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D. IS456 Column Design Principles

The IS456 Column Design Brief is for prismatic, rectangular solid, or circular shaped members.  

As with Beam Design, section properties must be taken from the User databases.  Members may be built up from multiple elements, but only the member end moments are considered, so only single height columns are recommended.

Note: Refer to the Section Property Calculator manual for setting up standard sections and User databases.

Principles

Columns are designed for axial force and biaxial end moments, Reinforcement is provided by the program based on the forces generated in the Analysis mode and the code clauses outlined below.

If the member has a reinforcement cage already defined, then the program checks the reinforcement for compliance.  Otherwise the program will select a reinforcement cage from the bars and specifications of the Design Brief to minimize the area of main reinforcement bars.

Tip: Selecting a column member from the design group and selecting Edit > Copy, copies the reinforcement cage which can be pasted on columns in the Design Group of the same dimensions, which can then be checked with that same reinforcement.

All selected load combinations are used to calculate the required reinforcement.  The combination which produces the maximum required reinforcement area is called the Guiding Load Case, and the bar arrangement is chosen based on that combination.  Detailed printout of the design calculations is also for that combination.

The program conforms to the following requirements of section 39 of IS456: 2000 in the design module:

Clause Content

39.2

Minimum Eccentricity

39.3

Short Axially Loaded member in compression

39.5

Members Subjected to Combined Axial Load and Uniaxial Bending.

39.6

Members Subjected to Combined Axial Load and Biaxial Bending.

39.7

Slender Compression Member

Note: Compression members with helical reinforcement (CL 39.4) are not considered by the program.

The design for compression members is based on Cl. 25 of IS456:2000. In addition to the assumptions made for beam design, the following assumptions are made for compression design:

  1. The maximum compressive strain in concrete in compression is 0.002
  2. When there is no tension in the concrete section and the section is subject to bending and axial compression, the maximum compressive strain at the highly compressed extreme fiber is taken as 0.0035 – 0.75 x strain in the least compressed extreme fiber.

Refer to the design brief for an explanation of the input parameters.

All compression members are designed for a minimum eccentricity in accordance with Cl. 25.4 of IS456. Compression members are treated as either ‘short’ or ‘slender’ in accordance with Cl. 25.1.2 of IS456. A compression member will be considered as short when the slenderness ratios :

lex/D < 12 and ley/ b < 12

where
lex
=
the effective length with respect to the major axis
ley
=
the effective length with respect to the minor axis

If either of the slenderness ratios exceeds 12, the compression member is considered to be slender. The axial load capacity of a "short" compression member is calculated as per Cl. 39.3 of IS456. Compression members subject to combined axial load and bending must also satisfy the interaction equation as per Cl. 39.6 of IS456:2000. In the case of slender compression members, any additional moments are automatically calculated and used as per Cl. 39.7 of IS456. The correction factor ‘k’ used is also automatically calculated as per Cl. 39.7.1.1 of IS456.

The design procedure used involves an iterative process where the neutral axis position for the design section is evaluated for a given combination of axial force and/or bending moment. The bar arrangement is chosen based on the Guiding Load Case. The program systematically goes through all the bar sizes given in the brief to evaluate a symmetrical arrangement of bars on all four sides of the section. For each of the design sections thus considered, the program attempts to evaluate the neutral axis position for the given set of design forces. The program considers the neutral axis position from being at the extreme compressive fibers (edge of the section with the highest compressive force) up to a distance equal to four times the section depth. If the neutral axis depth is greater than 4 x D, the program classifies the chosen section as failed. Once the exact position on the neutral axis is determined, the program then evaluates the section moment capacities for the given axial load, about both axes. Subsequent interaction checks are also performed if necessary. Among all the design sections that are deemed satisfactory for the given set of forces, the program automatically selects the most efficient bar arrangement and reports the details. Upon determining a suitable main bar arrangement, the program then evaluates the required shear reinforcement for the section.