Hi,
after reading RAMConcept manual and several posts on the topic, I have some questions.
On the Bentley post http://communities.bentley.com/products/structural/structural_analysis___design/w/structural_analysis_and_design__wiki/6191.ram-concept-load-history-calc-options-tn is defined the creep factor as 1 + 2.35 = 3.35, where the average value creep strain/elastic strain is 2.35.
According to AS3600 the basic creep coefficient (creep factor in RamConcept) for a 40Mpa concrete loaded at 28 days is 2.8, which is not really far from the 2.35 adopted by RamConcept.
The question is: why the creep factor input in the RamConcept Calc Options is 1+2.35? where the 1 comes from?
Moreover, in another post it has been said (by Karl G.) that "ACI 209 assumes that the initial loading is applied at 7 days. AS 3600 assumes a different time for initial load application, so the creep factor must be adjusted for an initial loading time of 7 days when that design code is selected".
Because AS3600 gives the basic creep coefficient only for a concrete loaded at 28 days, how do I get the relevant coefficient for a concrete loaded at 7 days?
Would it be the same if I input the coefficient 2.8 as creep factor and define 28 days as time of loading? I know the model adopted by RamConcept is the ACI model, so I am not quite sure on the implications of changing those parameters. But at the same time I wonder why RamConcept allows to modify those parameters if it would give incorrect results?
And the last question? Assumed I can input the 2.8 coefficient and 28 day loading time, is it 2.8 correct or do I have to input 1+2.8? if so, why?
Thank you
Davide
The creep factor in RAM Concept is defined as: (elastic strain + final creep strain)/elastic strain. This equation expands to elastic strain/elastic strain + final creep strain/elastic strain or 1 + final creep strain/elastic strain. 2.35 is the typical value for final creep strain/elastic strain.
AS 3600 uses the creep coefficient, which is defined in Clause 1.6.3.8 as the “mean value of the ratio of final creep strain to elastic strain …” In order to match the creep factor in RAM Concept which is defined as (elastic strain + final creep strain)/elastic strain, you would need to add elastic strain/elastic strain or 1 to the value. If the creep coefficient is 2.8, then you would use a creep factor of 1 + 2.8 = 3.35 in RAM Concept. The creep factor that is entered should represent the ultimate creep strain that is required by the design code being used.
The k3 factor in Table 3.1.8.3(b) can be used to modify the creep coefficient for time of loading. For an initial loading of 7 days, k3 is approximately 1.75. For an initial loading of 28 days, k3 = 1.1. The modifier to covert to an initial loading of 7 days would be 1.75/1.1 = 1.59.
RAM Concept is based on the ACI 209 creep model. In the ACI 209 model, the equation used to calculate creep at any time assumes an initial loading application of 7 days. ACI 209 has its own modification for the initial loading time for initial loading times other than 7 days that is automatically calculated by the program. This modifier is different from the initial loading time modifiers used in different codes. The implication of entering a creep factor of 2.8 and an initial loading time of 28 days is that a different initial load modifier would be calculated by RAM Concept and the adjusted creep at any given time would be adjusted and likely not match the creep predicted by the AS 3600 model.
Answer Verified By: Davide Rigo