Merging level in staad advanced concrete design (RCDC) cause reduction in reinforcements.

I have been trying to design columns in staad RCDC, My building model have some columns starting at level 0.0 and some other columns starting at level 3.0. Now when I choose to consider this level 3m, columns those who starting at 0m gives much higher steel then staad pro output. But when I merge that level 3m with the above floor level it gives steel near to what staad pro output gives. PS::0m and 3m are the levels at which footings of those respective columns started/lies.

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  • Hello,

    We would request you to mail us the STAAD RCDX file for our further checking.

    And also let us know the current STAAD RCDC version you using.

  • I'm unable to insert that file here, is there any other way to send this file/may be email id;

  • Hello,

    Thank you for the file,we will revert on your query as soon as possible.

  • Dear Sir

     

    We have gone through your shared files.

    RCDC identify the levels based on the Y co-ordinates available in STAAD file. So, 3.7m level is identified in RCDC. In column design, the effective length factor calculation is performed as per IS 456 for available levels. The floor to floor height at 6m level column is considered as 2.3m instead of 4.6m. refer attached image 1

    As story height plays an important role in calculating effective length factor, the values comes out more than 1.5 which is not the actual case. Refer attached image 2

    More the effective length factor, column are identified as slender column. thus column might show higher reinforcement due to additional of slenderness moment (major axis and minor axis). Thus it is always recommend to merge the levels which suits the analysis and design requirements to get the correct results.

     

    Please find the attached image 3 for,if levels are merged,

     RCDC does the calculation of effective length factor in detail. All the values used for the calculations are available in report. If you have any queries related to this please let us know. Also, we would request you to check the effective length factor used in STAAD and RCDC for result comparison.

  • Dear sir/madam,

    Thank you for your time to address my query. Actually that level 3.7 is the level at which some of the columns have their footings rested with 2.3m depth of footing. so basically they start from that 3.7 level, other columns have their footing rested at level -0.75m. in hilly terrain we usually have such cases of stepped buildings with columns having level differences. I have few more doubts/questions related to this:

    1. columns E1, for example have length 4.45 above level 1.55 with no node lying on it(also no other beam/member connecting) up level 6m, even then RCDC considered it to be broken there when levels are not merged, it should have automatically identify that this is a single member. merging the levels solved this issue but for other columns like E6,E7 etc starting level become 1.55 - 6 instead of 3.7-6. also I checked that there is considerable difference in effective length factors of these columns(E6,E7,F6,F7,G6,G7) before and after merging levels, Although their length is 2.3 in both cases. do we need to design columns by merging/demerging for different levelled columns(if yes, then it will be cumbersome in a case where there many steps in bulding(columns at different levels) in verticle profile with different depths of footings)? 
    2. In the image 1 that you sent, why the level 1.55-3.7 is skipped ( case when the levels are not merged).
    3. IN STAAD pro, it takes default value of 1 for effective length factor for all columns that's why there is difference I guess.
    4. Can we edit the combination of bars provided after design of columns, according to our needs? just like in the case of beams according to required reinforcement we can change the bar combination?
    5. In design output of RCDC it only shows the pt provided not the pt required, how can we check the pt required in RCDC?       
  • Hello,

    Please find our reply to your below queries.

    1)columns E1, for example have length 4.45 above level 1.55 with no node lying on it(also no other beam/member connecting) up level 6m, even then RCDC considered it to be broken there when levels are not merged, it should have automatically identify that this is a single member. merging the levels solved this issue but for other columns like E6,E7 etc starting level become 1.55 - 6 instead of 3.7-6. also I checked that there is considerable difference in effective length factors of these columns(E6,E7,F6,F7,G6,G7) before and after merging levels, Although their length is 2.3 in both cases. do we need to design columns by merging/demerging for different levelled columns(if yes, then it will be cumbersome in a case where there many steps in bulding(columns at different levels) in verticle profile with different depths of footings)? 

    Reply: The columns those are from 1.55 m to 6m level and doesn’t have any nodes in-between, RCDC is reading those members with length as 6m only as per STAAD data. RCDC doesn’t modify any geometry and result data provided by STAAD. Only the height considered for the calculation of effective length factor is as per Y co-ordinates available from STAAD file. Refer attached Image 1 from RCDC where levels are not merged, PDF

    Please note that, RCDC doesn’t modify the column geometry available from STAAD. It’s a user choice to merge the levels or not as per design requirements.

    2)In the image 1 that you sent, why the level 1.55-3.7 is skipped (case when the levels are not merged).

     Reply: For some columns, the level 3.7 doesn’t exists as explained in above points. Thus, columns are designed from 1.55 to 6m level.

    3)IN STAAD pro, it takes default value of 1 for effective length factor for all columns that's why there is difference, I guess.

    Reply: Might be one of reason.

    4)Can we edit the combination of bars provided after design of columns, according to our needs? just like in the case of beams according to required reinforcement we can change the bar combination?

    Reply: Yes, we would request you to try re-design option available. You can add or delete the rebars also. In Re-design, you can try any rebar arrangement with different diameters.

    5)In design output of RCDC it only shows the pt provided not the pt required, how can we check the pt required in RCDC? 

    Reply: RCDC performs the column design based on actual rebar arrangement and capacity is checked against required forces. Refer attached help content extracted from RCDC.

  • Thanks a lot for your efforts to clarify the doubts..Innocent

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