D8.F.2 Beam Design
Beams are designed for flexure, shear, and torsion. If required the effect of the axial force may be taken into consideration. For all these forces, all active beam loadings are prescanned to identify the critical load cases at different sections of the beams. The total number of sections considered is 13. All of these sections are scanned to determine the design force envelopes.
For design to be performed as per IS:13920 2016 the width of the member shall not be less than 200 mm (Clause 6.1.2). Also the member shall preferably have a width-to depth ratio of more than 0.3 (Clause 6.1.1).
The factored axial stress on the member should not exceed 0.08fck (Clause 6.1) for all active load cases. If it exceeds allowable axial stress no design will be performed.
Cl 6.1 of IS:13920-2016 is applicable for beams resisting earthquake induced effects. The program performs a load type check where it determines if the design load contains earthquake or dynamic load or not. If yes, it will perform check against Cl 6.1. Otherwise, the program will ignore the design load for check against this code provision.
D8.F.2.1 Design for Flexure
Design procedure is same as that for IS 456. However while designing following criteria are satisfied as per IS13920 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 tension steel ratio on any face, at any section, is given by (Clause 6.2.1b)
ρmin = 0.24fck/fy
The maximum steel ratio on any face, at any section, is given by (Clause 6.2.2)
ρmax = 0.025
- The longitudinal steel on bottom at a joint face must be at least equal to half the steel at its top at the same section. (Clause 6.2.3)
- The steel provided at each of the top and bottom face, at any section, shall at least be equal to one-fourth of the maximum negative moment steel provided at the face of either joint. (Clause 6.2.4)
- Beams shall have at least two 12 mm diameter bars each at the top and bottom faces.
D8.F.2.2 Design for Shear
The shear force to be resisted by vertical hoops is guided by the Clause 6.3.3 of IS 13920:2016 revision. Elastic sagging and hogging moments of resistance of the beam section at ends are considered while calculating shear force. Plastic sagging and hogging moments of resistance can also be considered for shear design if the PLASTIC parameter is used (refer to D8.F.4 Design Parameters).
Shear reinforcement is calculated to resist both shear forces and torsional moments. The procedure is the same as that used for IS 456.
- The spacing of vertical hoops over a length of 2d at either end of the beam shall not exceed
- d/4
- 6 times the diameter of the smallest longitudinal bars
- 100 mm
The spacing calculated from above, if less than that calculated from IS 456 considerations, is provided.
- While calculating shear force reinforcement the provision of Clause 6.3.4c (i.e. neglecting design shear capacity of concrete of RC section) is taken into consideration. The spacing of links is provided in accordance with clause 6.3.5.2 of IS-13920 2016.