Connections · 7 min read
Common bolt design mistakes (and how to avoid them)
The bolt mistakes that come up again and again in design checks: shear plane location, bearing on thin plates, edge distances, and group eccentricity.
Bolt design looks simple — pick a grade, count the bolts, divide the load. In practice, four mistakes show up over and over in connection checks. Avoid these and most bolt designs become routine.
Mistake 1 — Wrong shear plane assumption
Bolt shear resistance Fv,Rd depends on whether the shear plane passes through the threaded portion (As) or the unthreaded shank (A). The threaded area is roughly 78% of the gross area, and the resistance differs by the same ratio.
Fv,Rd = αv · fub · A ÷ γₘ₂ (αv = 0.6 typically; A or As)Mistake 2 — Forgetting bearing on the connected plate
Bolts almost never fail in pure shear. They fail by tearing out, splitting, or crushing the plate they bear against. The bearing resistance Fb,Rd is often the governing check — especially for thin plates or end plates with short edge distances.
Fb,Rd = k₁ · αb · fu · d · t ÷ γₘ₂Mistake 3 — Edge and end distances too small
EC3 minimum edge distance e₁ ≥ 1.2·d₀ and minimum spacing p₁ ≥ 2.2·d₀, where d₀ is the hole diameter. These minimums get ignored when bolts are squeezed in around stiffeners or near a flange tip — and the bearing resistance collapses.
- Minimum end distance e₁ ≥ 1.2·d₀ (loaded direction)
- Minimum edge distance e₂ ≥ 1.2·d₀ (perpendicular to load)
- Minimum spacing p₁ ≥ 2.2·d₀, p₂ ≥ 2.4·d₀
- Maximum p₁ ≤ 14·t or 200 mm (to prevent local buckling)
Mistake 4 — Ignoring group eccentricity
If the line of action of the load doesn't pass through the centroid of the bolt group, you have a moment in addition to shear. The outer bolts pick up a bigger share of the load. For a single-column-of-bolts shear connection with a beam reaction at the supported flange, the eccentricity is small and often ignored. For a fin plate or angle cleat, it's significant.
Bonus — Block tearing
Block tearing (block shear failure) is when a chunk of plate pulls out around a bolt group. It's often the silent killer of cleated connections to thin webs. Always check Veff,1,Rd or Veff,2,Rd from EC3 cl. 3.10.2 — it's quick and rules out a brittle failure mode.
Frequently asked questions
Are 8.8 bolts always enough?
For most ordinary structural connections, yes. 8.8 grade bolts give a good balance of strength and ductility. Move to 10.9 only when you need higher capacity in a tight space or for preloaded slip-resistant connections — and remember 10.9 needs more careful detailing for tension applications.
When do I need preloaded bolts?
When you can't tolerate any slip — for example, in seismic-resisting connections, wind-sensitive structures, or where slip would change load distribution (Category C in EC3). For most beam-to-column shear connections, non-preloaded bearing bolts are fine and far cheaper.
Do I include the bolt holes in plate strength checks?
Yes — for tension, use the net area (gross minus hole area). For shear, the gross section can usually be used unless the holes line up across the shear plane. Always check both gross and net section in tension.