Clause 7.3.2 gives the design strength of a tension member limited by rupture (fracture) at the net cross-section through bolt holes. The net area accounts for bolt holes and staggered pitch. For angles and channels connected through one leg, a reduction factor accounts for shear lag.
Key Requirements
•For plates: Tdn = 0.9 × An × fu / γm1
•An = net area after deducting bolt holes from gross area
•For staggered bolts: net width = gross width − Σ(hole diameters) + Σ(p²/4g) along the failure path
•For single angle connected through one leg: Tdn = 0.9 × Anc × fu / γm1 + β × Ago × fy / γm0
•β depends on the ratio of outstanding leg area to connected leg net area and the shear lag parameters
Formulas
Tdn = 0.9 × An × fu / γm1
Design strength due to rupture of net section for plates and members symmetrically connected
Tdn = Design tensile strength due to net section rupture (N)An = Net area of cross-section after deducting bolt holes (mm²)fu = Ultimate tensile stress (MPa)γm1 = Partial safety factor for ultimate stress = 1.25
Tdn = 0.9 × Anc × fu / γm1 + β × Ago × fy / γm0
Design strength for single angle (or channel) connected through one leg, accounting for shear lag
Anc = Net area of connected leg (mm²)Ago = Gross area of outstanding (unconnected) leg (mm²)β = Shear lag factor (depends on w/t ratio, number of bolts, and connection length)fu = Ultimate tensile stress (MPa)fy = Yield stress (MPa)
An = [b − n×dh + Σ(ps²/4g)] × t
Net area calculation for staggered bolt pattern
b = Width of the plate (mm)n = Number of bolt holes along the failure pathdh = Diameter of bolt hole (bolt diameter + 2 mm for standard holes)ps = Staggered pitch (longitudinal spacing between staggered bolts, mm)g = Gauge (transverse spacing between bolt lines, mm)t = Plate thickness (mm)
Practical Notes
✓The 0.9 factor in Tdn = 0.9 × An × fu / γm1 accounts for stress concentration at bolt holes and the fact that not all fibres reach ultimate stress simultaneously.
✓For angles connected by one leg (very common in trusses), shear lag significantly reduces the effective area. A single-bolt connection can reduce capacity by 40–50% compared to the gross section.
✓Bolt hole diameter = nominal bolt diameter + 2 mm (for standard clearance holes per Cl. 10.2.1). For M20 bolts, hole = 22 mm.
Common Mistakes
⚠Forgetting the 0.9 factor in the net section rupture formula — this factor is explicitly required by Cl. 7.3.2.
⚠Using fy instead of fu for net section rupture — rupture is governed by ultimate stress, not yield stress.
⚠Not considering all possible failure paths for staggered bolt patterns — the critical path is the one giving the minimum net area.