15-11-2012, 02:20 PM
HIGH PERFORMANCE STEEL FOR HIGHWAY BRIDGES
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ABSTRACT
All steels possess a combination of properties that determines how well steel performs.
Strength, weldability, toughness, ductility, corrosion resistance, and formability are all
important to determine how well 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.
INTRODUCTION
The U.S. Transportation system represents huge investments on the part of governments and
taxpayers. There is a widespread concern over the state of infrastructure. Despite indications of
increased investment, it is clear that funds available are not likely to meet all of the needs of this
sector in the long run. More than ever, wise investment decisions concerning roads and bridges
will be crucial to the future of transportation.
The problem we face today, such as the aging infrastructure, stretches our resources thin and
challenges our creativity. As a critical part of the infrastructure, deficient bridges represent a
major impediment to mobility on our highways. The resultant time lost to congestion is a drag
on our nation’s productivity. Innovative materials, such as high performance steel, will play an
increasingly important role as we attempt to meet all of the transportation challenges of the
future, including enhancing and expanding our bridge infrastructure. We will be more
dependent on high performance materials such as High Performance Steel to give us structures
that have 100-year design lives and that will help us with our goal to improve mobility by
eliminating deficient bridges.
DEVELOPMENT OF HIGH PERFORMANCE STEEL
Two new grades of high-performance structural steel, HPS-70W and HPS-50W, are developed
and now commercially available for highway bridge construction. Experiences on the HPS
demonstration projects are very promising, and it 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 cost-effectiveness of highway bridge
construction. FHWA, U.S. Navy, AISI, various universities, and state DOTs have played key
roles in this program. Results are being touted as “the fastest ever technology transfer within
the bridge construction industry in North America.” The payoff from this research should be
quickly realized through cost savings, and more durable and reliable bridges.
WHAT IS HIGH PERFORMANCE STEEL?
All steels possess a combination of properties that determines 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
can be defined as having an optimized balance of these properties to give maximum
performance in bridge structures while remaining cost-effective.
• High Performance Steel has low levels of carbon and carbon equivalents to provide
good weldability. It is weldable with reduced or no preheat and without expensive
welding techniques.
• It has a high level of fracture toughness (Zone 3 minimum) to improve structure
reliability. It provides better than adequate material ductility.
• It has atmospheric corrosion resistance characteristics that will eliminate the need for
coating steel bridges in most environments.
WEATHERING CHARACTERISTIC OF HPS
It was part of the initial research objective to develop HPS with "weathering characteristic",
meaning HPS should have the ability to perform without painting under normal atmospheric
conditions. HPS has slightly better atmospheric corrosion resistance than the conventional
grade 50W or 70W steels. For example, as measured in accordance with ASTM G101, the
atmospheric corrosion resistance index (CI) for conventional Grade 70W is 6.0, while the index
for HPS 70W is 6.5.
WEDABILITY OF HPS
Weldability is a property that is somewhat difficult to define. During welding, the conventional
70 ksi steels typically require preheating of plates, control of temperature of 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.
FRACTURE RESISTANCE OF HPS
High Performance Steel has much higher fracture toughness than the conventional grades of
steel used for bridge construction. Figure 1 shows the Charpy V-Notch (CVN) transition curves
for HPS 70W and conventional AASHTO M270 Grade 50W steel. The brittle-ductile transition
of HPS occurs at a much lower temperature than conventional Grade 50W steel. This means
that HPS 70W(485W) remains fully ductile at lower temperatures where conventional Grade
50W steel begins to show brittle behavior. This has the beneficial effect of eliminating sudden
brittle failure. As a result, reliability is enhanced as the net section can be relied on to resist
applied forces should any crack develop in a structure.
STEEL SPECIFICATIONS
High performance Steel is a quenched and tempered plate product available in thickness to 4
inches for use in bridges. It conforms to ASTM A709/A709M-01 specification requirements. It
is also a Thermo-Mechanical-Controlled-Processed (TMCP) (Non-Q&T) product and is
available in thickness to 2 inches for use in bridges.
AS DELIVERED BASE METAL
Owners, Designers and Fabricators must be aware that the length of HPS70W and HPS50W
Quenched and Tempered (Q&T) steel plates is limited to 50 feet maximum as a result of the
heat-treating process, regardless of the manufacturer. Use of Q&T material may result in
introduction of additional weld splices in girder webs and flanges. The goal is to eliminate the
need for quenching and tempering in production process. Once this is achieved, the plate length
limitation of about 50 ft. will be eliminated. Currently, HPS70W (Thermo-Mechanical-
Controlled-Processing) TMCP plates are available in thickness to 2 inches, with widths and
lengths similar to that of Grade 50W steel plates.
FATIGUE PRONE DETAILS
The fatigue resistance of high performance steels is controlled by fatigue-prone details. Tests of
welded high performance steel connections conclude that the fatigue categories given in LRFD,
Section 6.6.1 also apply to high performance steels. The effective use of high performance
steels can be improved by avoiding fatigue-prone details.
Among the characteristics most likely to improve the fatigue resistance of high-performance
steel is the enhanced reliability due to ease of fabrication. Cracking of the base metal due to
hydrogen is largely eliminated. The ability to weld with reduced or no preheat improves the
working environment so that more reliable welds are produced.