RAM Structural System & RAM Concept column moments differ

Hi,

When I design concrete columns in RAM Concrete, it uses the cracked factors of the slab and column in RAM Modeler. If I model the slabs with bending cracked factor of 0.25 and column cracked factor of 0.7 per ACI, I get significantly higher moments in the column than in the RAM Concept model (assuming this is a single floor model).

I am modeling the slab in RAM Concept without crack factors (default 2-way slab). Is this the reason why the column moments in RAM Concept are so much smaller? Should we be putting the crack factors for the slab in RAM Concept?

Does the strength design of the slab in RAM Concept account for load history cracking? Are load histories only used for deflections?

Thanks,

Andrew

Parents
  • RAM Concept does not use the element stiffness reductions from the load history analysis in the main calculations by the Strength Designer. The load history calculations are intended for service deflection checks only. One reason for that is that tension stiffening is considered in the load history analysis and that behavior should not be used in ultimate strength checks where the concrete in tension is ignored.

    If you are looking at the column design moments in RAM Concrete Column and comparing them to the reactions in RAM Concept, then moment magnification and minimum eccentricity requirements are the likely cause of the higher moments. More on that subject can be found here.

    ACI 318-14 R6.3.1.1 has discussion on stiffness reduction. If you are using the load history analysis in RAM Concept, you will want to use full element stiffness (no cracked section factors). Otherwise, you will be double-counting the cracking effect. More on that can be found on this web page.



  • The columns I'm designing are not slender so there aren't any magnifications. I'm presume that the difference in column moments between RAM Concrete and RAM Concept is due to the crack factor of the slab. 

    If RAM Concept strength design does not use the load history cracking factors, I presume we should be using the 0.25 cracked factor imported from RAM Structural for slab strength design. Is this correct?

  • I would use the ACI provision to decide what cracked section factor is appropriate. Using 0.25 would be too low for a PT slab. I might even be too low for a reinforced slab. Note that the corresponding provision in ACI 318-11 R8.7.1 mentions using EcIg for all members as a common assumption for all members of braced frames.

    It is not clear what magnitude of difference you are seeing with moments. If you are seeing extremely large differences, perhaps we should review your model. Feel free to share it using our Secure File Upload and post the file name and one column to review here after it is uploaded.



  • Crack factor of 0.25 is from ACI 318-14 Table 6.6.3.1.1(a) for flat slabs.

    The moments are about 50% higher in RAM Concrete. 

  • The factors in ACI 318-14 are geared to lateral load analysis and are intended to capture an appropriate sway deflection so P-Delta effects are not underestimated. RAM Concept is using a single floor analysis and, likely based on the governing load combination that I see, the gravity loading and not the lateral loads are the concern here. I would pause for just a moment before applying the 6.6.3.1.1(a) factors to the RAM Concept analysis. 

    I do not have enough details about your structure to given you a definite answer whether you should use 0.25 or not, which is why I pointed you initially to ACI 318-14 R6.3.1.1. Also, note that there are several different assumptions that might be appropriate. For example, for the previous mentioned lateral analysis, ACI 318-14 6.6.3.1.2 permits you to use I=0.5Ig for all members instead of the reduction factors in Table 6.6.3.1.1(a) that you cited.

    If you are uncertain what stiffness reduction to use, a good approach is to use lower bound and upper bound values in separate runs in an attempt to envelope the design. That approach should give you confidence that your design is safe.

    If you would like use to dig further or give better guidance on cracked section factors, please share your model so we have more information on the floor that you are designing.



    Answer Verified By: Andrew Yu 

Reply
  • The factors in ACI 318-14 are geared to lateral load analysis and are intended to capture an appropriate sway deflection so P-Delta effects are not underestimated. RAM Concept is using a single floor analysis and, likely based on the governing load combination that I see, the gravity loading and not the lateral loads are the concern here. I would pause for just a moment before applying the 6.6.3.1.1(a) factors to the RAM Concept analysis. 

    I do not have enough details about your structure to given you a definite answer whether you should use 0.25 or not, which is why I pointed you initially to ACI 318-14 R6.3.1.1. Also, note that there are several different assumptions that might be appropriate. For example, for the previous mentioned lateral analysis, ACI 318-14 6.6.3.1.2 permits you to use I=0.5Ig for all members instead of the reduction factors in Table 6.6.3.1.1(a) that you cited.

    If you are uncertain what stiffness reduction to use, a good approach is to use lower bound and upper bound values in separate runs in an attempt to envelope the design. That approach should give you confidence that your design is safe.

    If you would like use to dig further or give better guidance on cracked section factors, please share your model so we have more information on the floor that you are designing.



    Answer Verified By: Andrew Yu 

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