Current Revision posted to RAM | STAAD | OpenTower Wiki by Karl G on 7/7/2020 1:17:06 AM
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| | 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.
Creep Coefficient Correction Factors
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 always match the actual initial loading time of the real structure. The initial loading time will 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.
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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.
It should be understood that this calibration factor does not correspond to a single given loading time, but rather will be used by RAM Concept to try to adjust the ACI 209 curve (represented by the red line in the image above) for all specified loading/unloading times. There is not a way to make it match exactly for many different loading/unloading times. Since the curve is reasonably flat, using the calibration factor for the initial loading time will give reasonable results.
Creep Example
Determine the creep factor for a 32 MPa, 200 mm concrete slab in a temperate inland factor environment. Initial loading time is 14 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 = 1.35 (from green line in calibration plot above for time of 14 days)
Creep Factor = 3.4*1.2*0.6*1.0*1.35 + 1 = 4.31
The actual initial loading time is 14 days. The Initial Load Application should be defined as 14 days in the Load History/ECR tab of the Calc Options dialog.
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Shrinkage Strain
In AS 3600, the final shrinkage strain is defined as the design shrinkage strain and consists of two parts: autogenous shrinkage and drying shrinkage. The shrinkage strain that is input in RAM Concept is the final shrinkage strain, or the sum of the autogenous and drying shrinkage strains.
All appropriate adjustment factors to account for environment conditions should be included in the final shrinkage strain. The time-dependent factor k1(Figure 3.1.7.2) is accounted for automatically by RAM Concept through the ACI 209 model.
For AS 3600, the input shrinkage strain is determined as follows:
- Calculate the final autogenous shrinkage strain using Equation 3.1.7.2(3)
- Determine the basic drying shrinkage strain using Equation 3.1.7.2(5)
- Factor the basic drying shrinkage strain by factors k1 and k4
- Calculate final shrinkage strain by adding strain in Step 1 and 3.
Moist Cure Duration
The ACI 209 shrinkage model used in RAM Concept assumes that no shrinkage occurs during the moist cure duration period. The standard moist cure duration period is 7 days and correction factors are applied for other durations (see ACI 209R-92 Table 2.5.3). RAM Concept automatically calculates this correction.
AS 3600 references a time “after the commencement of drying” with respect to the drying shrinkage and this is the equivalent of the moist duration period used in RAM Concept. The input moist duration period should be the time that has elapsed between concrete setting and the commencement of drying.
Note that moist cure duration correction will have the most significant effect on early age deflection. Because of the unpredictable nature and variability of early age shrinkage, early age deflections should be used with caution. Section 67.7 in the RAM Concept Manual has more discussion on this topic.
Shrinkage Example
Determine the shrinkage strain for a 32 Mpa, 200 mm concrete slab in a temperate inland environment. Assume a moist cure duration period of 7 days.
The final autogenous drying shrinkage strain from Equation 3.1.7.2(3) is:
[(0.06*32) – 1.0]*50*10^-6 = 0.000046
The final basic drying shrinkage is 1000 x 10-6.
The basic drying shrinkage strain from Equation 3.1.7.2(4) is:
[1.0 – (0.008*32)]*1000*10^-6 = 0.000744
k4 = 0.6 (for temperate inland environment)
k1 = 1.22 (see Figure 3.1.7.2)
The adjusted basic drying shrinkage is:
1.22*0.6*(0.000744) = 0.000545
Final shrinkage strain = 0.000545 + 0.000046 = 0.000591
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See Also
Load History Article
RAM Concept Load History Calc Options
Tags: shrinkage, SELECT Services, creep, long term deflection, RAM Concept, load history, AS3600
