Reinforced Concrete Design / Design basis and limit states
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.
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.
Checks needed for project use
- A concrete-design reviewer must check the stated first-generation clauses, National Annex choices, detailing rules and factor sets. Numerical agreement alone does not establish code compliance.
- EQU, STR/GEO and sliding use different factor sets. The uplift example must remain illustrative, not current normative EQU guidance; licensed standard review is required.
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
Load-combination errors can arise from the classification of actions as well as the arithmetic: 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 expected to act throughout the reference period, with negligible variation or a monotonic change towards a limiting value: self-weight of the structure, finishes, fixed services, permanent partitions, earth pressure on a retaining wall. Classify the action by its expected duration and variation over the reference period.
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.
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
- Illustrative uplift arithmetic using the stated factors. A real EQU check requires its own verified factor set; γG,inf = 1.0 is not a universal EQU value.
- 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 actions persist through the reference period, with negligible variation or monotonic change towards a limit
- 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
- Use the favourable-action factors for the selected design situation and check; favourable variable actions are generally omitted in these examples
- Stability checks are decided by classification, not arithmetic