Module 10
Plate girders, built-up members and local effects
When you build a member out of separate plates you can put the steel exactly where it earns its keep — and you introduce failures that a rolled section never has. Both halves of that bargain are worth understanding.
What this module covers
- Say what a plate girder buys over the largest available rolled section, in numbers
- Explain why a slender web buckles in shear long before it yields
- Describe tension-field action, and say what has to anchor it
- Say what a rigid end post is worth, and when it is worth nothing
- Recognise flange-induced buckling and name the right remedy
- Explain why a built-up member's critical load can never exceed its lacing's shear stiffness
Lessons
Depth is what carries moment, and depth is cheap in a welded girder. The price is a web too thin to behave like a plate in shear.
Start lesson →Where to put stiffeners and why, a failure that gets worse when you add flange steel, and why a laced column has a ceiling its chords cannot lift.
Start lesson →
Module checkpoint
Check what you have taken in
8 questions
Question 1
A web panel has τcr = 60 N/mm² and fyw = 355 N/mm². What is λ̄w?
Question 2
A web is 1200 mm deep and 8 mm thick. If it is thickened to 12 mm, by what factor does τcr increase?
Question 3
A built-up member has Ncr,ideal = 6000 kN and Sv = 12 000 kN. What is Ncr, in kN?
Question 4
For that member the lacing is stiffened so that Sv becomes very large. What is the maximum Ncr it could ever reach, in kN?
Question 5
Lacing diagonals are flattened so their length d increases by 50%. By what factor does Sv fall, all else equal? Give the factor by which it is divided.
Question 6
A 1500 × 10 mm web in S355 has χw = 0.435 when the stiffeners are widely spaced. What is Vb,Rd in kN, with γM1 = 1.0?
Question 7
A girder's shear resistance is inadequate. Which change is likely to be most effective per kilogram of added steel?
Question 8
Which statement about tension-field action is correct?