Steels for modern bridge construction

Steels for Modern Bridge Construction The steels used in today's bridges bear no resemblance to the cast iron , wrought iron and early carbon manganese steels. High performance steels with yield strengths of 355 and 460 Mpa are widely used, and steelmakers worldwide can now readily supply grades with yield strengths in excess of 690Mpa. The steel grades used in bridges have to meet certain requirements, which are defined in material standards. These standards vary around the world. However, rather than dwell on standards and specifications, this presentation focuses on the latest developments in steels for modern bridges. These include: Weathering steel. High performance steel. Longitudinally Profiled (LP) plate. High strength steel. Constant yield point steel. High toughness steel.

Yeonjong Bridge, Korea

Slochd Beag Bridge, Scotland

Weathering Steel Weathering steels are high strength low alloy steels, which under normal atmospheric conditions give an enhanced resistance to rusting compared with that of ordinary carbon manganese steels. Weathering steel bridges do not require painting. Periodic inspection and cleaning should be the only maintenance required to ensure the bridge continues to perform satisfactorily. Hence, weathering steel bridges are ideal where access is difficult or dangerous, and where future disruption needs to be minimised. Cost savings from the elimination of the protective paint system outweigh the additional material costs. Typically, the initial costs of weathering steel bridges are approximately 5% lower than conventional painted steel alternatives. In addition, The minimal future maintenance requirements of weathering steel bridges greatly reduces both the direct costs of the maintenance operations, and the indirect costs of traffic delays or rail possessions.

High Performance Steel High Performance Steel (HPS 70W) is a new high strength weathering steel that has been developed in the USA over the last decade. The key features of this steel are high strength, high toughness, good weldability, and enhanced durability. HPS 70W has proved to be very cost effective for highway bridges in the USA, and is becoming widely specified for new bridges.
	I-90 over Muitzes Kill Creek, New York State, USA

Example of LP-plates

Longitudinally Profiled (LP) Plate The demand to reduce self-weight was the starting point for the development of longitudinally profiled plates (LP-plates). By a special control of the rolling gaps during the rolling process, a lon¬gitudinal profile with a continuously varying thickness along the length of the plate can be given to a heavy plate. Various types of LP-plates with different geometries can be produced. Such plates allow an optimised adaptation of the plate thickness to the actual stress in the structure. In addition, the use of LP-plates can speed fabrication and reduce costs as complicated welds can often be avoided. Avoiding such welds also enhances the fatigue performance of the bridge structure. Today, LP-plates are used in bridge building all over Europe.

High Strength Steel Developments in rolling processes, combined with new chemistries, have allowed steelmakers around the world to produce grades suitable for bridge construction with higher strengths of 460Mpa, 690Mpa and above. The use of such steels on appropriate bridges offers many benefits. Plate thicknesses can be reduced leading to savings in material requirements and fabrication processing (for example, cutting, welding and drilling). The reduction in self-weight benefits transportation, handling and erection, and even facilitates longer spans. In addition, the resulting slimmer structures are generally more elegant and aesthetically pleasing.

Example of the application of high strength steel in a truss bridge

Constant Yield Point Steel In Japan, a steel has been developed that offers a constant yield strength through the range of 16mm to100mm. With ordinary steels, the yield strength reduces as the plate thickness increases, and this is reflected in the material standards. However, with this new steel (called “Steel with Constant Yield Point”) designers are able to utilise a higher yield stress for thicker plates. The use of such steel not only simplifies the design process, but also leads to more efficient steel bridges as flange thicknesses can be reduced.

Comparison of yield point between SM520 and SM520C-H

High Toughness Steel

Due to recent progress in production technology, it has become possible to manufacture steel plate with excellent toughness, which offers the following key benefits:

• Cold forming is possible with smaller bending radius.
• The steel products can be used in cold regions.

The cold bending of steel introduces strains which reduce the toughness of the steel section. Hence, specifications place restrictions on the inside bending radius to limit the strain hardening, typically 15 times the material thickness. However, with high toughness steel the inside bending radius can be reduced to 5 times the material thickness.

The toughness of steel products decreases at low temperature, and the steel becomes susceptible to brittle fracture. However, the use of high toughness steel plates allows the use of steel for bridges in very cold regions.

Improved bending radius, toughness comparison, and an example of cold formed steel pylons