Structural Steel Design / 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 you will understand
- 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
The girder, and the web it can afford
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.
Stiffeners, local effects, and members made of members
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.
Module summary
- A 1550 mm girder at 275 kg/m beat the largest rolled UB on stiffness at 57% of its weight
- Its flanges carry 80% of the second moment on 57% of the material — the web is nearly free
- But hw/tw = 150 means τcr = 54 N/mm² against a shear yield of 205
- The web buckles and does NOT fail: a tension field forms and the girder acts as a truss
- Only 52% of the plastic shear resistance was available; closer stiffeners recovered some
- A rigid end post was worth 22% — and nothing at all on a stocky web
- A bigger flange makes flange-induced buckling WORSE, not better
- 1/Ncr = 1/Ncr,ideal + 1/Sv — so Ncr can never exceed Sv, whatever the chords weigh
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
A web panel has τcr = 60 N/mm² and fyw = 355 N/mm². What is λ̄w?
Practice
A web is 1200 mm deep and 8 mm thick. If it is thickened to 12 mm, by what factor does τcr increase?
Practice
A built-up member has Ncr,ideal = 6000 kN and Sv = 12 000 kN. What is Ncr, in kN?
Practice
For that member the lacing is stiffened so that Sv becomes very large. What is the maximum Ncr it could ever reach, in kN?
Practice
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.
Practice
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?
Check yourself
A girder's shear resistance is inadequate. Which change is likely to be most effective per kilogram of added steel?
Check yourself
Which statement about tension-field action is correct?