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D. IS456 General Considerations

This section describes some of the material properties, section properties, and other assumptions made for the design of concrete beams and columns per IS456:2000 in RC Designer.

Materials

The following clauses from section 2 of IS 456:2000 are considered by the program:

  • Cl 5.3.3 – The size of aggregate is considered as 20 mm.
  • Cl. 5.6 – This implementation assumes that the reinforcement specified conforms to Cl. 5.6 of the code. The modulus of elasticity of the reinforcement is taken as 200,000 N/mm2 as per clause 5.6.3 of the code.
  • Cl. 6.2 – The program will only consider the design of elements that use ordinary concrete or Standard concrete.

    If you specify a high strength concrete grade (i.e., compressive strength greater than 55 N/mm2), the program will issue a warning alert you that there might be additional considerations that need to be taken into account in the design. The design strengths of concrete shall be as given in Table 2 of IS 456:2000.

  • Cl 6.2.2 – The tensile and flexural strength of concrete shall be taken as:

    fcr = 0.7⋅√fck

    where:

    • fck = the characteristic compressive strength of concrete.
  • Cl. 6.2.3.1 – The modulus of elasticity of concrete is taken:

    Ec = 5000*√ fck

Note: The effects of creep, shrinkage and thermal expansion are ignored.

General

This design is based on Limit state method. The Working Stress method in Annex B of IS 456:2000 is not considered for this implementation. The loads and forces considered are as per Clause 19 and are defined in STAAD.Pro. The density of concrete is taken as 24 KN/m3. The stability of the structure as a whole (i.e., resistance to overturning, sliding, or lateral sway) are not considered by the program for the design of these elements. This implementation assumes that the section dimensions comply with fire resistance according to Clause 21 of the code.

The following clauses from section 3 of IS 456:2000 are considered by the program:

  • Cl. 22.2 – The effective span for beams used is as defined in Cl. 22.2 (a) – (d) of the code. The clear span is taken as the node-to-node distance of a member (end nodes or supported nodes) in the STAAD.Pro model.

    Note: You will have the option to edit the effective length in the column design brief.
  • Cl. 22.3 – The relative stiffness for the members is based on the gross cross section where necessary. The design of the member is based on its gross cross section.
  • Cl. 22.6.2 – The critical section for shear is considered to be at the face of the support. Enhanced shear at sections up to a distance equal to the effective depth of the beam are also considered.
  • Cl. 23 – Refer to D. IS456 Beam Design Principles for details of beam design.
  • Cl. 25 – Refer to D. IS456 Column Design Principles for details on column design
  • Cl. 26 – Requirements for reinforcement and detailing:

    • Cl. 26.1.1 – The reinforcement provided in the design of beams is limited to four layers (i.e., two top & two bottom) for this implementation. Bundling of reinforcement will not be considered in this implementation.
    • 26.1.2 – Consideration will also be given to the detailing rules as per IS 13920.
    • 26.2.1 – The development length of bars is taken as:

      Ld = φ⋅σs / (4⋅τbd)

      where:

      • φ = nominal diameter of the bar
      • σs = stress in bar at the section considered at design load
      • τbd = design bond stress given in 26.2.1.1
      Note: Clause 26.2.1.1 gives the bond stress for bars in tension in the table. For bars in compression these values need to be increased by 25%. The anchorage length of straight bars in compression shall be in accordance with 26.2.1.
    • 26.2.2.5 – The bearing stress at bends is considered and is evaluated as:

      Bearing Stress = Fbt/ (r⋅φ)

      where:

      • Fbt = tensile force due to design loads in a bar or group of bars
      • r = internal radius of the bend
      • φ = size of bar or, in a bundle, the size of bar of equivalent area

      The bearing stress within a bend or a hook should not exceed 1.5 Fck/(1+2φ /a), where a is the center to center distance between bars.

    • Cl.26.2.3 - The curtailment of reinforcement shall be done as per the conditions given in cl. 26.2.3.1 to 26.2.34 of the code.
    • Cl.26.3 – The spacing of reinforcement shall be done as given in Cl.26.3.1 – 26.3.3 of the code.
    • Cl. 26.4 – The cover to reinforcement is specified by the user. However checks is done to make sure that the cover requirements as per Cl. 26.4.1 – 26.4.3 are met.
    • Cl.26.5 – The minimum reinforcement requirements as given in Cl. 26.5.1.1 is checked for beams and columns. Separate checks is done for tension, compression and side face reinforcements. Transverse reinforcements for beams will also be checked as per Cl. 26.5.1.4 - 26.5.1.8 of the code. The reinforcement requirements for columns is checked as per Cl. 26.5.3 of the code.