For Efficient Tubular Structure avoid Class 4 Tubes
SAISC TECHNICAL
Structural Hollow Sections:
For Efficient Tubular Structure avoid Class 4 Tubes
By Franco Mordini, Chairman of Technical Committee, ASTPM
Structural Hollow Sections remains very popular from highly Architectural application to Industrial applications. Regardless of the applications the major benefits for Structural Hollow Sections are the efficiency for resisting compression loads such as bracing elements, torsional strength, its high strength-to mass ratio and its ability to be fabricated and erected in long sections. Benefits like great aesthetically and reduced painting areas are secondary.
In steel design one of the most important advantages is to use the high strength of steel effectively. In too many cases Engineers still specify class 4 members that are both difficult to design, difficult to make but most importantly do not leverage the high strength of steel. Simply Class 4 members do not reach the yield strength before failure. Interestingly the theory does coincide with the theory in the manufacturing process. Class 4 members are more difficult to make as the strip tends to locally buckle between the rolling stands. Also import to note, in the cases of Class 4 members, higher strength does not lead to higher resistance. In fact the higher the strength the lower the permissible d / t ratio for Circular Hollow Section and the lower the permissible b / t ratio for square or rectangles for these Class 4 members. These sections should be avoided wherever possible.
In these cases rather change the section to at least a Class 3 section. In most cases this is done by simply going down in diameter, width or height or by increasing the thickness of the section. In most cases this will also reduce the cost of the structure and provide the end users with a more efficient structure. The definitions of the various classes for flexural compression for the standard grade of 355 are shown below.
Conclusion
Should you use Class 4 members it is important to use only the effective area for all the geometrical properties of the section. This will significantly reduce the strength of the section. In order to avoid these complications that also result in inefficient structures only specify Structural Hollow Sections that are Class 1 to 3 members.
Class definitions – SANS 10162-1 Maximum width-to-thickness ratio – elements in flexural compressions
Class Maximum for CHS Maximum for flange of SHS 1 d / t ≤ 13000 / fy = 36.6 b / t ≤ 420 / SQRT fy = 22.3 2 d / t ≤ 18000 / fy = 50.7 b / t ≤ 525 / SQRT fy = 27.9 3 d / t ≤ 66000 / fy = 185.9 b / t ≤ 670 / SQRT fy = 35.6 4 d / t > 185.9 b / t > 35.6
The maximum width-to-thickness ratio for axial compression Class Maximum for CHS Maximum for flange of SHS 4 d / t > 23000 / fy = 64.8 b / t > 670 / SQRT fy = 35.6
24 Steel Construction Vol. 40 No. 2 2016