Structural Steel

Manufacturing Methods and Effects

As the name suggests, the cross section of a cold-formed member is achieved by a bending operation at room temperature, rather than the hot rolling process used for the heavier structural steel shapes. The dominant cold forming process is known as roll-forming. In this process, a coil of steel is fed through a series of rolls, each of which bends the sheet progressively until the final shape is reached at the last roll stand. The number of roll stands may vary from 6 to 20, depending upon the complexity of the shape. Because the steel is fed in coil form, with successive coils weld-spliced as needed, the process can achieve speeds up to about 300 ft /min and is well suited for quantity production. Small quantities may be produced on a press-brake, particularly if the shape is simple, such as an angle or channel cross section. In its simplest form, a press brake consists of a male die which presses the steel sheet into a matching female die.
In general, the cold-forming operation is beneficial in that it increases the yield strength of the material in the region of the bend. The flat material between bends may also show an increase due to squeezing or stretching during roll forming. This increase in strength is attributable to cold working and strain aging effects as discussed in Art. 1.10. The strength increase, which may be small for sections with few bends, can be conservatively neglected.
Alternatively, subject to certain limitations, the AISI Specification includes provisions for using a section-average design yield stress that includes the strength increase from coldforming.
Either full section tension tests, full section stub column tests, or an analytical method can be employed. Important parameters include the tensile-strength-to-yield-stress  ratio of the virgin steel and the radius-to-thickness ratio of the bends. The forming operation may also induce residual stresses in the member but these effects are accounted for in the equations for member design.


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