Revision 51 posted to RAM | STAAD Wiki by Shreyanka Bhattacharjee on 5/2/2022 5:04:00 AM
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| Applies To | | |
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| Product(s): | STAAD Advanced Concrete Design | |
| Version(s): | Connect Edition | |
| Environment: | | |
| Area: | Concrete Design | |
| Subarea: | Column & Shear Wall Design | |
| Original Author: | Bentley Technical Support Group | |
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[[Detailed explanation for Gravity column design in RCDC]]
[[Why only 50% of longitudinal reinforcement considered For Column Shear Calculation?]]
[[Why RCDC shows message of “Elevation of Column has been Omitted” while generating elevation of combined wall?]]
[[There is option under shear wall for detailing as Boundary Element or Equi Spaced rebar. I wish to have detailing as equi-spaced rebar, but it never gives detailing as equi spaced and redesign consume lot of time]]
[[Does RCDC calculate the effective length factor based on ACI 318M -14 Fig. R6.2.5 (b) for sway frames?]]
[[On what basis does RCDC perform the Joint shear check based on the Aspect ratio of vertical member as per Clause 7.1.2 IS13920 - 2016 & Amendment no.1 - 2017?]]
[[Can RCDC handle columns of shape L, T and other odd shapes?]]
[[When equal number of bars are provided in all columns from plinth to top floors, after design it’s showing lesser bars at the bottom and more at top]]
[[Some of the columns are missing at lower level even if they have exist in Staad model]]
[[Forces for column do not match with analysis]]
[[In edit link arrangement all the internal links can be removed. It doesn’t affect the design of links in column design. Please clarify]]
[[Does RCDC calculate the Effective length factor automatically or it is user defined? Is it same for all columns in model or different?]]
[[Spacing of confining reinforcement for column as per IS13920 2016]]
[[In RCDC, we can group columns and design them. But, when we go on creating elevation there is no way to create a single elevation for all the columns in a group. This feature is available only for detailed drawing. Can we incorporate the same for elevation]]
[[How can we change the Un-supported length in RCDC when column is tied in one direction and is free in another direction?]]
[[In Failure Diagnostic We Can Get Only Reason Of Failure But Not Get The Calculation. How We Know The Failure Calculation Of Column?]]
[[RCDC consider 0.4% reinforcement for column minimum reinforcement by default. Is there any clause in IS code to reduce column minimum reinforcement from 0.8% to 0.4%?]]
[[A parametric wall is having different thickness at different levels. RCDC is unable to show the correct thickness. What may be the cause?]]
[[In my model I have grouped the columns. Instead of column name I want to see group names. How to do that?]]
[[Does RCDC designs shear wall for out of plane moments in addition to in plane moments?]]
- 1. Can RCDC handle columns of shape L, T and other odd shapes?
- Yes. RCDC can design any shape of column like L, T, I, E, Capsule and polygons up to 12 edges. It also has an option to create user defined sections. For more information refer Topic “Irregular Shaped Column” for column from help Content.
- 2. When equal number of bars are provided in all columns from plinth to top floors. After design it’s showing lesser bars at the bottom and more at top.
- RCDC designs the column floor-wise. If percentage reinforcement required at a given level is more than the lower floor then it could be because of higher moment at that floor. Normally this happens at terrace floor where moments are more as compared to lower floors. This can be checked as per efficiency ratios in the design table.
- 3. Some of the columns are missing at lower level even if they have exist in Staad model.
- If the model has column members with no joints at in between levels, then these columns will appear ‘Missing’ in those levels. This can be checked by generating column elevation.
- 4. Forces for column do not match with analysis.
- RCDC reads the forces only for Primary load cases. These will match with the analysis forces. The forces for combinations are computed within RCDC as per load factors and other conditions like (LLR) Live load reduction. If the conditions of LLR are matched in analysis and RCDC, the design forces will match. For foundation design, RCDC uses member forces and not support reactions. In case of rotated columns or columns with offset, the support reactions do not match with member forces.
- 5. In edit link arrangement all the internal links can be removed. It doesn’t affect the design of links in column design. Please clarify.
- Option of Edit link arrangement is provided to change the link arrangement. It is provided only as detailing tool and not linked to design. User is expected to check the shear requirements before modifying it. Also it may be noted that, diameter and spacing of links is not recalculated after editing of links. For more information refer Topic “Edit Link Arrangement” for column from help Content.
- 6. Does RCDC calculate the Effective length factor automatically or it is user defined? Is it same for all columns in model or different?
- RCDC identifies the sway or Non-sway frames factor as per storey stiffness and storey index. It calculates relative stiffness of the beams and columns as per column orientation. It calculates the effective length factors as per available charts for Sway and Non-sway. Effective length factor would be different for all columns as per calculations. Option of overriding effective factor is available to user. User also can apply single factor to all columns on one click. For more information refer Topic “Effective Length Factor” for column from help Content.
- 7. Does RCDC designs shear wall for out of plane moments in addition to in plane moments?
- RCDC designs the sections as per the forces from analysis. In addition to the forces from analysis, RCDC calculates moments due to minimum eccentricity in both major and minor directions and performs section design. If applicable, slenderness moments are added in the final design moments. For cross-section design of walls same principles as column design are followed.
- 8. Can we design shear wall/wall with single layer (mesh) of reinforcement?
- No. Shear wall design with single mesh is not available as it is designed for the axial force and biaxial moments.
- 9. RCDC uses the formula of column for the “Minimum Eccentricity Calculation” (in IS code), though it is different for shear walls as per Clause: 32.2.2 of IS 456-2000.
- Clause 32.2 in IS 456-2000, is for Empirical Design Method of walls. Clause 32.3 is for design of walls subjected to horizontal and vertical loads and same has been followed in design of wall in RCDC.
- 10. Please clarify the braced and un-braced design conditions.
- These are design principles chosen by structural engineer for design of buildings. These are beyond purview of RCDC. RCDC captures this as information for further calculations.
As per clause 39.7.1 (Notes), IS code allows user to design column based on Braced and Un-braced conditions. The end moments are calculated based on end conditions given in this clause.
In Euro code, the braced and Unbraced option is available for calculation of effective length factor.
- 11. Lateral ties diameter, spacing & no. of columns main bars are correlated by formula given in IS: 13920 for rectangular and circular column. Can any single parameter be provided to calculate remaining two parameters in RCDC, once the initial design is complete?
- Yes. After initial design is completed, user can change the numbers and diameter of main reinforcement in redesign option. Diameter and spacing of links can be changed. For any parameter changed in redesign process, RCDC would design/ check the column with all relevant clauses. For more information refer “Edit Local Column Design” from help Content.
- 12. Pl refer to Annexure A off IS 13920 which states walls are to be designed for uniaxial bending. Columns are considered as biaxial. Hence, their design cannot be clubbed under one set.
- Annex A of IS 13920 is about the calculation of moment of resistance of the web portion of rectangular wall section. This is further used only in calculation of effective axial force in boundary element due to major axis moment. It may please also be noted in IS 456 which is the main code for design there is no separate procedure for design of walls. Clause no 39.6 of IS 456 refers to members subjected combined axial and biaxial bending which includes column and walls. Thus for biaxial design RCDC follows the basic principle of plotting P-M curve for design and checks for boundary element using annex-A provided in IS 13920. For more information refer Topic “Technical discussion” for column from help Content.
- 13. Can we have calculations for boundary zone length in wall design?
- Boundary elements are provided for ductile walls as per IS 13920. Governing Criteria to provide Boundary element is if stress is more than 0.2*fck and Boundary wall terminates (along the height of wall) if stress is less than 0.15fck. Zoning of reinforcement is done around the boundary element. The initial length of the boundary element is arrived at as per procedure discussed in help. For more information refer Topic “Technical discussion” for column from help Content.
- 14. RCDC follows ductile detailing as per 13920 for outer ring of boundary element but reduces link dia. & spacing for inner links.
- As per ductile detailing, for the calculation of confining links (outer links in BE – BE main) of boundary zone, formulation of Ash as per IS 13920, clause 7.4.8 is used. The internal links (BE others) are provided at same spacing with lowest possible diameter to maintain the “h” value in calculation of Ash. At middle zone, the links are provided as per IS 456. As a standard practice RCDC provided links to all longitudinal reinforcement.
- 15. What is the basis for only 20% of vertical reinforcement is considered for the calculation of Shear capacity (Tc)?
- Based on the IS 456, Only tension reinforcement is to be considered for shear capacity (Tc) calculation. We can assume at- least 30 to 40% reinforcement would be under tension in a given load combinations. Also, the effective depth as per code is suggested to be considered as 0.8 x the total depth for the shear calculation. Considering both the criteria, it is assumed that the only 20% of the main reinforcement of entire wall would be in tension. It is also very difficult to identify the % tension reinforcement in wall for combination which is critical in shear. This has been discussed with the professor on the code panel in technical presentation of IS 13920-2016 and suggested to use only 20%of the main reinforcement in the shear calculation.
- 16. How and when Modulus of rupture check performed in RCDC?
- Modulus of rupture check is performed as per clause 6.2.2 of IS 456-2002. The option of performing this check is available in RCDC as per user’s choice. This check is performed only for the tensile axial force in the column against the tensile capacity of the column. Effect of moment is not considered as this check is for tensile strength of concrete. If the axial tension is more than 0.7xsqrt(fck) then RCDC shows the column failure. It is just a check performed in the RCDC, there is no impact on the final design of the column.
- 17. In which cases we should not consider perform slenderness check - When to and when not to consider slenderness?
Slenderness check is an option given in RCDC. If the structure is analyzed with the Non-linear load cases (P-Delta) it is not recommended to consider this check. If the structure is analyzed with linear load cases, it is recommended to consider this check. If this check is selected, slenderness check will be performed, and additional slenderness moments will be calculated if column is slender.If this check is not selected, slenderness check will not be performed.
- 18. Provide detailed explanation for identification of section as a wall with respect to Depth and Width of member in RCDC
- 19. Provide detailed explanation for Gravity column design in RCDC.
Gravity columns is a choice that you as structural engineer have to make in your system. To reflect this correctly in analysis, ideally these columns should not participate in lateral load resistance in carrying shear and bending moments. These should be defined as ‘pinned’ ends in lateral load analysis. As per current limitations in E-tabs and STAAD, this is not easily manageable. Hence, we have allowed the users to select the required columns in RCDC and treat them as ‘Gravity’ columns. RAM software has the option to analyse the structure with combination of Lateral and Gravity columns.
As per code, the Gravity columns should be designed for the forces from analysis as well as effects of lateral displacement (known as ‘displacement compatibility’). This is done by considering moment due to Pu (Axial load from Gravity load combinations with DL +LL) acting at R*Delta distance away (Delta is displacement due to lateral loads). We would request you to go through Clause 11 of IS 13920 (2016) for more details on this.
In RCDC, we follow the above procedure in detail. Please check detailed design calculation report for Gravity columns for more information.
Please note that, in RCDC the member can be defined as ‘Gravity column’ only if that qualifies as column based on D/B ratio. Walls can’t be design as Gravity members. It can be either Non-ductile or Ductile. Further, the Gravity columns are designed for Vertical gravity loads with effect of later displacement due to lateral loads as explained earlier. Click on below link to understand the implementation of Gravity column in RCDC.
https://communities.bentley.com/products/ram-staad/m/structural_analysis_and_design_gallery/274370
- 20.Why only 50% of longitudinal reinforcement considered For Column Shear Calculation?
- for column shear calculation, Tc shall be calculated for tension reinforcement. As column is mainly axial force carrying member, all the rebar would not be in tension. Also, when column is designed, rebars below the neutral axis are in tension. For shear check, identifying rebars those are in tension for combination which is critical in shear would be critical. Thus, RCDC assumes the 50% of the tension reinforcement for shear calculation
- 21. Why RCDC shows message of “Elevation of Column has been Omitted” while generating elevation of combined wall?
Combined walls are consisting of more than one walls. If the wall shape and size is same at all floors, RCDC generally generates the elevation of combined wall showing one face only.
If the wall shape and size is not same throughout the height of all floors, it is difficult to generate elevation of these walls. The combined junctions of walls are detailed separately to satisfy the percentage reinforcement in both the walls. Also showing elevation of each face of combined wall would be difficult in case of thickness changes, thus RCDC generally ignore or omit the elevation of combined walls for elevation. Cases like major variation of reinforcement along height of wall and if combined wall consist of column are omitted.
Generally, elevations are generated to show typical reinforcement detailing along the height of column/wall. User can still generate elevation of column and single walls for typical reinforcement details.
- 22.There is option under shear wall for detailing as Boundary Element or Equi Spaced rebar. I wish to have detailing as equi-spaced rebar, but it never gives detailing as equi spaced and redesign consume lot of time
When you select the option of “Detail with Equi-spaced rebar arrangement" RCDC design and detail the wall with Zones, however rebar spacing in all zones are maintain same. refer below snap showing rebar arrangement with same setting,
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if you unselect "Detail wall with Boundary element" it will design as a ductile wall without Boundary element but it will try to provide zones to optimized the reinforcement.
when you select both the options i.e. Boundary element and Equi-spaced, it will design as Boundary wall with equi-spaced rebar arrangement. This setting is applicable for Non-ductile wall if you want t provide equal spacing.
if you want to design wall as column and with same rebar having equal spacing, then you have to perform the redesign step, however in redesign column tool allows user to change the type from "Zonal" to "Equi" in one step for wall along height. refer below snap,
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for wall, RCDC tries to optimize the reinforcement by providing higher diameter at edges.
23.Does RCDC calculate the effective length factor based on ACI 318M -14 Fig. R6.2.5 (b) for sway frames?
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Reply:
Reply: RCDC calculates the column effective length factors as per above charts based on type of frame.
Type of frame is identified as per story height, axial load, relative displacement and story shear. Refer below snap,
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Based on the column at top and bottom along with beam stiffness of story considered, value of Ѱ is calculated. Refer below snap,
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As per Ѱ, effective length factor (k) is calculated based on fig. 6.2.5
24.On what basis does RCDC perform the Joint shear check based on the Aspect ratio of vertical member as per Clause 7.1.2 IS:13920 - 2016 & Amendment no.1 - 2017 ?
As per clause 7.1.2 IS;13920 - 2016, the vertical members of structure whose B/D >= 0.4 (or D/B <= 2.5), shall be designed as per requirements of Clause 9.
The first print of IS;13920 - 2016, 'Clause 8' talks about 'special confining reinforcement'. Later in Amendment no.1 - 2017 to IS;13920 - 2016 , this clause was renamed as 7.6. Hence, the old clause 9 now automatically becomes 8 and clause 10 (which is for shear walls) becomes 9.
Further, in the same amendment, they have mentioned that 0.45 in 7.1.2 should be replaced by 0.4. With this background, one can interpret 7.1.2 as below - It is preferred to provide D/B as 2.5 or less (B/D 0.4 or more) for columns. In clause for walls it is very clearly mentioned that D/B should be more than 4. Also,
So, for D/B between 2.5 and 4, the behavior is in between and should be avoided. However, if one has to use these ratios then those members can be designed as walls as per clause 9, and can be done in RCDC by changing the D/B ratio in Design Settings form.
25.In RCDC, we can group columns and design them. But, when we go on creating elevation there is no way to create a single elevation for all the columns in a group. This feature is available only for detailed drawing. Can we incorporate the same for elevation drawings also? If not, can you please let me know the reason?
RCDC groups the column based on the Design of column and not the Geometry arrangement of column.
So different columns can have different beams resting on column which may have different dimensions all in.
So, while generating the column elevation, RCDC generates elevation of column separately so that there is no mis-match in the beam data that is resting on the beam.
Further, there are 2 ways of grouping possible in RCDC which is considering all the design aspects of the column and not the geometrical arrangement in the Layout:
- Auto Grouping - This feature of RCDC auto identified similar column and groups them. Following points are considered while checking the similarity:
- Column Size
- Material used for design at all levels.
- Pt provided
- Start and end level of each column that is qualified to be grouped by auto design process.
- Manual Grouping performed by user -- This feature allows user to group any column by his judgement and requirement. During this process, RCDC doesn't check any parameter and rather the grouping decision is solely dependent on user.
26.How can we change the Un-supported length in RCDC when column is tied in one direction and is free in another direction?
Reply:
Considering below example when there is a case where a column is not tied along one direction and is tied along another direction, the unsupported length of the column along the direction where it is not tied is to be manually entered in RCDC using the 'Redesign Section' tool available in RCDC.
With this option, the actual unsupported length = (Floor height - the Beam depth) needs to be manually entered in RCDC.
Once this height is entered; the column section is redesigned and the redesigned data is accepted, the revised height of the column at selected floor will be displayed in the design calculation report as well.
Below are the snips for reference;
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27. IN FAILURE DIAGNOSTIC WE CAN GET ONLY REASON OF FAILURE BUT NOT GET THE CALCULATION. HOW WE KNOW THE FAILURE CALCULATION OF COLUMN?
Reply:
RCDC provides the reason for failure in the Failure Diagnostics report. There are multiple type of Failure suggested by RCDC as follows:
- Axial Failure - When the Average Stress in Column (Pu / Ag) for Pumax from seismic Load Combination exceeds 0.4 fck, then there is Axial Failure.
- This can be tackled by either Increasing the Column Size or by Increasing the Grade of Concrete. As both these parameters are set by user, RCDC cannot modify these parameters and shows as a Failure condition.
- Joint Flexure Failure - As per Clause 7.2 (IS:13920-2016), for a considered Beam Column Junction, when the Column Capacity is found to be Less the 1.4 Beam Capacity (considering all the beams at the joint), then there is Joint Flexure Failure.
- In this case, RCDC tries to increase the Ast provided for Column up to Maximum Pt considering the Max Diameter - Min spacing parameters that are set in RCDC. If still there is failure after increasing the Pt, then RCDC shows it as Failure only.
- You can handle this by either increasing the Column sizing or by controlling the moments in beam by providing some partial releases in beam or by decreasing the beam sizing to control the stiffness.
- Joint Shear Failure - As per Clause 9.1 (IS 13920-2016), the Nominal shear strength of concrete in the Beam - Column Joint is checked as mentioned in the code. Below image for reference:
- This check depends on the Beam-Column Joint area and the Concrete Grade. So, this can be tackled by either Increasing the Column Size or by Increasing the Grade of Concrete. As, RCDC cannot modify both these parameters it is shown as a Failure condition.
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- Detailing Failure - This type of Failure occurs in RCDC when the 'Maximum Diameter' and 'Maximum allowable bars' for a column section is provided and still with this configuration of Pt provided is less than the Max Pt allowed.
- In this case RCDC has already tried to reach the maximum possible diameter and number of bars. So, this can be further handled from user's end by allowing the Maximum diameter available in the list and / or increasing the Number of Bars for the column section. The number of bars for column section can be increased from the Zone & Rebar settings in RCDC.
- Rupture Failure - Modulus of rupture check is performed as per clause 6.2.2 of IS 456-2002. The option of performing this check is available in RCDC as per user’s choice. This check is performed only for the tensile axial force in the column against the tensile capacity of the column. Effect of moment is not considered as this check is for tensile strength of concrete. If the axial tension is more than 0.7xsqrt(fck) then RCDC shows the column failure. It is just a check performed in the RCDC, there is no impact on the final design of the column.
- In this case, the check is performed only when user selects to perform this check and there is tension in the column.
28. RCDC consider 0.4% reinforcement for column minimum reinforcement by default. Is there any clause in IS code to reduce column minimum reinforcement from 0.8% to 0.4%?
As per code, Min As = 0.8% of the required C/s area of the column. RCDC follows the same.
RCDC internally calculates the c/s area required for the Maximum Pu and then takes 0.8% of the Required c/s area.
The 0.4% mentioned on the reinforcement setting form is used to calculate the Asmin with the Provided c/s area of the column.
Finally the Asmin is considered as Max (0.8*Required c/s area of column, 0.4*Provided c/s area of column) where the required c/s area of column is calculated in RCDC internally with Max Pu.
29. A parametric wall is having different thickness at different levels. RCDC is unable to show the correct thickness. What may be the cause?
Note that whenever a single parametric surface is modeled and 2 different plate thicknesses are found to be assigned to a single surface, RCDC considers the minimum of the thicknesses obtained. If it is desired to have different thicknesses at different levels, then you should model separate parametric surfaces and then assign the desired thickness to different surfaces. Then only RCDC will read and show the correct thickness at different levels.
30. In my model I have grouped the columns. Instead of column name I want to see group names. How to do that?
This can be achieved by changing the Drawing Style. To do that go to Modify > Drawing Style > Modify Drawing Style as shown below.
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Now a new window will open. There you need to check the box against "Use Group Names in Output" under Display Style. This will show the group names instead of column names.
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Tags: analysis, Column, IS 13920-2016, RCDC, FAQ's, Column Design, STAAD, IS 456, Shear Wall
