Hollow sections are commonly used in welded steel frames where members experience loads from multiple directions – hollow sections have uniform geometry and thus uniform characteristics with excellent resistance to torsion making them a good choice for columns. They can also be used for beams, though conventional hot-rolled I-shapes are generally more efficient for this application.
In steel structures hollow sections are often preferred due to their aesthetic properties in exposed areas. Hollow sections also have the benefits of a higher strength to weight ratio due to their strength in compression. For the same reason, they are often used as bracing members working in tandem with conventional hot-rolled steel shapes.
One headache with hollow sections is bolted connections due to the lack of access to the inside of the section. However, in this series of articles we'll take a look at some of the many options available including:
Splices and Joints
Hollow sections may be either Square, Rectangular, or Circular. In the United States the term HSS (Hollow Structural Section) is used to define all hollow section shapes which are then prefixed with the shape, i.e. Square HSS, Rectangular HSS, or Circular HSS. HSS is often mistakenly referenced as Hollow Structural Steel.
In Europe and the UK, or other countries which follow EU / UK construction or engineering terminology, the term HSS is not used. Rather, the three basic shapes are referenced as SHS, RHS, and CHS - being Square, Rectangular and Circular hollow sections. Typically, these designations will also relate to metric sizes, thus the dimensions and tolerances differ slightly from HSS.
There are 3 basic processes for forming the shapes:
- The steel sheet is hot formed into a circular shape. The open ends are then automatically welded using the ERW process and then cold-formed into the required square, rectangular or circular shape.
- The steel plate is hot-formed into the required shape through forming dies where again, the open ends are welded using the ERW process. The section is then cooled and driven through a series of sizing stations.
- Two pieces of steel strip are formed into identical halves which are then welded together using the Submerged Arc process.
Hollow Sections may be supplied as either Hot Finished (or Formed) or Cold Formed
Hot Finished (Formed)
Hot formed hollow pieces are created by superheating sheets of steel up to temperatures of up to 900° C, making it more malleable. While heated, the steel is processed through a series of rollers to form it into a desired shape.
Hot formed steel is cooled slowly back to an ambient temperature. While cooling, the steel reconfigures to have a consistent microstructure.
As heated steel is much easier to shape, hot formed hollow steel sections require less processing. The heating process also eliminates a steel sheet’s inconsistencies or internal stresses. Having undergone intense processing, hot formed hollow sections stand up to stress from additional fabrication, such as welding and riveting.
Hot Finished hollow sections shrink somewhat during cooling and corners become rounded, making tolerances a concern. In addition, the cooling process causes surface scaling. Because of its formability during processing, it’s also difficult to maintain a precise thickness along an entire length.
Cold formed hollow sections start out exactly the same as hot formed hollow sections.
The key difference between is that their shaping doesn’t occur until after the steel is cooled.
Like hot rolled steel, cold rolled steel is processed through rollers to create form. However, because cold rolled steel is less malleable, it requires additional pressure and several passes through shaping elements.
Despite taking more effort to create, cold formed hollow sections can be manufactured to tighter tolerances. Unlike hot rolled steel, cold rolled holds its shape after processing, providing a more dimensionally accurate finished product. Cold rolled steel pieces also have a uniform thickness and smoothness.
With more additional steps in production, cold rolled steel pieces take more effort and time to produce. Because of its rigidity, cold rolled steel faces shaping limitations -- though your manufacturer’s skill level is also a factor. During shaping, the steel is stretched, making cold formed hollow sections prone to corner breaks.
Comparing the strength of hot formed vs. cold formed hollow sections, cold formed is the clear winner.
The additional strength is attributed to strain hardening, which increases cold formed hollow sections’ resistance to bending by up to 20%.
While able to withstand great amounts of stress, hot rolled steel products are more apt to bend if there are any impurities within the metal.
Specifications and Materials
ASTM A500 Grade C is most common when specifying square, rectangular and round HSS. These specifications cover cold-formed production of both welded and seamless HSS; ASTM A847 offers atmospheric corrosion resistance properties similar to that of ASTM A588 for W-shapes. Pipe-size rounds (P, PX and PXX) are also available in ASTM A53 Grade B material.
ASTM A1085 is a new specification for HSS. It offers improved material properties and design wall thickness equal to nominal wall thickness, among other desirable characteristics.
External References: https://www.aisc.org/technical-resources/hss/
Specifications and materials for Hot Finished hollow sections are covered by the EU specification: EN 10210
Specifications and materials for Cold Formed hollow sections are covered by the EU specification: EN 10219
EN 10210 - (PDF Download)
EN 10219 - (PDF Download)