D8.F.3 Column Design

Columns are designed for axial forces and biaxial moments per IS 456:2000. Columns are also designed for shear forces as per Clause 7.3.4. All major criteria for selecting longitudinal and transverse reinforcement as stipulated by IS:456 are performed by the column design of STAAD.Pro. However following clauses have been satisfied to incorporate provisions of IS 13920 2016:

The minimum grade of concrete shall preferably be M20. (Clause 5.2)
Steel reinforcements of grade Fe415 or less only shall be used. (Clause 5.3.1)
The minimum dimension of column member shall not be less than:
1. 20 times the diameter of the largest longitudinal reinforcement in the beam passing through or anchoring into the column at the joint, or
2. 300 mm (Clause 7.1.1)
The ratio of the shortest cross-sectional dimension to the perpendicular dimension shall preferably be not less than 0.45. (Clause 7.1.2)
The spacing of hoops shall not exceed half the least lateral dimension of the column, except where special confining reinforcement is provided. (Clause 7.4.2d)
The minimum diameter of the rectangular hoop is 8 mm when diameter of the longitudinal bar is less than or equal to 32 mm, and 10 mm when diameter of longitudinal bar is more than 32 mm. (Clause 7.4.2a)
Special confining reinforcement shall be provided over a length l_o from each joint face, towards mid span, and on either side of any section, where flexural yielding may occur. The length l_o shall not be less than:
1. larger lateral dimension of the member at the section where yielding occurs,
2. 1/6 of clear span of the member,
3. 450 mm (Clause 8.1a)
The spacing of hoops used as special confining reinforcement shall not exceed:
1. ¼ of minimum member dimension,
2. 6 times diameter of the smallest longitudinal bars, or
3. 100 mm (Clause 8.1b)
The area of cross-section of hoops provided are checked against the provisions for minimum area of cross-section of the bar forming rectangular, circular, or spiral hoops, to be used as special confining reinforcement. (Clause 8.1c)

Cl 7.1 of IS:13920-2016 is applicable for columns resisting earthquake induced effects. The program will check whether the design load is gravity load case (as defined by the GLD parameter) or earthquake load. If yes, the program will perform this code check. Otherwise, the program will ignore the design load case for check against this code provision.

Note: In previous releases, the design performed checks against this clause for all types of design load cases, irrespective of whether they contain earthquake load or not. This logic is now automated to include only earthquake or gravity load case.

Relative Strengths of Beams and Columns at a Joint

Clause 7.2 of IS:13920-2016 checks the relative strength of beams and columns at a joint to evaluate if any column at a joint is acting as a gravity column or is a part of lateral load resisting system. After the concrete design is completed per IS 13920-2016, then this relative strength check is automatically performed at joints in which all connecting beams and columns were designed. Thus, no new commands are required for this check to be performed so long as all concrete members at a joint are designed. Per Cl 7.2, the sum of all column moment resistances at a joint shall be greater that 1.4× the sum of all beam moment resistances at the same point.

Note: STAAD.Pro does not have designate columns as gravity load structural elements. Rather, the program considers all columns as lateral load resisting. If this check fails at any joint, the column dimensions, beam dimension, or both should be changed to make the column suitable for the lateral load resisting system.

It is recommend that the design of all concrete beams and columns be performed in one concrete block (a "concrete block" is the information contained in the START CONCRETE DESIGN … END CONCRETE DESIGN input section) so this check can be performed. If multiple concrete blocks are defined, all previous relative strength check result will be deleted since multiple design of beams and column designs may change the relative strength of beams and columns at a joint. The strength check performed after the last concrete block will be considered valid and is available as result.

The output file will issue a note regarding the results of this check. A detailed output of the checks can be included in the output using the TRACK 4 parameter in the concrete design. The detailed output of the joint relative strength checks are also included in the filename_13920.txt file (which is generated regardless of the TRACK parameter used or check status).

Note: The program assumes that all beams connected to a column are horiztonal and parallel to global X and Z axes. If beams do not meet this requirement a warning message is issued in the analysis output.