The Chartership Forum

Structural Steel Design / module 13

Connection and joint design

A joint is a chain of components in series. That one idea settles both questions it raises — the weakest component sets the resistance, and every component sets the stiffness — and they are almost never the same component.

What you will understand

  • Recognise the T-stub in an end plate, a column flange and an angle cleat
  • Derive the three T-stub failure modes and say which is ductile
  • Explain prying as a consequence of the flange bending, not a code allowance
  • Assemble components in series and say why the weakest and the softest differ
  • Calculate a joint's moment resistance and initial stiffness
  • Say when the initial stiffness is the wrong number to have used

Lessons

  1. The T-stub, and the three ways it fails

    Almost every bolted joint contains a piece of plate that bends to deliver tension into a bolt. One model covers all of them, and its three failure modes are derivable.

  2. The component method: weakest, softest, and why they differ

    Components in series. The weakest sets the resistance, every one sets the stiffness, and improving the wrong one is the most common wasted effort in joint design.

Module summary
  • The T-stub is the recurring model for anything that bends to feed a bolt
  • Mode 1 = 4Mpl/m from virtual work; flange yielding alone does not certify joint rotation capacity.
  • Mode 2 = (2Mpl + nΣFt,Rd)/(m+n), derived from equilibrium with prying
  • Mode 3 is bolt failure without credited prying in this idealisation.
  • The assumed 15 mm flange gives 233 kN in mode 2; the 20 mm flange reaches the 282 kN bolt limit.
  • Beyond 20 mm extra flange bought nothing at all — the bolts are what fail
  • The weakest component set 380 kN; a DIFFERENT one supplied 39% of the flexibility
  • Sj,ini goes with z², so a 20% deeper joint is 44% stiffer whatever its components
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 notes

Check your understanding

Practice

A T-stub has Mpl = 6.0 kNm and m = 50 mm. What is its mode 1 resistance in kN?

Practice

The same T-stub has n = 40 mm and ΣFt,Rd = 400 kN. What is its mode 2 resistance in kN?

Practice

Four components have k values of 5, 10, 20 and 40 mm. What is keq?

Practice

In that group the k = 5 component is made rigid. What is the new keq?

Practice

A joint's lever arm is increased from 350 mm to 450 mm with no other change. By what factor does Sj,ini increase?

Practice

A bolt row develops 250 kN at a lever arm of 380 mm. What moment does it contribute, in kNm?

Check yourself

Which T-stub mode gives the least warning before failure?

Check yourself

Why must keq always be smaller than the smallest individual ki?