Welded hollow sections

Welded hollow sections can be produced by a number of different methods. The most common is known as the conversion method in which round hollow sections are produced and then converted to the required shape by re-rolling. The welding can be carried out by butt or continuous welding, electric weld, spiral weld, and submerged arc welding.

A method used less commonly in the UK, but more extensively in the United States and Japan, is the direct or brake press method in which the rectangular or square section is formed by bending the parent plate and then welded. However, these sections are visually less acceptable.

Butt or continuous weld process

In the butt or continuous weld process, which is now seldom used, hot-rolled strip is heated almost to welding temperature and bent into a horseshoe shape forming a nearly closed tube. The strip edges are then heated locally and pressed together to make the weld. The

11.1 Strip being formed and welded into a tube hot tube then goes through sizing rolls which reduce the outside diameter to within the specified tolerance. This method is now only used for sections of up to 48 mm diameter.

Electric weld process

The majority of hollow sections used in buildings in the UK are produced by the electric weld (EW) method, which can produce circular sections from 48 mm diameter up to 508 mm diameter, square sections from 40 mm up to 400 mm, and rectangular sections from 50 x 30 mm up to 500 x 300 mm. The thicknesses produced depend on the size of the section, and range from 2.5 mm to 6.3 mm in the small sizes up to 6.3 mm to 16.0 mm in the larger sizes.

In the EW process, the strip is progressively formed into a round, nearly closed tube shape and then passes through a high-frequency induction coil which raises the strip edges to fusion temperature. The edges are then pressed together, forming a weld without the use of any filler (electrode) material. The round hollow is formed to a diameter which will create the finished section size (see Figure 11.1).

After welding, the external weld flash or bead is removed, but the internal bead is normally left untrimmed. The weld bead is regularly checked for uniformity and integrity.

All shapes of hollow sections can be produced as hot-finished or cold-formed sections. Both processes use hot-finished strip as their feed-stock, and are initially formed into cold round sections and then welded as described above. Cold-formed sections are then finished in their 'cold' or normal state into the desired shape. Hot-finished sections are heated and formed into a circular, square or rectangular shape whilst in the normalising temperature range. Hot-finished and cold-formed products are different in their mechanical performance and each has its own design code and product standard.

Forming the finished shape

A circular tube is the starting point for making a CHS, SHS or RHS. The circular tube is first heated and then passed through a series of rolls to produce the correct dimensions.

RHSs and SHSs are formed by passing the circular sections through a series of rolls which change the profile shape gradually into the required shape, as indicated in Figure 11.2. 11.2 Forming the finished shape of a square section

Stretch reduction

The standard 170 mm nominal diameter tubes produced by the EW process can be stretch-reduced to produce other structural hollow section sizes up to the following nominal sizes:

• CHSs — 140 mm nominal outside diameter

In all cases, the maximum thickness of the section by this process is 8 mm. Stretch reduction is the main method of producing sections up to these dimensions.

Hot-finished versus cold-formed sections

Cold-formed hollow sections differ slightly in shape and form from hot-finished hollow sections. The principal differences are:

• square or rectangular cold-formed sections have larger and more rounded corner radii which may give them a less crisp appearance than hot-finished sections

• the seam weld profile is often more pronounced in cold-formed sections.

Hot-finished hollow sections are supplied to BS EN 10 210. Cold-

formed hollow sections are supplied to BS ENV 10 219. The structural properties of cold-formed sections are less than the equivalent hot-finished sections of the same nominal dimensions. Therefore, direct substitution of cold-formed sections for hot-finished sections should not be carried out without careful checking. It is also important that the correct specification is made at the design stage.

Submerged arc welded process

This process is generally used for tubes over 500 mm diameter and up to 2134 mm diameter. Larger sizes are formed from two semi-

circular rolled plates, smaller sizes from a single circular rolled plate with the final weld being made by the submerged arc welding (SAW) process.

Spiral welded process

Spiral welded tubes are made by helically forming the strip which is then welded by submerged arc welding or other CO2 processes. The method is generally used to produce large diameter thin wall tubes, for example, that are used in bored foundation piles. However, Corus no longer produces tubes by this method.

'Cigar'-shaped columns

These are made out of steel plate/cones which are welded down the seam and then in turn butt-welded onto the next piece. These are non-standard items and, as such, are relatively costly, as they have to be made to order for a particular project. A good example is shown in Figure 4.26.

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