The development and deployment of high-performance steel (HPS) for highway bridges originated with a joint effort of the Federal Highway Administration (FHWA), the Navy and the American Iron & Steel Institute in 1992. The effort was a resounding success. More than 200 HPS bridges now carry traffic in 43 states.
This collaboration acted to develop an affordable steel grade with higher strength, improved weldability and greater toughness while enhancing overall quality and ease of fabrication.
Now high performing
Work continues on improving the capability of HPS grades. For example, the yield strength of HPS 70W, the most widely used of the HPS grades, tends to drop off with plate thickness. To compensate, the steel’s manganese content can be increased. For HPS 70W plate thicknesses greater than 2.5 in., specifications now permit increasing the manganese maximum level from 1.35% to 1.5%. This change avoids the need for re-heat treating the plate to maintain minimum yield strength requirements.
The most recent development of high-performing bridge steels is the HPS 100W grade. Development work is under way to increase toughness for plate thicknesses of this grade beyond 2.5 in., which may require increasing the plate’s nickel content. The steel chemistries being studied are based on the U.S. Navy’s 100 thousands of lb per sq in. (ksi) grades.
A recent demonstration project in Nebraska represents the first steel bridge that makes extensive use of 100 ksi steel. This bridge, located near Grand Island, Neb., crosses over I-80. It’s a two-span steel box bridge with equal spans of 139 ft in length. The original design for the bridge made use of steel box girders in a hybrid arrangement, with bottom flanges of the box sections using 70 ksi HPS and webs and top flanges using conventional 50 ksi steel. Then designers substituted 100 ksi HPS for all webs and flanges to demonstrate that fabrication and construction would proceed normally.
Use of HPS allowed designers to increase the span length of each girder beyond the traditional 120 ft, while keeping the total weight of each girder below the crane capacity of the local fabricators. In addition, the HPS permitted thinner bottom flanges and reduced web depth. The webs of the HPS girders are perpendicular to the bottom flanges rather than sloped. This significantly reduces fabrication time and cost. The fabricator can use equipment and practices typical for I-beam plate girders.
The Nebraska Department of Roads implemented another innovation for this bridge. Designers initially configured each of the HPS girders as simple spans (abutment to pier). They then made the simple spans continuous by connecting each of the two in-line girders with a concrete diaphragm at the pier, making the three HPS bridge girders continuous for live and superimposed loads. The simple-made-continuous technique eliminated bolted field splicing of the girders, which would normally take place away from the pier and over the traffic below. The concrete “splice” significantly reduced the interruption of traffic and accelerated construction time. In this case the I-80 highway had to be closed for only 90 minutes for placing the three girders for each span.
The bridge opened to traffic in October 2003, becoming the first 100-ksi HPS bridge in the U.S.
New optimized shapes, designed to replace routine box and I-girder shapes, will further realize the full benefit of the strength and weldability of HPS.
In the mid-1990s, the Advanced Technology for Large Structural Systems (ATLSS) Center at Lehigh University and Modjeski and Masters Inc., with funding by the FHWA, began studying nontraditional steel bridge beam configurations.
One candidate configuration studied was an I-girder with a corrugated steel web. The corrugated web increases web stability, allowing a reduced web thickness without the need for web stiffeners. Increased stability benefits fabrication and erection. Fewer attachments to the web and flanges also improved fatigue performance.
Following initial studies, the next step was to design and build a demonstration bridge based on design equations and details using finite-element analysis, fabrication studies and applied laboratory research. Research for the extended project included:
- Selecting the optimum corrugated shape (trapezoidal or sinusoidal), considering structural performance, fabrication and manufacturing processes;
- Conducting shear and flexural tests to verify design capacities; and
- Testing bolted splices and determining fatigue properties.
Production processes tested included robotic welders on full-size test specimens. Fabricators studied two different robotic systems: one with the robot stationary while the steel girder advanced and one vice versa. This work represents the first step in using robotic welding for bridge girders in the U.S.
Bradford County, Pa., became the site of a demonstration HPS bridge with a corrugated web, which was designed by the Pennsylvania Department of Transportation (PennDOT). According to Tom Macioce, P.E., a bridge engineer with PennDOT, this two-lane demonstration bridge has two spans. The four lines of steel girders for this bridge have corrugated webs with a trapezoidal configuration. HPS 70W constitutes the web, flanges and splice plates. The bridge opened for service in July 2005.
In another development, steels with superior corrosion resistance are being evaluated for challenging bridge applications. ASTM A1010 steels, which have 12% chromium content, now find use in such applications as coal rail cars and coal processing equipment. Accelerated laboratory tests and exposure panels in seaside locations indicate that A1010 outperforms weathering and galvanized steels in wet and dry saltwater environments.
A cellular box girder bridge located in a corrosion-prone environment of Colusa County, Calif., uses 0.16-in.-thick A1010 steel. Research continues to develop production practices for more traditional bridge applications requiring thicker plate. A1010 steel can be considered when life-cycle costs are the paramount criterion. These steels, however, are about twice the cost of grade 50W steels.
Steel’s legacy
Recent federal highway legislation known as the “Safe Accountable Flexible Efficient Transportation Equity Act: A Legacy for Users” (SAFETEA-LU) provides funding for research directed at high-performing steel bridges. The research has two goals: finding low-cost, corrosion-resistant grades of steel and reducing maintenance costs through longer lasting coatings. The legislation provides $4.1 million per year for four years for this research.
