Contents
Design requirements of RCC slab as per standards | structural design | Building design
A concrete slab is a basic structural elements used in building construction. The thickness varies between 100mm to 500mm for basic buildings. In the slab section reinforcement bars plays an important role in strength. The reinforcement in slab section are classified into two categories one is main reinforcement and second one is related to the distribution reinforcement. Design requirements in the RCC slab section will help to calculate the proper steel reinforcement under application of various loading conditions.
The following are the seven basic requirements considered in the slab design as per standards.
- Effective span
- Limiting stiffness
- Main reinforcement
- Maximum diameter of the bars
- Spacing of main reinforcement
- Distribution reinforcement
- Cover to the reinforcement
Effective span in slab design
The effective span is defined as the distance between the supports or clear distance between the supports plus the depth of beam or slab section.
In the slab section effective span is considered in two cases one is related to the simply supported slab and second one is cantilever slab. For the simply supported slab members the effective span value is taken as the sum of minimum clear span and effective depth values. In case of the cantilever slab section the effective span of the slab section is taken as the sum of the length up to the face of support and half of the effective depth.
Limiting stiffness
The value of the limiting stiffness varies for the three types of the slabs namely cantilever slab, simply supported slabs or continuous slabs.
For the cantilever slab the value of limiting stiffness is taken as 7 value, in case of simply supported section the value is considered as 20 and finally for the continuous slab section the limiting stiffness value is 26 as per the Indian standard code provisions.
Minimum reinforcement
The reinforcement values in the slabs are classified in to two types one is main reinforcement and second is distribution reinforcement as per the Indian standard code provisions 456-2000 the minimum slab reinforcement value in RCC slab is considered as less than of 0.5percentage.
Maximum diameter of the bars
The maximum diameter of the reinforcement bars in RCC slab is should not greater than (1/8) of slab thickness as per IS code provision standards.
For example let us consider a building which is having 120mm thickness of slab, for that particular structure we need to take maximum reinforcement bar diameter is (1/8) of 120mm which is 15mm.
In the same process let us consider a building which is having 150mm thickness of slab, for that particular structure we need to take maximum reinforcement bar diameter is (1/8) of 150mm which is 18.75mm. Approximately we will take 20mm maximum diameter of reinforced steel bars.
In the same process let us consider a building which is having 200mm thickness of slab, for that particular structure we need to take maximum reinforcement bar diameter is (1/8) of 200mm which is 25mm diameter.
Spacing of the main reinforcement bars
Basically the spacing is the center to center distance between the main reinforcement bars in slab section. The spacing of the main bars is depending upon the following two factors
- 3 times of the effective depth
- 300mm
The lesser of the above specified value is considered as spacing of main reinforcement in the slab section.
Percentage of distribution reinforcement
The distribution reinforcement is perpendicular to the main reinforcement in slab section. The percentage of distribution reinforcement in the RCC slab is not less than 0.15percenatge.
Cover to the RCC slab
The cover is the distance between the edge of slab to reinforcing bar. As per the standards the cover for the RCC slab section is considered as minimum of 20mm thickness.
Requirements of RCC Slab as per standards
| S. No | Design Requirements | Values as per standards |
| 1 | Effective span | For SSB = min clear span + effective depth
C/C distance For Cantilever = Length up to face of support + (1/2) effective depth |
| 2 | Limiting stiffness | Cantilever – 7
Simply supported -20 Continuous -26 |
| 3 | Main reinforcement | Less than 0.5% |
| 4 | Maximum Diameter of bars | Not greater than 1/8 of slab thickness |
| 5 | Spacing of main reinforcement | 3 effective depth and 300mm
Lesser of two is taken |
| 6 | Distribution reinforcement | Should not less than 0.15 percentage |
| 7 | Cover to reinforcement | Minimum of 20mm |
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Conclusions on design requirements of RCC slab as per standards
Well now the above explained concepts are related to design requirements of RCC slab as per Standards. The design standards are classified in to seven types which are effective span, limiting stiffness, minimum percentage of main reinforcement, maximum diameter of bars, spacing of main reinforcement bars, minimum percentage of distribution reinforcement and cover to the reinforcement.
The minimum cover is taken as 20mm, distribution reinforcement value is should not be less than 0.15 percentage, spacing of main reinforcement is taken as minimum of 3 effective depth and 300mm values, main reinforcement values should not less than 0.5 percentage value, the limiting stiffness value is considered as 7 for the cantilever slab, 20 for the simply supported slab and 26 for the continuous slab section and finally the effective span value is the is taken as the sum of minimum clear span and effective depth values in case of simply supported slab and sum of the length up to the face of support and half of the effective depth in the cantilever case.
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