Structural Steel Design / Fabrication, erection and durability
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.
Lesson review: expert verification remains open
This lesson has received a full-context editorial review and a check of its technical explanations. Selected arithmetic has been checked separately. This is not independent engineering sign-off.
Corrections checked in this pass
- Historical SC/PC escalation and fixed NDT/cost claims removed from lessons; public execution-class calculator already withheld. Current first-generation selection basis and EN1090-2 amendment identified; traceability distinguished from sign-off.
Checks needed for project use
- A steel-design reviewer must check the first-generation calculation profiles, second-order analysis assumptions, connection rules and execution references against the applicable standards and amendments.
- Project execution specification, applicable edition/National Annex, weld-specific inspection/tolerance requirements and actual fabrication controls. Legacy executionClass function remains in library but is not imported by the public lab; do not restore it as a current recommendation engine. Full independent engineering sign-off remains open.
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 notesWhy this matters
Seventeen modules have produced resistances. Every one of them rests on an assumption nobody has stated: that the steel arrives with the properties assumed, that the welds are the size drawn, that the bolts are tightened as specified, and that the frame is put up close enough to plumb for the imperfection allowance to hold.
None of that happens automatically. It happens because the design specified an execution class, and the execution class bought a level of inspection. That specification is a design decision with a cost, and it is one of very few decisions in this course where getting it too high is also wrong.
By the end of this lesson you should be able to
- Say what the three inputs to an execution class are
- Explain what the class buys and what it costs
- Say why over-specifying is a real error and not a safe one
- Relate erection tolerance to the sway imperfection of Module 4
What you should already know
- Consequence classes and reliability differentiation (EN 1990, Module 1)
- Sway imperfections and equivalent horizontal forces (Module 4)
- Fatigue as a service condition (Module 17)
Specify the execution requirements
Record the applicable design and execution standards, edition and National Annex, the structure’s consequence class, loading conditions and project requirements. Select the execution class using that stated basis and document any component-specific provisions.
The older SC/PC matrix is historical. The later first-generation EN 1993-1-1 Annex C and UK National Annex changed the selection route; a simple rule that increments EXC for each input is not valid. Verify the governing edition for the project, particularly during the transition to second-generation Eurocodes.
Inspection follows the specified work
Execution class informs requirements for fabrication, welding, tolerances and inspection. NDT extent also depends on the weld, loading, utilisation, inspection method and applicable provisions; there is no universal 10%, 20%, 100% rule for every butt weld.
Additional assurance can be justified by risk or project requirements. It does not automatically raise calculated resistance, but it can have value. Specify the required assurance and review its cost without treating a higher class as either automatically necessary or pointless.
Worked example
Preparing two execution specifications
Given
- Two members, identical in every structural respect: same sections, same welds, same fabricator
- One is a floor beam in an ordinary office; the other is the crane girder of Module 17
- Both are in a normal building, so consequence class CC2
- The project’s standards editions and final execution classes have not yet been supplied.
Find
Identify the information needed to specify execution; do not assign an execution class from incomplete inputs.
Try it
Execution-class model under review
This exercise used a historical service/production-category model. Its class selection, inspection percentages and cost multipliers have not been verified against the applicable standards.
Calculated recommendations are withheld while this model is reviewed.
Expert review required
Use the project execution specification and the applicable EN 1993 and EN 1090 provisions to determine the class, inspection scope and tolerances. The historical examples in this lesson do not establish current requirements.
Tolerance, and what the design already assumed
Module 4 introduced the sway imperfection: a frame is not built perfectly plumb, so the analysis carries an initial lean of about 1/200 before reduction, applied as equivalent horizontal forces.
Erection tolerance is a separate thing, and the relationship between them is worth getting right.
A frame is permitted to be out of plumb by roughly h/300 per storey. Note the direction: the permitted tolerance (1/300) is tighter than the imperfection the design assumed (1/200). That is not an accident — it is why the allowance works. A frame built at the very edge of its permitted tolerance still sits inside what the analysis allowed for, using about two thirds of it.
So what about a frame erected outside tolerance? A 4 m storey measured 25 mm out of plumb is 1/160 — worse than permitted, and worse than the design assumed.
Practice
Arithmetic exercise only: for a height of four 4 m storeys, calculate H/300 in mm. This is not a verified multi-storey erection tolerance; check the relevant EN 1090 table and tolerance class.
Check yourself
A designer specifies EXC4 for an ordinary office building on the grounds that higher quality is always safer. What is wrong with this?
Predict first
A structure is described as CC3 with static loading and historical PC2 welding. Can those labels alone establish its current execution class?
Summary
- The displayed SC/PC framework is historical; determine execution class using the applicable standards and project requirements
- Weld inspection requirements depend on the applicable provisions and details, not a universal percentage by class.
- Execution-class selection requires a recorded standards edition and project basis.
- Additional assurance should be justified by project risks and requirements; higher class is not automatically necessary or automatically pointless
- The permitted erection tolerance (1/300) is TIGHTER than the design imperfection (1/200)
- That is why the imperfection allowance works — and why it does not cover a frame built outside tolerance