The stepwise procedure for beam design is given here.
1. Beam sections should be designed for:
- Moment values at the column face (not the value at centre line as per analysis)
- Shear values at distance of 'd' from the column face. (not the value at centre line as per analysis)
- Moment redistribution is allowed for static loads only.
- For beams spanning between the columns about the weak axis, the moments at the end support shall be reduced more and distributed and the span moments shall be increased accordingly to account for the above reduction.
- Moment distribution shall be done in such a way that 15% of the support moments shall be added to the span moment without the support moments getting reduced.
- The section within the span shall be designed for the increased span moment which will account for the concentrated & isolated loading that may act within one span.
- Moment redistribution is not allowed if
- moment co-efficient taken from code table
- designed for earthquake forces and for lateral loads.
- At least 1/3 of the +ve moment reinforcement in SIMPLE SUPPORTS & ¼ the +ve moment reinforcement in CONTINUOUS MEMBERS shall extend along the same face of the member into the support, to a length equal to Ld/3. (Ld-development length)
- Use higher grade of concrete if most of the beams are doubly reinforced. Also when Mu/bd^2 goes above 6.0.
- Try to design a minimum width for beams so that the all beam reinforcement passes through the columns. This is for the reason that any reinforcement outside the column will be ineffective in resisting compression.
- Restrict the spacing of stirrups to 8″(200mm) or ¾ of effective depth whichever is less.(for static loads)
- Whenever possible try to use T-beam or L-beam concept so as to avoid compression reinforcement.
- Use a min. of 0.2% for compression reinforcement to aid in controlling the deflection, creep and other long term deflections.
- Bars of Secondary beam shall rest on the bars of the Primary beam if the beams are of the same depth. The kinking of bars shall be shown clearly on the drawing.
- Length of curtailment shall be checked with the required development length.
- Keep the higher diameter bars away from the N.A(i.e. layer nearest to the tension face) so that max. lever arm will be available.
- Hanger bars shall be provided on the main beam whenever heavy secondary beam rests on the main beam.(Try to avoid the hanger bar if secondary beam has less depth than the main beam, as there are enough cushions available).
- The detailing for the secondary beam shall be done so that it does not induce any TORSION on the main beam.
- For cantilever beams reinforcement at the support shall be given a little more and the development length shall be given 25% more.
- As a short cut, bending moment for a beam (partially continuous or fully continuous) can be assumed as wl^2/10 and the same reinforcement can be detailed at span and support. This thumb rule should not be applied for simply supported beams.
SPAN/OVERALL DEPTH RATIO
15 TO 18
18 TO 20
12 TO 15
20 TO 30