Super structures: an update on high-strength steel design

Super-strength steels up to 1000 MPa allow engineers to push the boundaries of structural design. Nancy Baddoo and Tak-Ming Chan, editors of new themed issues of ICE’s Structures and Buildings journal, report.

By using high-strength steel, the main roof trusses at Friends Arena, Stockholm, were 13% lighter and 15% cheaper than using conventional strength steel
By using high-strength steel, the main roof trusses at Friends Arena, Stockholm, were 13% lighter and 15% cheaper than using conventional strength steel

High-strength steels are increasingly being used in structural applications. Key properties such as toughness and weldability continue to improve as advances are made in production processes and metallurgy.

Although they cost more than normal strength steels, in many design situations the cost penalty is easily outweighed by the strength increase. For example S460 steels are 30% stronger than S355 steels, yet the cost premium is only about 10%.

Additional benefits of using smaller sections are less welding and painting are required, as well as lower transportation costs. An increase in net-to-gross area ratio for each floor of a building is also advantageous, especially for commercial buildings.

In general, high-strength steels are most suitable for heavily loaded members, often where the steel elements would otherwise be very thick. Typical applications include relatively stocky, gravity columns for buildings taller than about four to seven storeys. Weight savings can also be made using high-strength steels for lateral stability systems, transfer beams and bracing members in high rise buildings.

New standards

Demand from construction specifiers has led to European product standards for hot-rolled products (EN 10025 and EN 10149) now covering steels with strengths up to 960 MPa and the forthcoming revisions of the standards for hollow sections (EN 10210 and EN 10219) will do likewise.

Running in parallel with developments to product standards, the scope of the main part of the European steel design standard, Eurocode 3 (EN 1993-1-1) is being extended to cover steels with yield strengths up to 700 MPa.

The possibility of developing a new part of Eurocode 3 giving supplementary rules for steels with strengths up to 960 MPa is currently under discussion.

To keep its members up to date with the latest developments in high-strength steel structures, the Institution of Civil Engineers has just published the first part of a two-part themed issue of its Structures and Building journal.

Section tests

The first paper (Ma et al., 2017) presents a review of recent tests on built up, hot-finished and cold-formed hollow sections made from steel with strengths of 690 MPa and higher. The applicability of European, American and Australian design rules to these members are discussed.

The second paper (Yang et al., 2017) uses two different experimental techniques to investigate the distribution of residual stresses in welded circular hollow sections made from steel with yield strength of 690 MPa. Comparisons are made with normal strength steels. The sensitivity of the residual stresses to galvanising, and varying the diameter-to-thickness ratio, are also studied.

The third paper (Schillo and Feldman, 2017) presents an experimental and numerical study of the rotational capacity of high-strength steel I-shaped beams. While the specimens with yield strength 960 MPa showed very low rotational capacity, the results for the specimens with strength 700 MPa achieved their plastic moment resistance and exhibited good rotation capacity, demonstrating the potential for using plastic global analysis methods for steels stronger than 460 MPa.

The fourth paper (Jiang et al., 2017) presents a numerical investigation of how the fabrication process affects the axial compression strength of high-strength steel welded box columns, including a simulation of the welding process in order to generate residual stress patterns. The effect of post-weld heat treatment on residual stresses are also studied.

Seismic resilience

The fifth and sixth papers study the use of high-strength steels to enhance the resistance of steel frames to seismic loading. The fifth paper (Chen et al., 2017) is an analytical study of the benefit of using high-strength steels in the novel ‘tension only concentrically braced steel beam-through frames' structural system in which the frame is designed to remain elastic while concentrating damage on the replaceable braces during an earthquake.

The sixth paper (Ke et al., 2017) describes the development of a seismic damage control evaluation method for a system comprising high-strength steel frames with energy dissipation bays. The method is calibrated against a large-scale laboratory test of a two-storey, two-bay frame with energy dissipation bays.

Finally, the seventh paper (Šmak et al., 2017) presents an experimental study of the strength and hardness of welded joints between steels of strength 700 MPa and steels of strength 235 and 355 MPa, investigating the effect of different electrodes and weld parameters.

I hope that reading the papers will help to inform and equip civil and structural engineers for their future designs using high-strength steel.

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