How To Design RCC Building Manually
Let’s design for a typical plan of a 2BHK. And let’s only take dead and live loads. Add wind loads and seismic loads according to zone after practicing with dead and live load.
The idea is simple – Slab rests on beams. Beams rest on columns and Columns rest on footings.
So design slab first, then use the load from load to design beams. Beams will put load on columns. And columns on footings.
IS codes – 456, 875 (I and II)
MS Excel – for faster calculations. Many design offices use Excel template sheets. Don’t see them as designing software. They are just for faster calculations and minimize errors.
1. Mark columns.
2. Join columns to mark primary beams. There will be tertiary beams resting on primary beams.
3. Mark all beams. Number them.
4. Now mark slabs. Slabs with similar dimensions will be marked the same.
5. Design slabs. Use loads from IS:875 part 2, call it ‘w’. Find the depth and area of steel. We’ll do detailing later.
6. Now beams will be calculated each individually. First, let’s calculate for tertiary beams, i.e beams resting on other beams. Assume, the depth and width of all beams are 500 mm and 300 mm respectively. Draw force dia for each beam.
Slab load: For two way slab – load on beam of length ‘l’ = WL/3. Calculate slab load intensity for all 4 beams on which 2 way slab is resting. Units in KN/m (UDL). For one way slab ‘WL/2’ for the beams supporting the slab.
Wall Load Intensity: The density of wall is 18 KN/m3. Multiply by height of wall and thickness of wall. Assume wall height 2.5m and width 300mm for external and 230 mm for internal. Multiply by 1.5 (factor of safety). Now wall intensity is in units KN/m (UDL).
Weight of concrete = Self-weight x Density (25KN/m3).
Self-weight = Width of beam x Height of beam
= 500 mm x 300 mm = 0.5 m x 0.3 m
Weight of concrete = 25 x 0.5 x 0.3 = 3.75 x 1.5 = 5.62 KN/m
Where 1.5 is factor of safety.
Add them. That is your net UDL. Find moment and reaction forces at support.
For primary beams repeat the same process. But now add the reactions of tertiary beams as point loads at according distance. Then, Use Substitute frame analysis to calculate span moments and support moments. Simultaneously calculate reactions at supports.
Find the area of steel at top and bottom.
7. Columns: Assume dimensions 500 mm x 300 mm. Use moments and reactions from beams resting on column to find the area of steel. Don’t forget to add column’s self-weight.
8. Net load from columns will be used to design footings.
9. Detailing: Detailing of steel as per SP 34.