AD 271: Strongbox® 235 Hollow Sections and Structural Applications
Although the issues relating to specifying Hot Finished (BS EN 10210) and Cold Formed (BS EN 10219) structural hollow sections have been addressed in AD 259 we continue to receive questions in this area; in particular, concerning the use of Strongbox® 235 in structural applications.
Strongbox® 235 is the brand name of a range of cold-formed hollow sections manufactured by Corus Tubes for the general engineering market requiring a lowgrade ‘box’. It is strongly recommended that this range of hollow sections should not be used in structural applications. Corus Tubes does however manufacture two alternative ranges of hollow sections for the structural sector, Hybox® and Celsius® 355.
Since Strongbox® 235 is intended for the general engineering market it is not manufactured to a particular product standard and hence the only mechanical property guaranteed is its minimum yield strength ReH of 235 N/mm². None of the other mechanical properties required of a structural steel are guaranteed; namely, minimum tensile strength Rm, Charpy Impact properties at specified test temperatures and minimum percentage elongation requirements. Further, hollow sections in the Strongbox® 235 range are supplied with a test report type 2.2 (not subject to ‘specific’ inspection and test requirements) and hence do not conform to clause 2.1.3 of BS 5950-2: 2001. This clause requires that all steel used in structural applications in buildings must be subjected to ‘specific’ inspection and testing requirements, which is satisfied by a 3.1.B Inspection Certificate. Reference should be made to the product standards for structural steels, BS EN 10021 and BS EN 10204 for an understanding of ‘specific’ and ‘non-specific’ inspection and testing requirements at manufacture as well as the accompanying certificates and reports.
In addition, Strongbox® 235 hollow sections are manufactured to the dimensional tolerances of BS EN 10219 with the exception of the weight tolerance. The lack of a weight tolerance means that the section properties of the Strongbox® 235 hollow section range are smaller than the section properties of Cold Formed hollow sections, which fully conform to BS EN 10219. Corus Tubes does provide tables of section properties for Strongbox® 235 for situations where ‘designs’ are not intended to comply with the building regulations and/or design codes.
In view of the above it is strongly recommended that Strongbox® 235 hollow sections should not be used in structural applications; in particular, where designs must comply with the building regulations and/or design codes.
The two fully compliant ranges of hollow sections manufactured by Corus Tubes for use in structural applications are:
• Cold Formed Sections to BS EN 10219 S355J2H: Brand name – Hybox®
• Hot Finished Sections to BS EN 10210 S355J2H: Brand name – Celsius® 355