Creep strains occur over time and a number of models are available to predict the development of creep as a function of time. The following creep models are implemented in RAM Concept and can be selected for use in the Load History / ECR tab of the Calc Options dialog: ACI 209R-92 (ECR values), ACI209.2 – GL 2000, AS 3600-2018, and Eurocode 2-2004. See Creep and Shrinkage Models for a brief description of each model.

Creep strains are assumed to be a linear factor of the elastic strain for a particular load. In order to consider loads that are applied at different times, the assumption is made that creep strains of like or opposing signs can be superimposed. These assumptions are likely reasonable for the normal range of service loads.

An ageing coefficient χ is used as a modifier of creep to account for the rate of application loading, its effect on

the creep and the variation of concrete strength over the time period. While the rigorous calculation of the coefficient is rather involved, this value can normally be taken as 0.8 with little loss in accuracy.

Creep models generally include an adjustment factor for a volume-to-surface ratio or an effective cross section thickness. The Exposure property in the Load History / ECR tab of the Calc Options dialog defines the element surfaces that are exposed and used to determine the exposed perimeter for that adjustment. When "Top and Bottom" is selected, both top and bottom surfaces of the element are assumed exposed. When "Top Only" or "Bottom Only" is selected, only the top or bottom surface of the element is assumed exposed, respectively.