| Applies To | |||
| Product(s): | RAM Concept | ||
| Version(s): | Various | ||
| Environment: | N/A | ||
| Area: | Analysis | ||
| Original Author: | Bentley Technical Support Group |
This tech note discusses how to determine the Creep Factor and Initial Load Application for the AS 3600 design code. RAM Concept uses the ACI 209 model for creep and shrinkage. The creep and shrinkage parameters outlined in AS 3600 need to be modified to fit this model.
Creep Factor
In general, the final (30-year) creep value after being modified by all appropriate adjustment factors should be input as the creep factor.
AS 3600 references a creep coefficient, which is defined as the ratio of creep strain to elastic strain. The creep factor in RAM Concept is defined as the ratio of total strain (elastic strain + creep strain) to elastic strain, or 1 + creep strain/elastic strain. The creep factor in RAM Concept then is equal to 1 + creep coefficient.
For AS 3600, the input creep factor is determined as follows:
- Find the basic creep coefficient from Table 3.1.8.2
- Factor basic creep coefficient by correction factors k2, k4, and k5
- Add 1 to convert the creep coefficient to a creep factor
- Use a correction factor to convert the maturity coefficient k3 to the ACI 209 age coefficient
See “Creep Coefficient Correction Factors” for more discussion on Steps 2 and 4 above.
AS 3600 uses the correction factors k2, k3, k4, and k5 to convert the basic creep coefficient to a design creep coefficient at any time. See Equation 3.1.8.3 in AS 3600-09.
Factor k2 should be based on a 30 year time after loading.
Factor k3 is a maturity coefficient that accounts for the initial load application. This parameter is analogous to the parameter in ACI 209, which is automatically accounted for by RAM Concept. A correction factor should be calculated based on the discussion in the Initial Load Application section below to calibrate the ACI 209 model used by RAM Concept to match the AS 3600 model.
Factor k4 accounts for environmental factors and will vary from project to project.
Factor k5 is a modification for high strength concrete and is a function of the concrete strength.
Initial Load Application
ACI 209 is based on a 7-day initial load application time, while AS 3600 is based on a 28-day initial load application time. The Initial Loading Application value that is input into RAM Concept should be based on the construction and shoring schedule of your project and not the standard values specified in the design code. For example, it is common in the United States for formwork to be removed anywhere between 3-7 days and this is what should drive the input of the Initial Load Application value in RAM Concept.
ACI 209 uses a modification factor to account for initial load application times other than 7 days. This factor is automatically determined by the program and should not be incorporated into the input creep value. In AS 3600, the k3 correction factor accounts for the initial load time. A correction factor should be applied to the input creep factor to match the k3 factor to the age coefficient in the ACI 209 model.
In the plot below, the red line represents the equation for the maturity coefficient in ACI 209. The blue line represents the equation for the k3 factor in AS 3600. The green line is the ratio of the coefficient in AS 3600 to the coefficient in ACI 209 and represents the calibration factor that should be applied to the input creep factor to calibrate the ACI 209 model to the AS 3600 model.
For a single load application time, the calibration factor determined from the green line is easily determined. However, most real load histories are more complicated and include many loading/unloading times. For these more complex cases, the initial load application time is generally the most influential on creep deflection and the factor associated with this time from the green line plotted above should be used. For example, if the actual initial load application time is 7 days, then the correction factor is between 1.4 and 1.5.
Example
Determine the creep factor for a 32 MPa, 200 mm concrete slab in a temperate inland factor environment. Initial loading time is 28 days.
The basic creep coefficient from Table 3.1.8.2 is 3.4.
k2 = 1.2 (see Figure 3.1.8.3(a) at 30 years after loading)
k4 = 0.6 (for temperate inland environment, see 3.1.8.3)
k5 = 1.0 (f’c < 50 MPa, see 3.1.8.3)
calibration factor for k3 = 1.3 (from green line in calibration plot above for time of 28 days)
Creep Factor = 3.4*1.2*0.6*1.0*1.3 + 1 = 4.18
The initial loading time is 28 days. The Initial Load Application should be defined as 28 days in the Load History/ECR tab of the Calc Options dialog.
See Also
RAM Concept Load History Calc Options