|
|
|
High-Performance Steel: Research to Practice
by William Wright
A new grade of high-performance structural steel, HPS-485W, is now commercially available for highway bridge construction. This is the first product developed under a highly successful cooperative research program between the Federal Highway Administration (FHWA), the U.S. Navy, and the American Iron and Steel Institute. The new steel possesses superior weldability and toughness compared to conventional steels of this strength range.
Conventional 485W steel has been available for bridge construction for years, but its use has been limited because it requires more precise control of welding and fabrication practices than lower strength steels. Although research has shown that many bridges can be designed more efficiently with higher strength steel, few bridge owners were willing to risk potential problems in fabrication. Consequently, only a few bridges have been built with steel of this grade.
The new HPS-485W grade promises to overcome these drawbacks and allows the full potential of this strength grade to be realized.
This article presents an overview of the research program that developed this new product. Also discussed are the first demonstration bridge projects that are now underway to use this steel in the "real world."
What Is High-Performance Steel?
All steels possess a combination of properties that determine how well a steel performs its intended function. Strength, weldability, toughness, ductility, corrosion resistance, and formability are all important to determine how well a steel performs. High-performance steel (HPS) can be defined as having an optimized balance of these properties to give maximum performance in bridge structures while remaining cost-effective. The main two differences compared to conventional 485-megapascal (MPa) steels are improved weldability and toughness. Other properties such as corrosion resistance and ductility will be essentially the same.
Weldability is a property that is somewhat difficult to define. Conventional 485-MPa steels typically require preheating of plates, control of temperature between weld passes, controlled handling of welding consumables, precisely controlled energy input, and post-weld heat treatment in some cases. When all of these operations are performed correctly, it is usually possible to produce high-quality welds in conventional high-strength steel. Difficulties can arise, however, when one or more of these operations deviate from prescribed procedures. Minor differences in procedure and quality control are the norm for bridge construction, where many different fabricators in different parts of the country work under different climates and conditions. The result is that conventional high-strength steels have experienced a higher percentage of weld problems compared to lower strength steels. Another disadvantage is that these controls, particularly the control of temperature, add significantly to the cost and time required for welding. The goal in developing HPS grades is to provide a steel that is forgiving enough to be welded under a variety of conditions without requiring excessive weld-process controls that increase costs.
The American Association of State Highway and Transportation Officials (AASHTO) sets the minimum specifications for toughness required by a steel. The specifications are based on climate (zones I, II, and III) and use (fracture critical vs. non-fracture critical). For fracture-critical members in the most severe climate (zone III), AASHTO currently requires a minimum charpy-vee-notch (CVN) energy of 35 ft-lb @ -10 F. Toughness values reported from the first heat of HPS-485W ranged from a minimum of about 120 ft-lb to a maximum of 240 ft-lb @ -10 F.(1) This far exceeds the current AASHTO minimum requirements. This "extra toughness" should provide a very high resistance to brittle fracture that will allow structures to tolerate a high level of damage without risk of sudden failure. This will provide added confidence to enable designers to use the full strength of this steel.
A final feature of high performance is high corrosion resistance. The HPS-485W grade satisfies the composition requirements listed in ASTM specification G-101 to allow classification as a "weathering" steel suitable for use in the unpainted condition. This gives designers the option to eliminate painting in many bridge locations and requires only limited painting in others. Because experience has shown that the life-cycle cost of many steel structures can be reduced by eliminating painting, this property is considered essential to call a steel "high performance."
Steel Development
Many approaches were tried to develop a steel with high performance and 485-MPa strength.(2) Both processing methods and alloy composition were varied until the optimum combination for HPS-485W was selected. The optimum alloy turned out to be a modified version of the existing A709 grade 485W quenched and tempered steel. The big difference is that the carbon level was greatly reduced, thereby providing the large boosts in weldability and toughness. Table 1 shows the chemistry differences between HPS-485W and conventional A709-485W steel. As shown, the carbon is reduced from a maximum of 0.19 percent to about 0.10 percent. Alloy adjustments, micro-alloy additions, and processing changes enable strength to be maintained. This low carbon level is the primary reason for the great improvements in weldability and toughness.
Table 1 _ Composition of HPS-485W Compared to Conventional Steel (by percentage)
| C | Mn | P | S | Si | Cu | Ni | Cr | Mo | V | B | Al | |
| A709-485W | 0.19 max | 0.80 to 1.35 | 0.035 max | 0.04 max | 0.02 to 0.65 | 0.20 to 0.40 | 0.50 max | 0.40 to 0.70 | - | 0.02 to 0.10 | - | - |
| HPS-485W | 0.10 | 1.2 | 0.006 | 0.001 | 0.39 | 0.33 | 0.31 | 0.52 | 0.06 | 0.06 | 0 | 0.013 |
It can be noted that the HPS-485W grade also satisfies the requirements of A709-485W. This means the new steel can be ordered under the existing specifications, with a supplemental requirement for the low carbon chemistry. Such a specification is currently being developed by the research team.
Testing and Evaluation
An extensive effort is underway to evaluate the properties, weldability, and structural performance of bridge members fabricated from HPS-485W. Standard tension and CVN tests are being performed on each plate that has become available through this program. This process will be continued as future plates are produced to develop a comprehensive database on material properties and variability. These basic tests are being supplemented by more advanced fracture mechanics tests being conducted at the FHWA and Navy laboratories.
Weldability is being studied through a series of tests. Fundamental performance is being evaluated using the gapped bead-on-plate (G-BOP) test, Y-groove test, and Implant tests. These tests study the performance of all parts of the weld, including the base metal, weld metal, and heat-affected zone between the two. These tests are still in progress. Additionally, a series of standard procedure qualification plates have been welded according to procedures in the American Welding Society's Bridge Welding Code (AWS-D1.5-95). These tests are used to qualify welding procedures prior to welding in the fabrication shop. Preliminary results indicate that HPS-485W may be capable of being welded without preheat and interpass control, using both the submerged arc welding (SAW) and shielded metal arc welding (SMAW) processes. More work is being done to verify this performance and develop updated code provisions for the new steel.
Structural performance of welded steel bridge members is also being evaluated through a series of full-scale tests. The FHWA Structures Laboratory in McLean, Va., is performing a series of full-scale fatigue and fracture tests to determine what effect the high toughness has on performance. It should be possible to show that the new steels make members more resistant to fatigue and fracture due to the high toughness. Static bending tests are being performed at the University of Nebraska at Lincoln to determine the strength and ductility of bridge I-girders. This research will be the basis for optimizing the AASHTO specifications to take full advantage of the properties of HPS.
Demonstration Bridges
Construction contracts have been awarded for the first two high-performance steel bridges using the new HPS-485W grade steel. Two states, Tennessee and Nebraska, are partnering with FHWA through the demonstration projects program to smooth the way for the introduction of this new product. The construction cost of the bridges is being borne by the states, as is the case for normal federal-aid jobs. FHWA is providing additional funds for supporting research, documentation, quality control, and field monitoring of the completed structures. The fabrication and erection of these two bridges began in February 1997. It is anticipated that additional projects will be initiated in 1997 as more bridge owners become aware of the availability of HPS. Michigan, Pennsylvania, and West Virginia have expressed interest in developing future projects.
The bridge in Tennessee is a two-span continuous structure located on state Route 53 over Martin Creek in Jackson County. The structure will provide an 8.5-meter-wide roadway over two spans, both 72 meters long. Three welded 2-meter-deep plate girders fabricated from HPS-485W form the cross section, and they will act compositely with a cast-in-place concrete deck slab. The design was fully optimized for the 485-MPa steel, using the new AASHTO Load and Resistance Factor (LRFD) Bridge Design Specifications.
Cost estimates prepared by the Tennessee Department of Transportation indicate that the steel weight was reduced by almost 25 percent compared to the original grade 345W design. Because HPS-485W currently costs slightly more than grade 345W steel, this resulted in a 16 percent reduction in the total cost to fabricate and erect steel for this bridge. This results in a cost savings of about $78,000, according to estimates prepared by the Tennessee Department of Transportation.
Nebraska has set on a more ambitious demonstration of HPS, consisting of a series of three bridges, over the next several years. The Nebraska Department of Roads has partnered with FHWA and the University of Nebraska at Lincoln to implement this effort. The first step is a direct substitution of grade 485W for grade 345W in a simple-span bridge. The design of this bridge has not been optimized for the higher strength, the focus being on optimizing welding and fabrication procedures. The second bridge will be a multispan structure fully optimized for HPS-485W at a site yet to be determined. Finally, a third phase, which will implement one of the new innovative bridge designs that are currently being researched, has been conceptually agreed upon.
The contract has been awarded for the first bridge, a 46-meter simple-span bridge on state Route 79, located at Snyder South, Dodge County, Neb. The bridge provides a 10.8-meter clear roadway on a concrete deck supported by five 1.4-meter-deep welded plate girders. Fabrication is scheduled to begin in early spring, and construction will follow in summer 1997. Comparative cost estimates for this project are not meaningful because no design optimization was performed. However, the knowledge gained on fabrication performance will provide an essential basis to justify fully optimized designs that will follow in the future.
Photo 2: Completed set up for testing a steel beam in FHWA's Structures Laboratory.
Summary
The demonstration project in Tennessee is showing that HPS can reduce the first cost of steel bridges by reducing the weight of steel in the structure. These savings are being realized even though many of the projected cost savings associated with fabrication efficiency are not yet available for this job. Therefore, even greater savings are projected for the future when welding and fabrication procedures are totally optimized. It is difficult to project the overall savings potential because it will vary with bridge type and span length. However, the experiences of the first demonstration projects are very promising, and HPS should have a significant impact on the bridge industry. This research and development effort has been a model partnership between government, industry, and academia to improve the cost-effectiveness of highway bridge construction. FHWA, U.S. Navy, American Iron and Steel Institute, various universities, and state transportation departments have contributed key pieces of the program. The results are being touted as "the fastest ever technology transfer within the bridge construction industry in North America."(3) The payoff from this research should be quickly realized through cost savings.
Back to Public Roads Table of Contents
References
 |
TFHRC Home | FHWA
Home | Feedback
United States Department of Transportation - Federal Highway Administration |