Structural Steel Specifications, Properties, and Seismic Selection for Building Structures

A technical article by structural engineer Chen Cheng-ping, published in the Taipei Structural Engineers Association Forum, systematically examines manufacturing methods, mechanical properties, and seismic selection criteria for structural steels used in buildings. While steel frames provide light self-weight, workability, and large spans, joint details, weld heat input, and steel grades directly affect structural safety. Early high-strength steels suffered from poor weldability due to high carbon equivalents; however, developments since the 1970s—such as microalloying, controlled rolling, thermo-mechanical controlled processing (TMCP), and inline quenching-tempering—have enabled higher strength alongside refined grains and reduced carbon equivalents. Mechanical requirements encompass strength, ductility, toughness, and workability, with seismic applications demanding tight yield strength limits, low yield ratios, and verified impact toughness, especially against lamellar tearing in thick built-up box columns under electroslag welding.

Key takeaways

• Steel plate manufacturing has evolved from conventional rolling and normalization to controlled rolling, TMCP, and inline quenching, refining grains to allow 60–70 kg/mm2 grades to balance strength, impact toughness, and weldability.
• In seismic regions such as Taiwan, framing members require upper limits on yield strength and yield ratio to ensure uniform plastic hinge formation and intended energy dissipation.
• Impact toughness is evaluated using Charpy impact testing via absorbed energy or shear fracture ratio; due to rolling directionality, through-thickness toughness requires strict verification based on operating temperatures.
• Built-up box columns using high-heat electroslag welding face grain coarsening and lamellar tearing in thick plates (e.g., ≥25mm); specifying through-thickness reduction of area and steels like CNS SN-C or SM570MC HW is recommended.