Glossary · Steel structures
Effective cross-section
The part of a thin-walled section that still carries compression after its walls buckle locally; computed per EN 1993-1-5 and EN 1993-1-3.
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Definition
A thin plate in compression — a web or flange of a C-profile — buckles before the steel yields. After that the middle of the plate does little work and the strips next to the edges carry the load. The effective section is the section with those inactive parts removed. Its area and section modulus are smaller than the gross ones.
The width of each plate is reduced by a factor ρ that depends on its slenderness λ̄p (EN 1993-1-5:2006, cl. 4.4):
λ̄p = (b/t) / (28,4 · ε · √kσ), ε = √(235 / fy)kσ is the plate buckling factor: for an internal plate it depends on the stress ratio across it, ψ (Table 4.1). For an outstand (a flange with a free edge) EN 1993-1-5 gives ρ = 1 at λ̄p ≤ 0.748. A lipped cold-formed section also needs a distortional buckling check, the factor χd per DSTU-N B EN 1993-1-3:2012 (eq. 5.12): χd = 1 at λ̄d ≤ 0.65.
The thinner the wall, the smaller the effective part. That is why swapping 41×41×2.5 for 41×41×1.4 cuts resistance more than the change in thickness alone suggests. The effective section also depends on the load: in bending only part of the web is compressed.
Why this is not in the catalogue. Profile makers usually publish gross properties — area, second moments of area, section moduli. For a check to DSTU-N B EN 1993-1-3:2012 these are not enough: effective properties depend on the stresses and differ for compression and for bending in each plane. So a check 'from the catalogue' overstates the resistance. Perforation reduces the section further. When comparing profiles from different makers, look at wall thickness and steel grade, not only at the section modulus from the table.
How Solispec applies it
The Solispec engine computes the effective section of the C-profile for each truss member and each force combination: effective widths of web, flanges and lips per EN 1993-1-5:2006 (cl. 4.4) and DSTU-N B EN 1993-1-3:2012 (cl. 5.5), including distortional buckling. The resulting properties feed the compression, bending and buckling checks.
Norms and clauses
- EN 1993-1-5:2006, cl. 4.4, Table 4.1
- DSTU-N B EN 1993-1-3:2012, cl. 5.5
Related terms
- Thin-walled cold-formed C-profileA bent steel section with thin walls that buckles locally before yielding; it is designed to DSTU-N B EN 1993-1-3:2012.
- Buckling reduction factor φ (χ)A factor that reduces a compressed member’s resistance for flexural buckling: φ in DBN V.2.6-198, χ in Eurocode 3.
- Profile perforation and bolts in slotsFactory slotted holes of a mounting C-profile; bolts go into the slots without drilling, and the maker confirms a slotted bolt’s resistance.
- Steel partial factors γM and γmFactors that divide the characteristic steel resistance: γ_M per Eurocode for C-profiles, γ_m per DBN V.2.6-198 for tubes.
- DSTU-N B EN 1993-1-3:2012: designing C-profiles for PV mountingNorms explained
Solispec’s calculation is preliminary: for a design it has to be checked by an engineer. The values and clauses follow the current editions of the norms on the date of the page.