Cold formed sections
Material S355J2 Stress level 0.75fy
Temperature (°C) -70 -80 -90 -100 -110 -120 -130 -140 -150 -160 -170 -180
Thickness (mm) 54 40 30 32 18 15 12 10 8 7 6 5
Table 2: Limiting thicknesses for low temperatures
Adjustment (Tmd + ΔTr) ΔTRD ΔTRg ΔTRT ΔTRσ ΔTRS ΔTε ΔTεcf Total
Temperature (°C) -15 -30 0 0 0 0 0 -136 -181
Table 3: Temperature adjustments for determining limiting thickness
0 < t ≤ 6 -181 5
6 < t ≤10 -145 9
10 < t -124 13
0 < t ≤ 6 -171 6
6 < t ≤10 -135 11
10 < t -114 16
0 < t ≤ 6 1.6T to 2.4T 0.6 to 1.4 not given
6 < t ≤10 2.0T to 3.0T 1.0 to 2.0 6
10 < t 2.4T to 3.6T 1.4 to 2.6 6 (out of range)
Table 4: Detail classed as welded very severe (equivalent to -30 °C in NA.1)
Table 5: Detail which is classed as welded severe (equivalent to -20 °C in NA.1)
Table 6: Maximum thickness for welding related to the product standard
Technical Digest 2018
As an example consider a cold formed section of steel grade S355J2 with
thickness in the range 0 to 6 mm used in an external building environment
where fatigue is not a design consideration, with a high design stress (σEd
> 0.5fy ), no gross stress concentration and with a welded detail classed as
“Welded very severe”.
Temperature adjustments are given in Table 3.
From Table 2, the limiting thickness of the cold formed section is 5 mm.
Limiting thicknesses for sections in the higher thickness ranges in the
product standard are given in Tables 4 and 5 for details classed as “welded
very severe” and “welded severe”.
Table 5 also applies to a detail which is classed as “welded very severe”
and has a design stress of less than 0.3fy .
An examination of the sizes in the Blue Book shows that certain sections
should not be used if the attributes of a connection detail correspond to
those in Table 4. If the detail corresponds to the description in Table 5, there
is no restriction on the catalogue sizes which could be used.
The strain involved in cold forming circular hollow sections is much less
than that at the corners of square and rectangular sections (about 10% in
the worst case) and there is consequently no restriction on the choice of
cold formed circular hollow sections, even with the presence of a gross
Designers will also remember that BS EN 1993-1-8:2005 Clause 4.14 and
Table 4.2 imposes restrictions on welding near cold formed zones. The table
is entitled ‘Conditions for welding cold formed zones and adjacent material’
and gives maximum thicknesses based on an r/t ratio or strain due to cold
forming. Unhelpfully, the radius considered in the clause is the internal
radius of the corner, whereas the product standard BS EN 10219-2 uses the
external radius (external corner profile). The corresponding r/t values and
limiting thicknesses are given in Table 6 (right).
The clause therefore prohibits welding within 5 times the wall thickness
of the corners of many square and rectangular cold formed sections, unless
the steel is “fully killed Aluminium-killed steel (Al ≥ 0.02%)”, with limits on
carbon (C ≤ 0.18%,), phosphorous (P ≤ 0.02%) and sulphur (S ≤ 0.012%).
Alternatively, tests must have been carried out to show that welding is
Table A1 in Annex A of the product standard indicates the steel is fully
killed steel with a minimum 0.02% of total aluminium. The table gives the
chemical composition of the steel and includes maximum percentages
Design detail Thickness
no fatigue; external
steelwork, welded very
severe, high design stress
(>0.5fy ) no gross stress
Design detail Thickness
no fatigue; external
steelwork, welded severe,
high design stress (>0.5fy ) no
gross stress concentration,
by mass of carbon (C ≤ 0.22%), phosphorous (P ≤ 0.03%) and sulphur (S
≤ 0.03%). The material therefore satisfies the requirement for fully-killed
aluminium killed steel but allows the percentage of carbon, phosphorous
and sulphur to fall outside the limits specified in Table 4.2.
This restriction prohibits the adoption of a welded end plate or base plate
for most rectangular and square cold formed hollow sections which comply
with the product standard but do not meet the tighter requirements for
carbon, phosphorous and sulphur in Table 4.2.
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