RAM Concept - Load History / Construction Sequencing Discussion

Hi all, 

I'd like to have a discussion about the best way to set up the Load History Parameters and realistic Construction Sequencing. 

I tuned into a Bentley Webinar a few months ago and the presenter went through a Load History setup which was more comprehensive than the default file setup. I can't locate that webinar material. Perhaps one of the Support Team can explain/detail it further? I remember it separating the different stages of construction (slab poured, slab backpropped, partitions installed, occupancy etc).

An example of a post-tensioned highrise tower floor plate is below:

Load applied after 7 days (just after the final stressing). 

Backpropping loads (say 2.5 kPa average) applied for the next 28 days (say we need to backprop 4 levels and there is a week cycle time). 

Construction Loading (1.5 kPa) applied for the next 21 to allow for materials being stacked. 

Partitions installed (SDL applied) 

Occupancy (SDL + LL) after say 3 months from partitions installed. At this point we can calculate short term deflection. 

Then we can go on to calculate Long Term deflections. 

What are other people's approach to this? Have you set up your own Load History steps? If so would you like to share them? 

PS - Does Concept account for the fact the concrete hasn't reached it's final design Elastic Modulus (E) value when loaded between 0-28 days? 

Cheers

Parents
  • This is probably the webinar you saw: communities.bentley.com/.../ram-concept-webinar-resolving-serviceability-concerns-in-thin-concrete-floor-slabs-register-today the links within the post have video recordings.
    Your question about the impact of E is a valid one. For clarity, RAM Concept does not consider any variation of E in load history deflection calculations. The E used always corresponds to the 28-day concrete strength (or the input Ec value). This is true for the linear elastic global analysis as well as the detailed cross section curvature calculations. This will normally not cause huge errors in deflection calculations. Assuming the concrete strength at time of first loading is approximately 75% of design strength, this would create a variation in elastic modulus of approximately 10% for normal strength concretes. While it is true that this would impact any subsequent creep strains, this magnitude on elastic and subsequent creep strains is fairly small compared to the effect of cracking. For ACI, the effect of lower modulus upon application of loading and subsequent creep strains is ignored. For EC2, the intent appears to be to calculate the creep strains based upon the 28-day modulus, so RAM Concept’s calculations for creep might be more directly applicable for this standard. But you are correct in observing that there appears to be a difference in the standards related to treatment of Ec that is not reflected in the calculations in RAM Concept.



Reply
  • This is probably the webinar you saw: communities.bentley.com/.../ram-concept-webinar-resolving-serviceability-concerns-in-thin-concrete-floor-slabs-register-today the links within the post have video recordings.
    Your question about the impact of E is a valid one. For clarity, RAM Concept does not consider any variation of E in load history deflection calculations. The E used always corresponds to the 28-day concrete strength (or the input Ec value). This is true for the linear elastic global analysis as well as the detailed cross section curvature calculations. This will normally not cause huge errors in deflection calculations. Assuming the concrete strength at time of first loading is approximately 75% of design strength, this would create a variation in elastic modulus of approximately 10% for normal strength concretes. While it is true that this would impact any subsequent creep strains, this magnitude on elastic and subsequent creep strains is fairly small compared to the effect of cracking. For ACI, the effect of lower modulus upon application of loading and subsequent creep strains is ignored. For EC2, the intent appears to be to calculate the creep strains based upon the 28-day modulus, so RAM Concept’s calculations for creep might be more directly applicable for this standard. But you are correct in observing that there appears to be a difference in the standards related to treatment of Ec that is not reflected in the calculations in RAM Concept.



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