Structural Steel Design / module 12
Bolts, pins and welds
A fastener does not have a resistance. It has several, belonging to different failure modes in different materials — and which one governs decides what would actually help.
What you will understand
- Say what happens at a bolt in shear, and why bearing is a separate check
- Explain why a higher bolt grade often buys almost nothing
- Recognise punching shear and prying, the two modes people forget
- Explain what a preloaded bolt does differently, and why it is not chosen for strength
- Size a fillet weld on its throat, and say what the directional method adds
- Say what each detailing rule protects, rather than memorising the number
Lessons
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.
Preloaded bolts, welds, and what detailing protects
Preloaded connections transfer load by friction before slip. Fillet welds are assessed on their effective throat, and connection resistance depends on the bolt and weld arrangement.
Module summary
- An M20 8.8 in single shear gave 94 kN — and only 60 kN in a 6 mm ply
- Which mode governs decides the useful remedy, and it is not visible without checking both
- Grade 10.9 over 8.8 gave 4%, because the shear coefficient drops from 0.6 to 0.5
- Fitting a BIGGER bolt into an unchanged hole pattern made the connection weaker
- Punching shear governed an M24 in a 6 mm ply — 170 kN against the bolt's 203
- Prying added 19% to the bolt force with an 8 mm plate and 2% with a 20 mm one
- A preloaded bolt gave 69 kN in slip against 118 kN in bearing — it is not for strength
- A 6 mm fillet is a 4.2 mm weld, and transverse loading is worth 22.5% more
Teaching basis and review status
These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.
Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.
Read the lesson register and outstanding review notesCheck your understanding
Practice
An M24 grade 8.8 bolt has As = 353 mm², threads in the shear plane. What is Fv,Rd in kN? Take αv = 0.6, γM2 = 1.25.
Practice
A bolt group has bolt shear 94 kN and ply bearing 60 kN per bolt. Six bolts carry the load. What is the group's resistance, in kN?
Practice
What is the throat of a 10 mm equal-leg fillet weld, in mm?
Practice
A 6 mm fillet gives 1067 N/mm longitudinally. What does a 200 mm run on each side of a plate give in total, in kN?
Practice
A weld leg is doubled from 6 mm to 12 mm. By what factor does the deposited weld metal increase?
Practice
An M20 bolt takes a 22 mm hole. What is the minimum end distance at 1.2 d₀, in mm?
Check yourself
A bolted connection is governed by ply bearing. Which change would help most?
Check yourself
Why is a fillet weld's strength taken on its throat rather than its leg?