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Queensferry

Module 2 · Lesson 2.2

Actions, and getting the classification right

Permanent or variable, favourable or unfavourable — the classification decides the factor, and it is where most combination errors begin.

Why this matters

Almost nobody gets the arithmetic of a load combination wrong. What goes wrong is the classification: a load treated as permanent when it should be variable, a stabilising weight factored up when it should have been factored down, or a favourable variable action counted at all. These are not arithmetic slips — they are conceptual errors, they survive checking because the sums add up, and one of them can turn an unsafe structure into an apparently satisfactory calculation.

By the end of this lesson you should be able to

  • Classify an action as permanent, variable or accidental, and justify the choice
  • Decide whether an action is favourable or unfavourable for a given check
  • Explain why the same load can be both, in different checks on the same structure
  • Apply the correct factor in each case, including the awkward ones

Permanent, variable, accidental

Permanent actions (G) are those whose variation over time is negligible: self-weight of the structure, finishes, fixed services, permanent partitions, earth pressure on a retaining wall. The defining question is not 'is it heavy' but 'will it be there for the whole life of the structure, essentially unchanged?'

Variable actions (Q) vary significantly and unpredictably: imposed floor loading, wind, snow, thermal effects, loads during construction.

Accidental actions (A) are of low probability but potentially severe: impact, explosion, and the notional removal of a member in a robustness check.

The boundary is not always obvious, and two cases catch people out.

Movable partitions are a variable action, even though they feel permanent — the whole point of them is that a tenant may move them. They are usually handled as an equivalent uniformly distributed variable load.

Fixed partitions built off the slab are permanent. If they are demountable studwork, they are not.

Favourable and unfavourable

An action is unfavourable for a check if increasing it makes the check harder, and favourable if increasing it makes the check easier.

This is a property of the check, not of the load. The same self-weight is unfavourable when you check the sagging moment at midspan and favourable when you check uplift at a support. There is no such thing as a favourable load in the abstract.

The rules follow from what the factor is protecting against:

  • Unfavourable permanent: factor UP by γG,sup (typically 1.35). It might be heavier than you think, and that would be bad.
  • Favourable permanent: factor DOWN by γG,inf (typically 1.0). It might be lighter than you think, and now that is what would be bad.
  • Unfavourable variable: factor UP by γQ (typically 1.5).
  • Favourable variable: omit entirely. A variable action may simply not be present, and you cannot build a structure that relies on the furniture being in the room.
Load path from slab through beam and column to foundationslabbeamcolumnpadload → slab → beam → column → pad → ground
The same load, classified differently at each stage of its journey. What is a variable action on the slab arrives at the foundation combined with permanent actions accumulated on the way — and at the foundation, some of that weight may be favourable.

Worked example

A canopy in an uplift wind — where classification decides everything

Given

  • A projecting concrete canopy, self-weight 55 kN characteristic
  • Characteristic wind uplift on the canopy, 40 kN
  • Characteristic imposed load on the canopy roof, 30 kN
  • γG,sup = 1.35, γG,inf = 1.0, γQ = 1.5 (all nationally determined — see the note below)

Find

Whether the canopy is held down.

Assumptions

  • The check is static equilibrium: uplift against restoring weight
  • The canopy is not anchored down — only its own weight resists

    Practice

    A retaining wall is checked for sliding. The soil behind pushes it outward (characteristic 90 kN); friction under the base resists, proportional to the wall's self-weight (characteristic 140 kN, friction coefficient 0.5). Using γG,sup = 1.35 for the destabilising earth pressure and γG,inf = 1.0 for the stabilising weight, what is the design sliding resistance, in kN?

    Practice

    A 300 mm wide by 600 mm deep concrete beam. Taking reinforced concrete at 25 kN/m³, what is its self-weight, in kN/m?

    Check yourself

    A tall building is checked for overturning under wind. Which is the correct treatment of the imposed floor loading?

    Summary

    • Permanent means unchanged for the whole design life — not merely heavy
    • Movable partitions are variable; fixed partitions are permanent
    • Favourable and unfavourable are properties of the CHECK, not of the load
    • Split self-weight into separate actions wherever its distribution matters
    • Favourable permanent → 1.0; favourable variable → omitted entirely
    • Stability checks are decided by classification, not arithmetic
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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