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