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Queensferry

Module 18 · Lesson 18.1

Execution: what the drawing actually asks for

Every calculation in this course has assumed a structure built the way it was drawn. This lesson is about the machinery that makes that assumption true — and about who pays for it.

Why 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)

A determination, not a preference

The execution class comes from three things:

  • Consequence class — CC1 to CC3, from EN 1990: what happens if this structure fails.
  • Service category — SC1 where the action is predominantly static, SC2 where fatigue, seismic or significant dynamic action applies.
  • Production category — PC1 for non-welded or simple welded work, PC2 for welded members of higher grades, thick material, or site welding.

Those three combine to give EXC1 to EXC4. The combination is a determination: it follows from the structure, not from how careful the designer feels.

Rule classification: execution requirement. The class and its consequences are procedural rules for how work is carried out and verified. They are not derived from mechanics and they cannot be argued from it.

What the class buys

The class sets how much the work is verified — most visibly, how much of the welding gets volumetric non-destructive testing, meaning testing that looks inside the weld rather than at its surface.

The jump matters. Moving from EXC2 to EXC3 roughly doubles the proportion of butt welds tested, and EXC4 requires testing every one of them. In the model used here that is about 1.8 times the EXC2 inspection cost at EXC3, and about 3.5 times at EXC4.

Those multipliers are indicative — they exist to make the point that inspection is a real cost, not to price a job.

Why over-specifying is an error

This is worth stating plainly, because engineers reach for the higher class the way they reach for a bigger section.

  • Specifying a class lower than the determination requires is a non-conformity. The structure is not verified to the level its consequence demands.
  • Specifying a class higher than the determination requires is a real cost with no structural benefit. It does not make the structure stronger. It buys inspection the structure does not need, and on a large job that is a significant sum spent on nothing.

A bigger section at least carries more load. A higher execution class does not.

Worked example

The same steel, two execution classes

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
  • Both are shop-welded S355 in ordinary thicknesses, so production category PC1

Find

The execution class of each, and what caused any difference

    Try it

    What determines the execution class

    The execution class is not a quality aspiration you choose — it is a determination from three inputs. Change each in turn and watch where it lands, and what that costs.

    Consequence class

    Service category

    Production category

    Relative inspection cost of each execution classEXC10.6xEXC21.0xEXC31.8xEXC43.5xinspection cost relative to EXC2 - this structure is EXC2
    Execution class
    EXC2
    Volumetric NDT on butt welds
    10 %
    Welding quality certification
    required
    Relative inspection cost
    1.0 x EXC2
    Step 1
    CC2 gives a starting point of EXC2.
    Step 2
    SC1: predominantly static, so no escalation from the service category.
    Step 3
    PC1: non-welded or simple welded work, so no escalation from production.

    EXC2: volumetric NDT on 10% of butt welds, at about 1.0 times the inspection cost of EXC2. The class is a DETERMINATION from consequence, service and production — not a preference. Specifying higher than determined is a real cost with no structural benefit; specifying lower is a non-conformity.

    Things worth trying

    • Start at the defaults — CC2, SC1, PC1. That is the ordinary static building, and it lands on EXC2 at the baseline inspection cost.
    • Switch the service category to SC2. That is the crane girder of Module 17, and the SAME structure is now EXC3: twice the volumetric testing, at 1.8 times the inspection cost.
    • Note what happened there. Nothing about the steel changed. A decision about ACTIONS, taken in Module 2, has just changed how the fabricator is inspected.
    • Take the consequence class up to CC3 with PC2, leaving the service at SC1. It stops at EXC3 — demanding welding on a high-consequence structure does not on its own reach EXC4.
    • Now switch the service to SC2 as well. THAT reaches EXC4: full volumetric testing of every butt weld, at 3.5 times the EXC2 cost. EXC4 is for extreme consequences, and it should be hard to reach.
    • Read the steps in the readout. With CC3 and SC2 you are already at EXC4, and the readout says PC2 would have escalated further but cannot. A determination that quietly dropped one of its inputs would not be checkable.
    • Try CC1 with PC2. The class does not move — demanding welding on its own does not escalate a low-consequence structure.
    • The practical point: specifying a class higher than the determination requires is a real cost with no structural benefit. Specifying one lower is a non-conformity. Neither is a matter of preference.

    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

    A four-storey frame is measured after erection. Each storey is 4 m, and the top of the fourth storey is 12 mm out of plumb from the base. What is the permitted deviation over that height, in mm?

    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 CC3 with PC2 welding but entirely static loading. What execution class does it reach?

    Summary

    • The execution class is determined from consequence, service and production — not chosen
    • It buys verification: EXC3 roughly doubles the volumetric NDT of EXC2, at about 1.8× the cost
    • EXC4 needs extreme consequences AND dynamic or fatigue action, not just demanding welding
    • Specifying above the determination is a real cost with no structural benefit
    • 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
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    This is educational material. It uses simplified examples to teach principles, and must not be relied on for real design or safety-critical decisions. Module overview and checkpoint