RCDC-Column & Shear Wall FAQ's

RCDC-Column & Shear Wall

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.