Structural Steel Design / module 18
Fabrication, erection and durability
The point at which a design stops being a calculation and becomes an instruction to somebody else — and the two limit states that are reached on site rather than in service.
What you will understand
- Say what determines the execution class, and why it is a determination rather than a preference
- Explain what an execution class actually buys, and what it costs
- Treat corrosion protection as a design-life problem rather than a paint problem
- Identify the details that defeat a coating specification whatever is written on it
- Show why a partly erected frame can be several times less stable than the finished one
- State what the sway imperfection of Module 4 does and does not cover
Lessons
Execution: what the drawing actually asks for
Execution specifications set the fabrication, inspection and erection requirements. This lesson explains their role and identifies historical examples that still need standards review.
Durability and erection: the two limit states nobody analyses
One is reached slowly over decades, the other in an afternoon halfway through the programme. Neither appears in the frame analysis, and both can govern.
Module summary
- The execution class follows from consequence, service and production — it is determined, not chosen
- A crane girder is SC2, so the same steel that would be EXC2 becomes EXC3: twice the NDT at 1.8× the cost
- EXC4 should be hard to reach — demanding welding on a CC3 structure does not get there on its own
- Corrosion is a design-life problem: C3 with 200 µm gives 25 years against a 50-year life
- The same system gives 66 years in C2 and under 10 years in C5 — the site changed, not the paint
- A detail that traps water defeats the specification locally, and that is a drawing correction
- At 1 of 4 braced bays with 80% of the steel up, αcr is 3.75 against the finished frame's 12
- The design sway imperfection of 1/200 assumes a frame built inside its 1/300 tolerance
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
Moving from EXC2 to EXC3 changes the proportion of butt welds given volumetric NDT from 10% to 20%. By what factor does the tested proportion increase?
Practice
Unprotected steel in a C5 marine environment corrodes at about 120 µm per year. Over a 50-year design life, how much thickness is lost from each exposed face, in mm?
Practice
A frame will finish with 4 braced bays and αcr = 12. At one stage, 2 bays are braced and all the steel is standing. Estimate αcr at that stage.
Practice
A 4 m storey is permitted to be out of plumb by h/300. What is the permitted deviation, in mm?
Practice
A 200 µm system gives 25 years to first maintenance in C3. How many times must the building be repainted over a 50-year design life?