Creep

Wood as a structural material demonstrates many benefits over other structural materials. It is a naturally grown, aesthetically pleasing material with a high strength to weight ratio and has the added benefit of sequestrating carbon from the environment during its in-service life.

When using wood as a structural material, particularly in bending, careful consideration must be given to the long-term performance. When subjected to loaded situations for long periods of time, wood is susceptible to creep strains or deformations, and for timber structures, the creep deflection can be large relative to the instantaneous deflection. There are three primary components that contribute to the total creep deflection of a structural element. These three components are time-dependent or viscoelastic creep, mechano-sorptive creep due to moisture changes and pseudo-creep that is attributed to swelling and shrinkage of the timber.

Figure 1. Creep deflection of timber beams subjected to a sheltered external climate.
(Photo credit: “Martin Ansell, University of Bath”)

Viscoelastic creep in wood is described as additional strain with time at constant moisture content and constant environmental conditions under sustained loading. This additional strain manifests as an increase in deflection. It can be seen in Figure 2a, under constant relative humidity and temperature conditions in bending, the elastic (ue) and viscoelastic (uve) strain/deflection components contribute to the total deflection.

Wood and creep in wood are heavily influenced by the surrounding environment. Under changing relative humidity, hygro-expansion or swelling/shrinkage deformations of wood must be considered but also, under load, the creep behaviour of wood is accelerated with changing relative humidity. This behaviour is known as mechano-sorptive creep behaviour and is a strain/deformation due to an interaction between stress and moisture content change in wood. In Figure 2b, under variable relative humidity conditions, additional deflection due to the mechano-sorptive (ums) and swelling/shrinkage (us) components is presented. Typically, there is an increase in the deflection during each drying phase and a decrease during the wetting phase of a relative humidity cycle. In a variable climate, the total strain/deformation comprises the elastic, viscoelastic, mechano-sorptive and swelling/shrinkage components.

It is difficult to separate and characterise these individual components as they often occur simultaneously within a loaded structural element. In the design of such elements, factors are available which account for additional deflections with time. Eurocode 5 (CEN 2005) provides deformation modification factors or kdef factors which account for creep effects with time. The kdef factor is used to increase the initial elastic deflection of the designed element. These factors are dependent on the different service classes in order to account for viscoelastic creep behaviour in a constant climate and additional mechano-sorptive and swelling/shrinkage behaviour in variable and external climates.

                                                                                                "a"                                                                          "b"

 
Figure 2. Creep components in bending: ("a" Constant climate, "b" Variable climate, where ue = elastic component, uve = viscoelastic component, ums = mechano-sorptive component and us = swelling/shrinkage component.

Studies focused on the creep behaviour of wood and engineered wood products along with additional studies utilising wood within timber engineering applications may be found on the TERG (www.nuigalway.ie/terg) web-page (Timber Engineering Research Group at the National University of Ireland Galway).

 

Contributor:       Dr. Conan O'Ceallaigh