Class Rules

IACS Adopts Buckling Strength Assessment of Ship Structural Elements (UR S35)

Harmonized methodology provides for independent buckling toolbox regardless of ship type

Ship operations take place in hostile and corrosive environments meaning that the structural strength of the ship’s hull may deteriorate with time leading to buckling or structural instability.  Buckling has long been recognized as one of the main modes of failure of ship structural elements and IACS has, for many years, had in place a number of Resolutions to address this.

Recognizing, however, that the rules for buckling varied across different IACS Unified Requirements (URs), such as S11, S11A, S21, S21A and Common Structural Rules for Bulk Carriers and Oil Tankers (CSR for BC & OT), in 2018 IACS agreed to develop a standalone Unified Requirement (UR) dedicated for buckling strength assessment using the net thickness approach.  This has resulted in the publication of UR S35 “Buckling Strength Assessment of Ship Structural Elements” which will enter into force on 1 July 2024.

The new harmonized methodology for buckling has been developed applying the toolbox in the Common Structural Rules (CSR) for Bulk Carriers & Oil Tankers which are widely accepted as being technically sound and which have been implemented, maintained, and improved over the years with industry’s expertise and experience feeding into the continuous improvement of CSR.

In harmonizing the buckling methodology, improvements have been made to the global elastic buckling mode for stiffened panels subject to combined loads (biaxial loads, in-plane shear force, and lateral pressure), the torsional buckling mode for stiffeners, buckling strength of U-type stiffeners, plates with opening and also plate panels fitted with sniped stiffeners.

Thorough verification and consequence assessments have been carried out by extensive linear and nonlinear FE analyses, with the harmonized requirements showing increased accuracy and effectiveness.  The application of the buckling requirements such as scope, loading, and permissible utilization factor will be described in individual UR S series, e.g. in UR S21 on Evaluation of Scantlings of Hatch Covers and Hatch Coamings.

The new UR S35 will serve as an independent buckling toolbox regardless of ship type by collating, as far as possible general requirements on buckling together in one UR.  A further benefit arising from the development of UR S35 is to facilitate the merger of the UR S21 and UR S21A (which currently address different ship types) into a single UR S21 Rev.6 also with an entry-into-force date 01 July 2024 to align with that of UR S35.

Commenting on this new UR S35, IACS Secretary General, Robert Ashdown, said

the adoption of a unified approach to buckling is a significant step in making the rule application check of different ship types easier for Industry but also in enhancing the maintenance of future buckling rule improvements by IACS.  Furthermore, it also facilitates the further harmonization of other relevant IACS resolutions that will also be of benefit to the industry.

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