Technical
Cold formed sections
The forming process affects the toughness of cold formed sections and their use in external
structures. Welding is prohibited near the corners of cold formed sections in certain
circumstances. Richard Henderson of the SCI discusses the issues.
26 NSC
February 18
The toughness of steel is affected by the extent of strain it has
undergone as well as by other factors. This fact is taken into
account when determining the limiting thickness for materials
using BS EN 1993-1-10. The limiting thickness of plate or hot
rolled or hot finished structural sections does not, in general,
depend on the extent of strain because such elements are not
subject to plastic strain during their use, nor in the course of their
manufacture. This does not apply however to cold formed square
and rectangular hollow sections, which experience significant
strains at the corners of the profile. Neither does it apply to
beams which have been pre-cambered by cold bending.
The product standard for cold formed welded structural
hollow sections, BS EN 10219-1:2006 requires that for square or
rectangular sections, the test pieces for impact testing are taken
either longitudinally or transversely midway between the corners
from one of the sides not containing the weld. The impact values
therefore relate to material which is unaffected by cold forming,
thus tacitly acknowledging that the forming process affects the
material toughness. According to clause 6.7.2 of the product
standard, there is no requirement for impact tests for specified
thicknesses of less than 6 mm.
The effect of strain during cold forming must be taken into
account when determining the limiting thickness of material
of a given sub-grade. According to BS EN 1993-1-10 and its UK
National Annex, the reference temperature TEd for determining
the toughness of a steel element:
TEd = Tmd + Tr + T+ T+ T + T R cf
where (T+ΔT) considered together represent the minimum
mdreffective temperature of the steel part, ΔTR is a safety allowance,
ΔTε is an adjustment for strain rate and ΔT is an adjustment for
εcf
the extent of strain during cold forming.
The UK National Annex collects together factors affecting the
safety of elements and gives an equation for ΔTR as follows:
TR = TRD + TRg + TRT + TR + TRs
where ΔTRD is an adjustment for detail type, ΔTRg for gross
stress concentration, ΔTRT for Charpy test temperature, ΔTRσ
for stress level and ΔTRs for strength grade. The procedure is
consistent with ΔTσ = 0.
The temperature adjustment for cold forming is given in clause
2.3.1(2) of the standard as minus three times the percentage
strain expressed as degrees Celsius. A strain of 10% would
result in a temperature adjustment of −30 °C. This is potentially
significant when considering the adoption of cold formed
sections.
The strain resulting from cold forming SHS or RHS tubes can
be determined from the limiting dimensions in the product
standard as follows. Consider the corner of a box section as
shown in Figure 1. The external corner profile is determined in
the product standard by measuring dimensions C1 and C2 or R.
The length of the centre line is the original length before
forming. For one corner, the centre line length is:
L =
2
4
( T
R –
)
2 The outside length after forming is
2
4
R
The change in length ΔLL =
2
4
R –
T
4
2
4
( T
R –
)=
2
L
L
=
The strain is
T 4
2 4(R–T 2)
=
T
(2R–T)
The dimensional tolerances on the corner radius for different
thickness ranges is taken from the product standard and used to
determine the maximum percent strain due to cold forming in
Table 1 (over page) by substituting the minimum external radius
in the formula for strain.
The strain could therefore be as high as 45% for material less
than 6 mm thick bent to the tightest radius, giving a temperature
adjustment for cold forming of -3 × 45 = −135 °C when
determining the limiting thickness. Such an adjustment puts
Figure 1: Corner
dimensions
28
T
R
C2
C1
/Material_selection_and_product_specification#Fracture_toughness
/Steel_construction_products#Structural_hollow_sections
/Design_codes_and_standards#National_Annexes