Focus on high-performance

by Susan Lane, Eric Munley, Bill Wright, Marcia Simon,
and James D. Cooper

While there have been many advances in steel-making technology over the last 20 yrs., today structural steels in the United States are not very different from those used 20 years ago. This is particularly true for the higher-strength grades, with yield strengths greater than 355 MPa. While today’s steels possess high strength, they are only marginal in weldability, toughness, and corrosion resistance. Today’s steels can and have been used successfully, if all conditions are ideal. However, in many cases, problems have occurred. Consequently, the use of higher strengths has been extremely limited, due to the reluctance of owners and designers to risk potential problems.

The consequences of this are twofold:

1. There has been little impetus for innovation in steel designs using higher strengths, because the materials were questionable.

2. Industry has invested little to improve these steels, because they currently comprise less than 1% of the steel used in bridge construction.

The net result is that innovation has been limited in steel bridge design, and many potential gains in cost efficiency and long-term performance are being overlooked.

What is HPS?

For highway bridge construction, high-performance steel can be defined as one that possesses the optimum combination of properties required to build cost-effective structures that will be safe and durable throughout their service life. It is important to consider the more important properties and their significance to the Federal Highway Administration HPS research initiative.

Strength. There have been relatively few problems with today’s steels with strengths of 355 Mpa and below. Therefore, the FHWA is looking at the higher-strength grades (480 and 690 MPa) in its research program. These higher strengths can be immediately used to improve the structural efficiency of today’s bridge designs. Preliminary results also indicate that strengths as high as 900 MPa will be possible in the future. However, different bridge design concepts will be needed in order to use it.

Corrosion resistance. There is much more concern today for the long-term durability of bridges than there was a few years ago. Traditionally, corrosion protection was achieved by painting the bridge. Today, the high cost and adverse environmental impact of repainting make this approach impractical. Therefore, corrosion resistance is an essential property for HPS to be used for bridge construction. As a minimum, the HPS grades being currently developed will have corrosion resistance equal to or greater than today’s A588 weathering steel. Future research is needed in this area to gain further improvements in corrosion resistance without incurring the excessive cost of stainless steels.

Weldability. Today’s steels are weldable; however, the process must be highly controlled and inspected to ensure adequate quality for bridge construction. When conditions are not perfect, weld defects occur, and subsequent repair and reinspection adds significantly to the fabricated cost of bridges.

A primary goal of high-performance steels is to make them more weldable by reducing the carbon levels in the steel. The payoff will be steels that are more tolerant of welding process procedures and welding conditions. This can lead to significant improvements in fabrication efficiency, improved weld quality, and possible reductions in the level of quality assurance inspection that is currently required. Improving weldability also opens the door for using field welding for construction and repair of steel structures.

Toughness. Toughness is the ability of steel to ductilely deform under a load, rather than fracture brittlely like glass. High toughness allows structures to absorb and redistribute the impact of traffic loads. This becomes even more critical at low temperatures where steel becomes more brittle. Many of the HPS processes being researched today produce steels with excellent toughness. Tougher steels provide a higher reserve capacity to structures, and allow those structures to tolerate fabrication flaws and withstand extreme loading events such as earthquakes. Toughness may provide the confidence necessary for engineers to take a risk, and try innovative new bridge design concepts and systems.

Formability. Formability is a key factor in high-performance steel due to the increasing use of light-gauge structural members. Many of these new shapes may require steels that tolerate bending and forming operations.

HPS design issues

Innovation in bridge design has been slow to occur over the last 30 yrs. While the U.S. highway community used 690-MPa steel in the 1960s, problems related to welding and brittle fracture caused its use to be discontinued. Yet, while there were problems, it was noted that high-strength steels have higher strength-to-weight ratios and result in more efficient bridge designs. The improved properties of current HPS should open the door again for new innovations in U.S. bridge design.

The FHWA is also sponsoring design research to determine how much the current design practice can be improved through the use of HPS, and to investigate new design concepts that, when coupled with HPS, can result in more cost-effective bridge systems.

Preliminary results from this study indicate that steel strengths as high as 480 MPa can be used with a savings of up to 10% on the complete fabricated cost. Work is also underway to develop design concepts that use higher strengths and are even more cost effective.

The FHWA is also investigating new structural shapes, such as corrugated web plates and tubular flange members, in combination with other systems (such as prestressing), as an additional way to improve design efficiency. It may turn out that the bridge of tomorrow bears little resemblance to the bridges commonly built today.

It is also becoming apparent that many restrictions in the current American Association of State Highway and Transportation Officials bridge specification are potential barriers to the implementation of HPS in the U.S. These restrictions, which need to be reconsidered.

Future plans for HPS

Steel plates from the FHWA alloy development project are on a parallel track with another project for preliminary design recommendations. When the results are in, the two studies and their results will be merged, and a program to build prototype HPS bridge structures will be implemented. The FHWA is considering partnerships with state departments of transportation that are willing to have innovative bridge structures in their state.

The Civil Engineering Research Foundation has recently issued a report, High-Performance Construction Materials and Systems — An Essential Program for America and its Infrastructure, that includes a detailed evaluation of the future research needs for HPS. Among the many topics in this report are improved weathering steels, steel cladding, improved structural shapes, and improved composite systems with HPS and other materials. Improvements in these areas will lead to great improvements in the life-cycle cost and performance of highway bridges in the future.

Susan Lane, P.E., Eric Munley, P.E., Bill Wright, P.E., Marcia Simon, P.E., and James D. Cooper, P.E. represent the Turner-Fairbank Highway Research Center of the Federal Highway Administration. This article was adapted from a paper presented at the U.S., European Bridge Workshop in Barcelona, Spain. This article is the third in a three-part series.

Reprinted from Better Roads Magazine
May 1998