How to design column in RCDC so that lower level columns have more reinforcement than upper level columns?

Question

When we design columns in RCDC some times upper level columns have more reinforcement perhaps due to higher moments in upper storey . How to design columns so that lower storeys have equal or more reinforcement than upper level columns?

Abhijeet Gundaye · Accepted Answer

Hi RCDC is a design software. it gets the forces form analysis software. So, the design is based on the forces (Axial , Moment and shear forces) available from analysis software. Column design is based on the axial forces and bending moment in the column. RCDC design the column for these axial force and moments. As explained in the earlier thread, reinforcement in upper level columns is might be due to lesser axial force and high moments in the column. We would suggest following option to get lesser reinforcement at upper columns, 
 Option-1 
 As column design is based on forces available from analysis, we would suggest you to model the structure in such a way that it will give lesser reinforcement at upper floor 
 Option -2 
 in RCDC even you get the higher reinforcement at upper floor, you can still copy and paste the higher reinforcement to columns having lesser reinforcement.

Royal Civil Design Center · Answer

Royal Civil Design Center · Answer

The average stress in column Pu/Ag for Pu-max from seismic load combinations, shall not exceed 0.4 x Fck. In this case RCDC will report this as a &lsquo;Failure&rsquo; condition.

Royal Civil Design Center · Answer

Principle used in RCDC for column design is to check the capacity of a section of given size with given reinforcement arrangement against the design-demand. Thus, for a given size of column with specific arrangement of reinforcement (in terms of location and diameter), for given grades of concrete and reinforcement, the P-M curves are generated as 'section capacity' curves. Then for all design load combinations, values of Pu, Mux and Muy (from analysis) are used and after due process for checking of minimum eccentricity, slenderness and other relevant checks, final design values are worked out as Pu-d, Mux-d and Muy-d. Now each value of this Pu-d, Mux-d and Muy-d is checked against capacity available as per the P-M curve envelope.

Royal Civil Design Center · Answer

Royal Civil Design Center · Answer

Royal Civil Design Center · Answer

FAILURE DIAGNOSTICS

Project Name 
 : 
 Royal Civil Design Center

Client Name 
 : 
 Royal Civil Design Center

Engineer Name 
 : 
 Royal Civil Design Center

Analysis File 
 : 
 D:\P4_A_FOR COL.C40_JS CHECK UPDATE.STD

Analysis Last Modified 
 : 
 31-07-2021 17:38:15

Column Failure Report:

Column No. 
 Analysis Ref. 
 Level 
 LC 
 Loc 
 Design Forces 
 Pt 
 Int/Tensile ratio 
 Failure Type

Pu 
 Mx 
 My 
 Vx 
 Vy

(kN) 
 (kNm) 
 (kNm) 
 (kN) 
 (kN) 
 %

C50 
 188 
 3.749m 
 2 
 - 
 1077.87 
 249.55 
 -460.05 
 - 
 - 
 5.36 
 0.99 
 Detailing

Royal Civil Design Center · Answer

Royal Civil Design Center · Answer

Nidhi Valia · Answer

Hi, 
 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.

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&rsquo;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.

If you have any file specific concern, you can share your files here. To understand the procedure of uploading the files, refer thi link - 
 communities.bentley.com/.../communities-secure-file-upload

Nidhi Valia · Answer

Dear Mr. Astik, 
 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.

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&rsquo;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.

Thanks

Nidhi Valia · Answer

Hi, 
 To understand the procedure of uploading file, you can refer to this link -- communities.bentley.com/.../communities-secure-file-upload

Royal Civil Design Center · Answer

IS 13920 - 2016 1 Spacing of special confining reinforcement : 10.4 and 7.6 2 C/s area of special confining reinforcement : 7.6.1 3 Applicability of boundary element : 10.4.1 4 Muv-Moment Capacity of Web : Annex 5 Additional Compressive Force in BE : 10.4.2 6 Check for BE in tension and compression : 10.4.2.1 and 10.4.3 7 Shear include due to Beam : 7.5 8 Minimum Flexural Strength of Column : 7.2 9 Shear Check at Column Joint : 9.1 10 Length of wall to thickness ratio : 10.1.3 11 Type of wall & Minimum reinforcement : 10.1.4 12 Largest diameter of longitudinal steel bar : 10.1.8 13 Shear reinforcement in RC wall : 10.2.3 14 Special confinement reinforcement in Boundary Element : 10.4.4 15 Minimum vertical reinforcement across horizontal construction joint : 10.7

Nidhi Valia · Answer

Hi, 
 Please find below our pointwise reply to your queries;

(1) I would like to understand that how RCDC extract forces from STAAD model w.r.t member length? 
 RCDC reads the Beam end forces from the STAAD file for each of the member and considers the force at the Upper and Lower level. Refer below snip for clarification.

(2) Do we have to model only in physical modeler to use RCDC? 
 No, a regular Analytical model is read in RCDC and all the elements can be designed in RCDC. 
 (3) Does crack width check consider diagonal Acr for both between bar & at corner location? 
 Yes, it does. Refer below explanation on how RCDC on how RCDC computes the equilibrium location of Neutral Axis: 
 
 Please refer to adjacent figure, column is of size (B x D), with vertical reinforcement. 
 The column is subjected to following forces &ndash; 
 P = Axial Force 
 M-major = Moment about major axis = M @ LX 
 M-minor = Moment about minor axis = M @ LY 
 
 Eccentricity of force along both the axis can be calculated as: 
 Ex = M-minor/ P 
 Ey = M-major/ P 
 
 For equilibrium of forces acting on the cross-section,it is essential to understand the location of Neutral-axis. From vector forces, it can be easilyfound that effective moment acts at an angle &lsquo;Alpha&rsquo; with LX axis. 
 
 Tan (Alpha) = M-major/ M-minor 
 
 Hence, NA (neutral axis) is perpendicular to this angle. 
 
 Neutral axis location is calculated iteratively, based on the principles of equilibrium as below &ndash; 
 
 For final position of neutral axis, sum of axial forces of the section is equal to applied axial force, 
 
 Fc (concrete) + Fc (Steel) + Ft (Steel) = P 
 
 For final position of neutral axis, sum of moments about neutral axis is equal to moment of applied force about neutral axis, 
 
 Mc (concrete) + Mc (Steel) + Mt (Steel) = P * N 
 
 Computing Procedure: 
 
 The software works on an iterative algorithm, which calculates equilibrium position of neutral axis and establishes the force equilibrium. While performing calculations, stress is assumed to vary linearly from C1 to C2. 
 
 For NA-1 condition, stress at C1 is tensile and minimum. For NA-2 condition, stress at C1 is compressive and maximum. 
 
 Following are the principles for calculation of section forces &ndash;

M = Modular Ratio = 280 / (3 * Max. permissible stress in concrete) 
 Ec = Elastic modulus of concrete = 5000 * Square roof of Fck 
 The concrete in tension is neglected. 
 The stress in bars in compression = 1.5 * M * Ec * Equivalent stress in concrete at that location 
 The stress in bars in tension = M * Ec * Equivalent stress in concrete at that location 
 The stresses in individual bars of reinforcement are calculated as per their actual location and their perpendicular distance from neutral axis.

There are three possible locations of NA - 
 
 NA-1 = Neutral axis is beyond the section - Column is in tension 
 NA-2 = Neutral axis is within the section - Column is in partial tension 
 NA-3 = Neutral axis is below the section - Column is in compression

From above conditions, it is clear that for NA-3, section is un-cracked. For NA-1 and NA-2, following checks are performed before calculating crack-width &ndash; 
 
 Stress at corner C1 = Maximum compressive stress = This should be less than permissible 
 Stress in corner bar near C2 = Maximum tensile stress in reinforcement = This should be less than permissible 
 
 After finding the NA position, and confirming that stresses at C1 and for bar near C2 are within permissible limit, following calculations are performed for calculating crack-width and defining the adequacy &ndash; 
 
 Epsilon (strain) at C2 is calculated from NA position and stress at C2 
 Crack-width is calculated as = Wcr = 2.3 * Acr * Epsilon at C2 
 Crack-width check at C2 = This should be less than permissible value of crack-width.

(4) Is Acr calculated from longitudinal rebar face? and for circular column does it consider radial clear spacing for Acr derivation? 
 Yes. For circular column, Acr is calculated from the longitudinal rebar face. So acr = cover that is mentioned for the column design as cover in RCDC is the cover to Main reinforcement.

How to design column in RCDC so that lower level columns have more reinforcement than upper level columns?

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