Critical temperatures for compression members in the UK NA to BS EN 1993-1-2

Advisory Desk Note 546

Steel Construction Institute, 2025
Critical temperatures for compression members in the UK NA to BS EN 1993-1-2 Download document

Document Status

Current

Supplement

Civil & Structural Supplement

Series

Advisory Desk Note

Abstract

This SCI Advisory Note clarifies the limitations of using critical temperature values from the UK National Annex to BS EN 1993-1-2, particularly Table NA.1, for compression members in fire design. The tabulated values assume an imperfection factor of 0.49 (buckling curve 'c'), appropriate for UC sections in S355 buckling about their minor axis. However, this assumption is not conservative for members with lower imperfection factors—such as UB sections, hot finished hollow sections, or higher-grade steels—commonly found in trusses, bracing, or frames. The note presents replacement critical temperatures for hollow sections in S355 with a = 0.21 and advises designers that both UK NA and ASFP Yellow Book guidance may be unconservative for members not meeting the curve 'c' assumption.

History

July 2025 edition.

Subjects

  • Building systems/frame
  • Steel building systems/frame
  • Building structure
  • Materials
  • Metal sections/other items
  • Steel

Publisher History

With over 30 years experience, the Steel Construction Institute provide technical expertise and best practice to the steel construction sector.

Information

NBS

NBS has produced specification systems and information products for construction industry professionals for more than 40 years. The National Building Specification is the recognised UK national standard and is used by over 5,000 organisations. NBS is part of RIBA Enterprises Ltd.

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