Where do the default values for cracked and uncracked properties of concrete come from?
Tekla Structural Designer
QuestionWhere do the default values for cracked and uncracked properties of concrete come from?
1. Modification FactorsAnalyse ribbon > Options > Modification Factors > Concrete allows you to define the factors for use in the different analysis types performed.
These factors are applied to the various element types, based on whether the section is specified as cracked or not in their member properties. The factors reduce the full short term stiffnesses of the members to their long term values, which allows for long term effects, such as creep, cracking and shrinkage, and will generally have the effect of amplifying the deflections.
1.1 SlabsThere is various general guidance available from design guides and documents as to what these values should be set to, but these generally provide a range of values - typically a range of 0.16-0.25. We have selected a value of 0.2 (for cracked sections) as the default, as this sits within these ranges and is generally considered to be quite reasonable. The 0.25 value is considered by some to be slightly unconservative for the office situation, so 0.2 seems more reasonable here, with 0.16 typically used for storage. However, if you want to set it to something else, then that is your engineering decision.
As mentioned above, there are various guidance documents available, but below are some more commonly available ones:
- “How to design reinforced concrete flat slabs using finite element analysis” by O Brooker The Concrete Centre
- Technical Report (TR) 58
- CIRIA Report 110 (2nd edition)
1.2 WallsBoth the Eurocode and British Standard codes lack guidance in this area, however, the American codes provide more information and recommend doubling the factor for uncracked sections.
In the lack of any further guidance we have followed suit, but obviously you can change the value if you wish.