Creep is the slow, permanent deformation a material undergoes when a constant load rests on it for a long period. Load a beam and it deflects immediately — that is elastic deformation. Leave that load in place for years and the deflection keeps increasing slowly. That additional movement is creep.
Where creep matters
Concrete. Concrete creeps considerably. On a permanently loaded member the extra deformation can be several times the immediate elastic deflection; Eurocode 2 handles this with a creep coefficient, which — depending on humidity, member thickness and the age of the concrete when loaded — is typically in the order of 1.5 to 3. Creep is also why prestressing force is lost over time.
Timber. Timber creeps too, and damp timber especially so. Eurocode 5 covers this with the factor k(def). For solid timber it is 0.60 in service class 1 (heated internal), 0.80 in class 2 (covered external) and 2.00 in class 3 (fully exposed). In practice: a timber beam in an exposed external situation can end up deflecting around three times as far as it did on day one.
Plastics and bitumen creep strongly, particularly at higher temperatures — think of an insulation board under a heavy sustained load, or a pipe sagging between its clips.
What you meet on site
- Doors and windows that start to bind under a slowly deflecting floor or lintel — often years after completion.
- Cracks in non-loadbearing partitions built off a creeping floor. Detail a sliding or open joint at the head.
- Ponding on flat roofs, because the deck sags in the middle faster than the fall compensates for. Design the fall with margin — at least around 1:80, and preferably 1:60.
- Prestressed hollow-core slabs have a camber that changes with time; allow for it when screeding and tiling.
Creep cannot be avoided, but it can be managed: work at lower stress levels, let concrete gain strength before loading it heavily, and check deflection limits against the final long-term value rather than the instantaneous one.
Related terms
- Shrinkage
- Elastic deformation
- Plastic deformation
- Modulus of elasticity
- Concrete shrinkage
- Eurocode
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