AN UPDATE ON THE PREVIOUSLY
(Steel Construction March 2010)
By Franco P. Mordini, Robor Market
Development Engineer and Chairman
of the Technical Committee of the
Association of Steel Tube and Pipe
Manufacturers of South Africa
Grade 355 tubes, also commonly known as structural hollow sections, was officially launched in February 2010 in Cape Town, Durban and Johannesburg. The launch was a joint venture between the Association of Steel Tube and Pipe Manufacturers of South Africa and the Southern African Institute of Steel Construction. Engineers are now taking advantage of an increased minimum yield stress of 355MPa and an ultimate yield tensile stress of 450MPa for designs.
Grade 355 Tube has enhanced the existing benefits of structural hollow sections. Tubes now compete on an equal footing with other steel sections as well as competing with other construction materials.
Grade S355 is now the customary standard in the ‘small bore’ structural range, i.e. size up to CHS 219 x 6, RHS 200 x 100 x 6 and RHS 160 x 80 x 6. Sizes greater than these dimensions are available but typically with a longer leadtime of approximately eight weeks.
Structural benefits of tubes are apparent when one compares various different profiles required to resist the same compressive loads. In Table 1 below various different profiles are compared in resisting a compressive load of 800kN and an effective length (kL) of 3.0m. In the comparison the circular hollow section is lightest of all, followed by the square hollow section. The circular hollow section is 46% lighter than the 152 x 152 x 37 universal column. In members subject to mainly compression mass savings of up to 55% by mass are possible when compared to other profiles.
In simple terms the material in a tube is ideally positioned far away from the center of gravity of the profile and hence increases the radius of gyration. This reduces the slenderness ratio thus increasing the load carrying capacity of the cross–section. This material distribution results in a typically higher second moment of area with a higher section modulus. Another advantage of
tubes is that the closed section increases the St. Venant torsion constant (j) hence increasing the torsional resistance of the member. This results in high flexural stiffness in all directions combined with a high torsional stiffness.
Table 2 is a comparison between different profiles where a St. Venant torsion constant of 1.7 x 106mm4 is required to resist a torsion load.
In general, columns and beams made from tubes do not need to be checked for torsional-flexural buckling if they fall within the limits of the slenderness ratio or when the height to breadth of these sections does not exceed 2 to 1.
Another factor, often overlooked, is that because the tubular structure is lighter and therefore more efficient, a smaller foundation may result, with the potential of reducing costs. This is especially applicable where poor foundation conditions are encountered.
These structural efficiencies also give the architect and engineer options of reducing the number of columns in the structure. This effectively provides the developer with more usable space thus optimising the area under cover.
Low mass to strength ratio
The lower mass of the structure often provides the engineer with a competitive engineering solution. The lighter, yet stiffer structural members simplify the erection as longer spans can be pre-fabricated, thus increasing the speed and reducing the cost of erection.
From an environmental aspect less material is used without affecting the functionality of the structure. This is particularly important for long–span girders and trusses, where hot rolled section construction could require lateral stabilising during lifting.
The smooth exterior presented by hollow section members together with the absence of gussets, re-entrant corners, inaccessible surfaces (as in double angle members), bolt heads and nuts etc. can result in a major enhancement of the ease
with which corrosion resistance design may be achieved. Particular benefits can be gained on structures where access is difficult for maintenance painting such as footbridges or sign structures over motorways.
Galvanizing of tubular steel
The vast majority of the S355 Tube manufactured locally is made with galvanizing friendly steel. The silicon content ranges between 0.15 to 0.25 – an internationally well recognised range that will produce good galvanizing. This silicon range is the same as that of Grade 355 steel plate and conventional long products, hence aesthetics are compatible. Previously Grade 300 steel was aluminum killed. As expected this has resulted in the need to slightly modify the galvanizing parameters. Generally the finish, when compared to low silicon steel, is less shiny. Typically the coating thickness is slightly higher resulting in an increased corrosion protection of the structure.
The phosphorus maximum content is also controlled. Statistical data received to date has measured a maximum of phosphorus of 0.02% and the average of 0.012%.
Concrete filled Tubes – an interesting option
Tubes offer an option to increase their load carrying capacity by filling them with concrete. This capacity can further be increased by the addition of reinforcement. Not only does the additional strength come with little extra cost, the fire resistance is increased significantly. Internationally this method has found favour with columns in high-rise construction.
Guidance with regard of designing of concrete filled hollow sections is comprehensively covered in SANS 10162 Part 1. But remember to allow an escape hole to permit the steam that would be given off due to the conversion of the water of hydration by heat to steam. Failure to do this could negate the fire protection of the system.
Aesthetical attractive appearance
Aesthetically pleasing solutions are possible using tubular construction. This is derived not only from the simple outline of the profiles themselves, but also from the absence of such items as gussets, battens, lacing, sub-bracing, lateral ties, bolts and the like. Curving of members is easily facilitated with circular hollow sections, and to a lesser degree with square and rectangular hollow sections. Remember that thicker profiles are less likely to suffer from secondary distortion during curving.
Reduced paint areas, refer to Table 3, and reduced wind resistance, refer to Table 4.
In Table 3 the circular hollow section has 43% less surface area than the equivalent star angle strut configuration resisting the same load. The relative ease with which the painting maintenance can be done is also apparent when one studies the various member configurations.
Similarly Table 4 compares the effect of wind force for the same design example. A star angle strut attracts 64% more force than the equivalent circular hollow section.
GRADE 355 TUBE SIZE RANGE
In Table 5 the size range is shown. Stocking of the sizes varies from manufacturer to manufacturer.
TECHNOLOGICAL DEVELOPMENTS IN PROCESSING OF STEEL
Modern high definition plasma and laser profiling machines have arrived in South Africa, simplifying the assembly of connecting profiles radically. This, ever developing, technology makes the profiling of tube intersection simple. The machinery provides the fabricator with a clean, ready-to-assemble component that can be simply welded together with ease. The machines not only profile and/or make slots at the ends of the members but can cut any shape that may be required along the length of the member.
Plasma and laser machines are supplied with either a 2D head or a 3D head. The 3D head is typically required for thicker material where weld preparation is required at the end of the tube.
CONCLUSION – ECONOMIES OF TUBULAR STRUCTURES
We have presented a strong case for using hollow sections in steel structures. The argument against their more general utilisation is of course based on cost. There is no debate that the purchase price per ton of hollow sections is higher than that of hot rolled conventional long sections. The previously often quoted ‘high labour costs’ have significantly reduced but will still be higher than for hot rolled labour. But these increases will in many instances be more than offset by the lower mass of tube profiles needed when compared with the equivalent hot rolled profile solution.
The case of members subject mainly to compression forces, that is columns and bracing, tubular construction, is well documented and will surely be cheaper in tubular construction.
However for other structural components, if all the aspects are carefully considered and properly assessed, in a great many applications tubular construction will be very competitive.
Should you have any queries contact Franco Mordini on FrancoM@robor.co.za or +27 11 977 2029 or +27 82 800 7965.