Free Tool · EN 1993-1-8 §3.9

EN 1993-1-8 §3.9 Slip-Resistant Bolt Connection Calculator

Calculate preload force Fp,C and slip resistance Fs,Rd for HSFG preloaded bolts per EN 1993-1-8 §3.9. Covers Category B (slip at SLS) and Category C (slip at ULS), all surface classes A–D, four hole types, and the combined tension+shear reduction.

F_p,C μ (Class B) V Ed
Fs,Rd = (ks × n × μ / γM3) × Fp,C   [EN 1993-1-8 Eq. 3.6]   |   Fp,C = 0.7 × fub × As   [Eq. 3.7]
Connection Parameters
8.8 (f_ub = 800 MPa) or 10.9 (f_ub = 1000 MPa) — only preloaded bolts qualify as HSFG.
Nominal diameter M12–M36. Tensile stress area A_s from EN ISO 898-1.
Class A blast-cleaned μ=0.50, Class B μ=0.40, Class C μ=0.30 (hot-dip galvanised), Class D μ=0.20 (untreated).
Standard holes k_s=1.00, oversized k_s=0.85, short slot parallel k_s=0.76, short slot perp k_s=0.63, long slot parallel k_s=0.76, long slot perp k_s=0.55.
n=1 for single lap, n=2 for double cover plates (two slip planes).
Category B: slip-resistant at SLS, γ_M3=1.10. Category C: slip-resistant at ULS, γ_M3=1.25.
Total bolt count in the connection group.
Applied tension per bolt. Set 0 if no tension. Combined check per Eq. 3.8a: F_s,Rd reduced when F_t,Ed > 0.
Total shear on bolt group to compute utilisation.
Slip Resistance Results
109.76 kN
Fs,Rd per bolt
As 245 mm²
fub 1000 N/mm²
Fp,C (Eq. 3.7) 171.5 kN
ks 1
μ 0.4
γM3 1.25
Fs,Rd/bolt (Eq. 3.6) 109.76 kN
↓ Eq. 3.8a: F_p,C,eff = 171.5 kN → F_s,Rd,red = 109.76 kN (governs)
Fs,Rd,total (group) 658.56 kN
① Tensile stress area: A_s = 245 mm²
② f_ub — bolt ultimate strength: f_ub = 1000 N/mm²
③ Preload force F_p,C (Eq. 3.7): F_p,C = 0.7 × 1000 × 245 = 171.5 kN
④ k_s — hole coefficient (Table 3.6): k_s = 1
⑤ μ — slip factor (Table 3.7): μ = 0.4
⑥ γ_M3 — partial factor: γ_M3 = 1.25 (Category C)
⑦ Slip resistance per bolt F_s,Rd (Eq. 3.6): F_s,Rd = (1 × 2 × 0.4 / 1.25) × 171.5 = 109.76 kN
⑨ Total group resistance: F_s,Rd,total = 109.76 × 6 = 658.56 kN
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Frequently Asked Questions

What is the difference between Category B and Category C?

Category B connections are designed to be slip-resistant at the serviceability limit state (SLS) with γ_M3,ser = 1.10. Category C connections are slip-resistant at the ultimate limit state (ULS) with γ_M3 = 1.25. Cat C is stricter and required for fatigue-loaded connections or where slip must be avoided under full design loads.

What does the preload force F_p,C represent?

F_p,C is the minimum specified preload that must be induced in the bolt during tightening, per EN 1993-1-8 Eq. 3.7: F_p,C = 0.7 × f_ub × A_s. The 0.7 factor accounts for tightening losses. It is the clamping force that creates the friction between plies.

Why does external tension reduce slip resistance?

External tension F_t,Ed reduces the clamping force between the connection plies. Per Eq. 3.8a the effective preload is reduced by 0.8 × F_t,Ed, so F_s,Rd,red = (k_s × n × μ / γ_M3) × (F_p,C − 0.8 × F_t,Ed). If the tension exceeds F_p,C / 0.8, slip resistance goes to zero.

What surface preparation is required for Class A?

Class A requires blast-cleaning to Sa 2½ (ISO 8501-1) with no pitting, giving μ = 0.50. Class B covers blast-cleaned and subsequently hot-dip galvanised or metallised surfaces (μ = 0.40). Class C is hot-dip galvanised only (μ = 0.30).

Can I use oversized holes for slip-critical connections?

Yes, but at a penalty. Oversized holes have k_s = 0.85, which directly reduces F_s,Rd by 15%. Short slotted holes parallel to load give k_s = 0.76; perpendicular give k_s = 0.63. Verify the reduced resistance satisfies the design load.

What bolt grades qualify as HSFG?

EN 1993-1-8 §3.9 covers grade 8.8 (f_ub = 800 MPa) and 10.9 (f_ub = 1000 MPa) preloaded bolts. Grades 4.6 and 5.6 are not permitted for slip-resistant connections — they must be tightened to the specified preload using a calibrated torque wrench or direct tension indicators.