Structural Hollow Sections from an Engineer’s perspective: What size should I use?

By Franco Mordini

Structural Hollow Sections are being used more and more in structural applications as they have proven to be aesthetically pleasing and very efficient profiles. This has resulted in an increase in demand for structural tube. This includes both direct off mill and “drawn” sizes. Typically tube is used in applications where the member is subject to compression or part of a frame that may be subjected to both of compression or tension depending on load direction. In some cases, it may be used in an application where the member is subjected to biaxial bending. This typically results in lighter more efficient structures.

Other benefits include ease of erection as they are typically lighter and the increased stiffness makes it possible to fabricate longer sections and lastly but not lastly result in an aesthetically pleasing structure.

Design Class of Hollow Section

One common denominator regardless of production processed used is the structural behaviour of the tube. As with other steel structural members, the performance of the tube is dependent on the class of member, typically regarded as stocky or slender. Class 1 and 2 are regarded as stocky. Class 4 members are slender and should be avoided especially where the member is to be used as a structural element. One must bear in mind that Class 4 member are not structurally efficient as they will fail before the steel reaches its yield strength. In this case, it is recommended that one rather chooses a “stockier” size which may be smaller but more efficient, this would include Class 3 members.

Another interesting aspect of Class 4 members is that the higher the D/t or B/t ratios are the more difficult they are to manufacture. The difficulty will also increase, for the same ratios, the higher yield stress steel is as this ratio is inversely proportionally to the square root of the yield stress in the case of Square or Rectangular Hollow Sections; and inversely proportional to the yield stress in the case of Circular Hollow Sections. This correlates to the theory that thin plate or tube will elastically deform before reaching the yield stress, which may, in the end, result in severe deformation and in many cases results in failure of the member before it has reached its yield stress.

When Class 4 members are used the complex theoretically resistance of the member will need to be done from first principles in order to avoid unwanted failures.

Drawn Tube – what to look out for

Drawn tube is often needed especially for sizes that are unique or where volumes do not permit efficient direct of mill rolling. By definition, Drawn (tube) Hollow Sections are Circular (CHS), Square (SHS) or Rectangular (RHS) profiles that are converted “off line” from a circular “mother tube”. In this instance “line” refers to tube manufacturing mill in which the final (by size) product is made on a continuous tube production line. Simply described the process would be to take a previously formed (circular) hollow section and alter its shape, preferably without changing its diameter to make it smaller, into a square or rectangle which has the same perimeter as the mother tube.

The drawing process also allows for circular sections to be drawn down into non-standard diameters through dies.  The cost of rolls used to convert the tube is often low as typically they can be used to manufacture many different sizes, so the cost of changing an online mill set up to a new profile can thus be avoided, which is especially desirable when only small quantities are required.

One of the biggest advantages of Drawn tube is more than one size can be made from the Circular Hollow Sections input.  I.e. the width and height of the profile can be varied for the same input mother tube, e.g. A 219.1 diameter circular hollow section can be converted to an RHS 250×100, RHS 200×150 or to an SHS 175. When producing tubes on an online mill, minimum order quantity is often based on the length of a strip of coil. To produce a drawn tube only the availability of the number of lengths of mother tube required is considered, hence small quantities can be made on a given “Turk’s head” setting. The only limiting criteria is that the size of the Mother Tube must be dimensionally suited to the finished drawn size. In order to draw down tubes, it is necessary for jaws to grip one end of the tube to pull it through the machines, resulting in a small length of waste on each tube (there are “no free lunches” so someone is paying for the waste)

Figure 2 : Tubes with too large corner radii resulting in large and inconsistent corner radii

If a mother tube circumference is greater than the perimeter of the drawn size, it is necessary to draw down the circumference to suit the end product before conversion. If not done correctly it can result in a profile that will have very tight corner radii. See figure 1.  If the radius is excessively tight this may result in cracking in the internal corners. The additional energy required also results in work hardening, especially in the corners, that reduces the ductility of the profile.

Annealing (heating and slow cooling treatment) may be required when excessive work hardening has occurred in the drawing down process. This can however be expensive and should therefore be avoided where possible.

If a Mother Tube circumference is too small for perimeter of the drawn tube is used, the result a profile which has very rounded corner radii considerably in excess of the requirements of SANS 657 Part 1 see figure 2.  (The SANS document under which Tubes are produced). In many of these cases a compounding problem may occur in that the four rounded corners are often unequal and may present aesthetical problems. A good example is a SHS 175 that is drawn from a CHS 219.1, if 180 square is specified and product is drawn then corners will be very rounded and in most cases will result in unequal corner radii.

The simple and highly desirable solution is to specify the sizes that are a direct conversion, i.e. a size that does not need to be drawn before conversion. See Figure 3.
A good example where drawn tube was successfully specified – Standard Bank Facade in Rosebank
Figure 3 : Good example where drawn tube was successfully specified  – Standard Bank Façade in Rosebank

Drawn tube manufacturing does not have a place where volumes required reach a critical mass which justifies these sizes to be made directly, and more efficiently, using on line mills. Some examples, to name a few, are SHS 120, SHS 150, RHS 160 x 80, RHS 200 x 100.

The recommended (Preferred) large square and rectangles sizes in graded steel (typically S355) are shown below. Smaller sizes, provided they are “standard”, will typically already be manufactured direct using on line mills. Consult the SAISC Steel Construction Handbook (The Red Book) or your friendly steel supplier or tube manufacturer if in doubt.

 

Conclusion

Key for Engineers is to specify efficient members avoid class 4 members. Drawn tube serves an important part of the market where non-standard sizes are required or / and when quantities are small.  When specifying drawn sizes, where possible, ensure that you specify a size that does not require drawing down before conversion, that are compatible with the Mother Tube standard sizes which will result in a finished product that will conform to the dimensions and standards called up in SANS 657 Part 1. The sizes listed in the latest Red Handbook will typically avoid many of the above pitfalls when specifying tube. For availability it is best discussed with your local friendly merchant or tube mill. By sticking to these simple rules, the result will be a reduced cost and provide the end user with a better quality product.

For more information contact the Franco Mordini at Macsteel Tube and Pipe at (011) 897-2100 or on e-mail franco.mordini@mactube.co.za.