NSBA) recently honored the Texas Department of Transportation (TxDOT) with its inaugural Owner of the Year award. The recognition stems from TxDOT’s pioneering efforts in the realm of steel bridge design and construction, particularly showcased through innovative projects such as the Brazos River Bridge.

“TxDOT isn’t just implementing best practices for designing and building steel bridges, it is defining how an owner can maximize the potential of steel,” says NSBA Senior Director for Market Development Jeff Carlson. “Recent projects like the remarkably economical Brazos River Bridge demonstrate how TxDOT’s longstanding investment in steel bridge research is paying dividends for Texans today and tomorrow.”

Bridging Research with Practice

Thanks in large part to the Texas Steel Quality Council, a joint owner-industry forum, and years of research, TxDOT has spent decades developing best practices for achieving economical and easily constructed steel bridges. These initiatives underpinned the solutions implemented in the Brazos River Bridge project, which was included in a broader project that involved shifting Texas State Highway 105 (SH 105) to accommodate three bridge replacements across the Navasota and Brazos rivers and Coles Creek.

The need for replacement stemmed from the westward migration of the Brazos River, compromising the foundation of the bridge’s east approach and necessitating a shift to a more stable location. Relocating the Brazos River Bridge approximately 820 feet downstream (south) of the existing bridge necessitated realigning a portion of the SH 105 roadway as well. Because the Brazos River Bridge is in close proximity to bridges at the Navasota River and Coles Creek, modifications to the Brazos River Bridge’s alignment also affected these bridges. The project also aimed to address functional obsolescence concerns with the existing bridges, all of which have been in use since the 1950s, thereby enhancing safety and efficiency along SH 105.

Construction of the new bridges and roadway began in 2013, a $33.4 million project contracted to James Construction Group, but they weren’t opened for traffic until September 2021. The project experienced years of delays due to Hurricane Harvey and Brazos River flooding, which resulted in some engineering changes. Despite these challenges, at the heart of the project lies a robust foundation of research-driven decision-making.

Due to the bridge’s 300-foot main span, the project team was left with two options: steel I-girder or concrete spliced girder. The contractor ultimately chose steel due to its versatility, cost-effectiveness and compatibility with the envisioned design parameters.

Engineering Challenges and Solutions

One of the first issues to be addressed was the precise location of the replacement bridge. Due to the Brazos River’s slope-failure region, the new bridge couldn’t be placed adjacent to the existing structure. It also couldn’t be placed upstream of the existing bridge due to the channel migration to the south and the flow being severely skewed to the proposed bridge. The only rational location for the bridge was downstream of the existing bridge, as the river will likely continue to migrate to the west. Long spans with few disturbances to the environment were desired.

Engineering

TxDOT designers proposed that the bridge be supported by single-column bents founded on multishaft footings. They also wanted to keep the foundations out of the Brazos River as much as possible. Due to the bridge’s 300-foot main span, the project team was left with two options: steel I-girder or concrete spliced girder. The project was let for construction with both options, and the contractor ultimately chose steel due to its versatility, cost-effectiveness and compatibility with the envisioned design parameters.

The first step in the design process for the steel-girder option was to investigate the ideal type of girder (I-girder or trapezoidal box girder) and transverse cross-section (number of girders). The investigation focused on the most-economical means of fabrication, shipping, erection, maintenance, future inspection and deck construction. Three preliminary steel superstructure cross-section types were considered: four I-girders spaced at 13 feet, five I-girders spaced at 10 feet and three trapezoidal box girders (tub girders) spaced at 14 feet 11 inches. To ensure the girders were cost effective from a material standpoint, TxDOT contacted AFCO Steel, a steel fabricator whose input on flange transitions—as well as plate thickness and length—provided valuable information for the preliminary design process. The final steel superstructure chosen was the five-line I-girder system with a constant web depth of 92 inches and a constant flange width of 24 inches. All steel was fabricated using Grade 5OW.

Unique Attributes

The Brazos River Bridge stands as a testament to TxDOT’s culture of innovation and continuous improvement, as standard TxDOT bearing designs were modified to meet the structural needs of the steel unit. The cross-frames were a costly structural component in the overall steel bridge unit. The bridge’s designers considered TxDOT research project 0-1772, “Cross-Frame and Diaphragm Behavior for Steel Bridges with Skewed Supports,” and put the research into practice on a nonskewed bridge. By integrating lean-on bracing principles into the bridge’s design, TxDOT reduced the number of cross-frames significantly from 148 to 80. This is the first TxDOT bridge without bents oriented on a skew angle to use the lean-on bracing concept. Previous bridges using the lean-on concept all had skewed supports.

Cross-frames often are difficult to install due to fit-up problems and can attract significant live-load forces that may lead to fatigue problems. Minimizing the number of cross-frames on the bridge led to a better design and reduced maintenance costs. The TxDOT “Bridge Design Manual—LRFD” was revised to allow the use of the lean-on bracing design after TxDOT completed research project 0-1772.

Lean-on bracing concepts allowed the project team to brace several girders across the width of the bridge using a minimal number of cross-frames. (Texas Department of Transportation)

Typically, for the steel I-girder framing system, cross-frames would be placed between each girder at a uniform spacing along the length of the girders. Using lean-on bracing concepts allows several girders to be braced across the width of the bridge by a minimal number of cross-frames. Lean-on bracing provides multiple benefits to bridge owners, fabricators and erectors, including improved structural performance and long-term durability, simplified inspections, lower cost, and easier fabrication and erection, as detailed in the National Steel Bridge Alliance’s “Lean-on Bracing Reference Guide.”

To develop the lean-on bracing system configuration for the Brazos River Bridge, the erection sequence of steel girders had to be considered during the bridge design phase. After consulting with a local steel erector, a solid assumption could be formed about how the bridge’s steel portion would be built. The lean-on bracing configuration assumed the bridge would be erected beginning at the first and last bent of the steel unit, with a drop-in section at mid span of the middle span of the steel unit. The framing plan was developed assuming girder No. 1 (far-left girder) is placed first, following with girder Nos. 2 through 5. The bridge was designed using software from MDX, and spreadsheets were developed to check the stiffness at each line of bracing across the width of the bridge. The bridge was modeled in UT Bridge software, developed by researchers at the Ferguson Structural Engineering Laboratory at the University of Texas at Austin, for the construction phases.

The substructure for the Brazos River Bridge consists of single-column bents supported by a footing and drilled shafts. A single-column support was chosen to minimize disturbances to the surrounding environment. Due to the soil type and concerns with scour, the columns couldn’t be rectangular; therefore, each column has a large, chamfered edge. The aesthetic concept captures the true meaning of the Brazos River, “The River of the Arms of God.” The single column splits into two columns near the top with a recessed concrete area between them to appear as a void or window. The two columns represent two arms reaching up and supporting the bridge over the river. The aesthetic interior bents provide support for most of the bridge, but the abutments, which support the beginning and ends of the bridge, also are unique. As stated previously, the river has a history of meandering. With this in mind, the bridge abutments were designed as bent caps, which will allow for future lengthening of the bridge if necessary.

A view from the top of the bridge shows the steel girders and bracing installed. (Texas Department of Transportation)

A side view shows the bridge’s steel superstructure. (Texas Department of Transportation)

Pioneering the Future

The Brazos River Bridge project epitomizes TxDOT’s commitment to excellence in research, engineering and innovation. A steel I-girder superstructure was well suited to the project, and the bridge’s geometry, which included a straight steel I-girder unit, contributed to the decision to design the bridge using lean-on bracing concepts. Decreasing the number of cross-frames saved on material and fabrication costs as well as simplified construction. Advanced software modeling and meticulous planning facilitated the efficient erection of steel components, showcasing the synergy between theoretical knowledge and onsite implementation. By harnessing the power of steel and integrating cutting-edge design concepts, TxDOT set a new standard for bridge construction.

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