I would like to have a question about how should apply “Load history parameters and AS3600” as per link below:
https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/36046/load-history-parameters-and-as-3600
To clarify I would like to separate some queries following:
I ask this question because sometime in my country they use additive to push concrete age early mature, and they usually take the propped at 7 days. But at this time because of the additive the concrete age is reach full compressive strength at 28 days. So the “Initial load application” is 7 days (remove propped), but the concrete age is actually 28 days.So what is the key point in this situation, “the day of apply load” or “the day of concrete age”?
> So what is the key point in this situation, “the day of apply load” or “the day of concrete age”?
And what should we use in Ram? For above example: 7 or 28 days?
But the picture show input Ram Concept showed 14 days? Is it correct?
If it is correct, mean that you have to put it 14 days by default for any case? Or else?
Below is picture captured in the web:
4. It is said that: “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.” Mean that if I design with ACI code I need to put 7 days and creep factor corresponding. And If I want to apply load at 28 days ( or 3), I just need to change “Initial load application” to 28 (or 3), and leave creep factor as it is?
5. In the link I see a plot of calibration of k3 factor. But the AS3600 curve (blue line) seem not so corresponding with the one state in AS3600 – 2009 code.
For example:
7 days
28 days
365 days
Blue line (in web)
~1.45
~1.1
~0.75
AS3600 – 2009
1.75
1.1
0.9
6. I am list here step by step to apply creep factor for Load history in Ram Concept from what I understand so far, please advise if any:
a. Use the equation 3.1.8.3 in AS3600 to calculate design creep coefficient with k2, k4, k5 as per code. For k3, I take the value from green line above at 7 days which k3~1.45
b. Use this value and + 1, then put into creep factor in Ram.
c. Input 7 days in “Initial load application”.
And if I want to apply load at 28 days ( or 3 days), I just need to change “Initial load application” to 28 (or 3), and leave creep factor in step b above as is.
Thank in advanced,
1. Since AS 3600 defines a creep coefficient as creep strain/elastic strain and the creep factor in RAM Concept is defined as total strain/elastic strain, a value of 1 should always be added to the creep coefficient determined from Equation 3.1.8.3.
2. The initial loading time should always match the actual loading of the structure. This will correspond to the age when the shoring is removed and not when the concrete has reached its 28-day strength. Note that RAM Concept does not vary the concrete modulus to account for early age concrete. The effect of early age strength on the modulus are on the order of 10-15% and the differences on cracked concrete deflections based on this difference are even less. Cracking and creep tend to include deflections by 20x that or more, so in the grand scheme of things, the effect of early age concrete strength is relatively insignificant (but not 0). Also, note that early age deflections are vary difficult to predict due to variability fo cracking and the steep gradients in the early age creep and shrinkage curves. As noted in the RAM Concept Manual (see Section 67.7 “Final Word of Caution”), load history deflections for ages less than 90 days should be used with caution.
3. Sorry, that was a typo. We have corrected the image in the tech note. Thank you for bringing this to our attention.
4. Yes, that is correct. The initial loading time should always match the actual loading of the structure. The green line in the calibration graph should be used to determine a calibration factor that will convert the determined AS 3600 creep factor to the ACI 209 model. All other adjustments for loading time will then be automatically accounted for by the program.
5. The equation for k3 was updated in an amendment to the original published AS-3600. The equation that is used is 2.7/(1 + log(t)).
6. Yes, that is correct. If the initial loading time is 3 days or 28 days, you would change the input initial loading time and adjust the correction factor read from the green line in the graphic. So, the adjustment value would no longer be the 1.45 factor used for the 7-day initial loading time.
Answer Verified By: Tri Le