Module 12 · Lesson 12.1
Bolts: several resistances, not one
The bolt can shear, the plate can tear, the head can punch through, and a flexible plate can pry. Each is a different calculation with a different remedy.
Why this matters
Ask what an M20 grade 8.8 bolt is good for and the honest answer is a question: in what, and how thick? The same bolt gave 94 kN in one connection in this module and 60 kN in another, and the difference was the plate. Connections are where steel design becomes concrete and where most of the real errors live — not because the mechanics is hard, but because there are several mechanisms at once and the calculation that reports the largest number is rarely the one that matters.
By the end of this lesson you should be able to
- Calculate bolt shear and ply bearing, and say which governs
- Explain why the useful remedy depends on that answer
- Recognise punching shear and prying
- Say what a preloaded bolt does that a bearing-type bolt does not
What you should already know
- Net section and block tearing (Module 6)
- Ultimate strength fu against yield fy, and why connections use fu (Module 1)
- Partial factors, and why γM2 is larger than γM0 (Module 2)
Two things can fail, and they are in different materials
Put a bolt through two plates and pull them apart. Two quite different things can happen.
The bolt can shear. The resistance is a strength times an area — but which area? A bolt has a plain shank and a threaded portion, and the threaded portion is about 22% smaller. Whether the threads fall in the shear plane depends on how long the bolt is against how thick the plies are, which is a drawing decision with a resistance attached to it.
The plate can tear. The bolt bears against the side of its hole, and if the plate is thin or the bolt is close to an edge, the plate deforms and tears out rather than the bolt breaking. This is bearing, and it is a check on the plate, not on the bolt.
For an M20 8.8 in S355 plate with 40 mm end distance:
| Ply thickness | Bolt shear | Ply bearing | Governs |
|---|---|---|---|
| 6 mm | 94.1 kN | 60.4 kN | bearing |
| 8 mm | 94.1 kN | 80.5 kN | bearing |
| 10 mm | 94.1 kN | 100.6 kN | bolt |
| 20 mm | 94.1 kN | 201.3 kN | bolt |
The bolt's own resistance never changes. The connection's does, by a factor of more than three across that range — and the crossover is at about 9.4 mm, which is squarely in the middle of the plate thicknesses a real connection uses.
Try it
Which mode governs, and what actually helps
One bolt through S355 plate. The top two bars are the two failure modes; the third is what the connection actually has. Watch them cross as the ply thickness changes.
Bolt grade
Threads in the shear plane
- Clearance hole d0
- 22 mm
- Bolt shear Fv,Rd
- 94.1 kN
- Ply bearing Fb,Rd
- 100.6 kN
- Governing mode
- BOLT SHEAR
- Resistance FRd
- 94.1 kN
- Bearing factor k1
- 2.12
- Bearing factor αb
- 0.606
- A larger bolt would give
- 73.9 kN
- A higher grade would give
- 98.0 kN
- A thicker ply would give
- 94.1 kN
- More end distance would give
- 94.1 kN
- Best single change
- a higher grade
- Spacings all satisfied
- yes
Bolt shear 94 kN against ply bearing 101 kN, so bolt shear governs at 94 kN. The bolt itself is the weaker part, so a higher grade or a larger diameter would raise the resistance directly.
Things worth trying
- Start at M20 8.8 in a 10 mm ply. Bolt shear governs at 94.1 kN, just below bearing's 100.6 — a 6% margin.
- Take the ply down to 6 mm. Bearing collapses to 60.4 kN and takes over. The bolt has not changed at all.
- Now look at the four remedies at 6 mm. A higher grade gives nothing, a larger bolt makes it WORSE, and only a thicker ply or more end distance help.
- Take the ply to 20 mm and look at the same four. The ranking has completely inverted: now a larger bolt is much the best and the plate changes do nothing.
- Try the larger bolt at 6 mm directly — set the diameter to 24 and watch the answer FALL. The bigger hole tightens every spacing ratio, and the extra diameter cannot make it up.
- Switch the grade from 8.8 to 10.9 with a thick ply. The gain is about 4%, not the 25% the grade names suggest, because the shear coefficient drops from 0.6 to 0.5.
- Move the threads out of the shear plane instead: 28%, from a detailing decision that costs nothing.
- Take the end distance down to 20 mm and watch αb fall away — and the spacing check turn red. The minimum spacings are what make the bearing calculation valid in the first place.
Worked example
An M20 bolt in a 10 mm splice plate
Given
- M20 grade 8.8 bolts, fub = 800 N/mm², threads in the shear plane
- 10 mm S355 plates, fu = 490 N/mm²
- End distance 40 mm, edge distance 40 mm, pitch 60 mm, gauge 60 mm
- Single shear, γM2 = 1.25
Find
The resistance of one bolt, and what would raise it.
Assumptions
- αv, k₁ and αb are all calibrated, and the profile's values are unverified here
- Block tearing of the plate is a separate check, covered in Module 6
Bolts in tension: two more modes
A bolt in tension can break — that is the obvious one, at 0.9 fub As, where the 0.9 allows for the bending and stress concentration a real bolt suffers.
Two others are less obvious and both are commonly missed.
Punching shear. The head or nut can pull through the plate, shearing out a plug of it. The resistance goes with the plate thickness, so a thin plate with a large bolt is where it appears — and a thin end plate with large bolts is a perfectly reasonable-looking detail. An M24 in a 6 mm ply gives 170 kN in punching against the bolt's own 203 kN: punching governs, and nothing about the bolt would fix it.
Prying. If the plate the bolt passes through is flexible, it bends away from the bolt line under load, and its edge bears against the supporting member. That contact force has to be balanced — by more tension in the bolt than the load applied. A rigid plate does not bend, its edge never bears, and there is nothing to pry against.
| Plate thickness | Bolt force, for 100 kN applied |
|---|---|
| 8 mm | 119 kN |
| 12 mm | 108 kN |
| 20 mm | 102 kN |
| 30 mm | 101 kN |
Prying is controlled by the PLATE's stiffness, not by the bolt's strength. It is the first case in this course where the fix for an overloaded bolt is to change something else entirely — and Module 13's T-stub is where that idea is developed properly.
Predict first
A connection governed by bearing at 60 kN is respecified with M24 bolts instead of M20, in the same hole pattern and the same 6 mm plate. What happens?
Practice
An M20 grade 8.8 bolt has As = 245 mm², with the threads in the shear plane. What is Fv,Rd in kN? Take αv = 0.6 and γM2 = 1.25.
Practice
The same bolt bears on a 10 mm ply with fu = 490 N/mm², k₁ = 2.12 and αb = 0.606. What is Fb,Rd in kN?
Practice
An M20 bolt takes a 22 mm hole. With an end distance of 40 mm, what is e₁/3d₀?
Practice
An M24 bolt in a 6 mm ply has Ft,Rd = 203 kN and Bp,Rd = 170 kN. What is the governing tension resistance, in kN?
Check yourself
Why does changing from grade 8.8 to grade 10.9 give only about 4% more shear resistance, rather than 25%?
Summary
- A bolt has several resistances, not one, and they are in different materials
- Bolt shear 94 kN was fixed; ply bearing ran from 60 kN at 6 mm to 201 kN at 20 mm
- The crossover was at about 9.4 mm — the middle of the range real plates use
- Which mode governs decides the useful remedy, and the two lists barely overlap
- Grade 10.9 over 8.8 was worth 4%, because αv falls from 0.6 to 0.5
- A LARGER bolt in an unchanged hole pattern made the connection weaker
- Punching shear governed an M24 in a 6 mm ply, and no bolt change would fix it
- Prying is controlled by the plate's stiffness: 19% at 8 mm, 2% at 20 mm
This is educational material. It uses simplified examples to teach principles, and must not be relied on for real design or safety-critical decisions. Module overview and checkpoint