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Various steps used in the two way slab design as per IS 456 2000 code standards?
Slab design is the process of determining the amount of the steel that is required to resist the total amount of the load which is acting on the structure. In this article you can able to learn about detailed concepts of various types of steps used in the design of two way slab as per the IS 456 2000 code standard.
The below specified 7 steps are generally used to design the two way slab.
- Calculation of ratio of the length of long span to length of short span
- Calculation of effective depth
- Calculation of the loads
- Bending moment calculation
- Check for the depth
- Calculation of area of steel
- Check for the deflection
Calculation of ratio of the length of long span to length of short span
This is the first step in the two way slab design as per IS 456 2000 code standards. In this we will consider the length of the slab member and width of the slab member. Then calculate the ratio of the long span to shorter span for that particular slab member. If the ratio is less than 2 we will consider two way slab design considerations and if the ratio is greater than 2 consider it as one way slab design.
For example let us take an example slab which is consisting of 3.5mX2.5m dimensions. The ration of longer span to shorter span is 3.5/2.5 which is 1.4 and is less than 2, consider it as two way slab design.
For example let us take an example slab which is consisting of 4.5mX2.0m dimensions. The ration of longer span to shorter span is 4.5/2.0 which is 2.25 and is greater than 2, consider it as one way slab design.
Calculation of effective depth in two way slab design
Before calculate the effective depth initially assume the thickness of the slab d value. The value of the effective depth of the RCC slab is determined by using below expression
Effective depth = d – (cover +(dia/2))
Where d= thickness of the slab
Dia = diameter of the reinforcing bar assumed.
Calculation of the loads
In the designing of two way slab or one way slab we will consider the loads which are related to the self weight of slab member, floor loads and live loads. The combination of these loads are considered in the reinforcement design. Finally factored load is determined by considering extra 50% loads. The example load calculation is shown below.
- Self weight of 100mm slab = 0.1X25 = 2.5 kN/m2.
- Floor load of 100mm thick = 0.05X20 = 1 kN/m2.
- Live load (assumed value ) = 2 kN/m2
Total load = 2.5+1+ 2 = 5.5 kN/m2
Factored load = Mu = 1.5 X 5.5 = 8.25 kN/m2.
Bending moment calculation
This is the fourth step in the two way slab design. The positive and negative bending moments are calculated in both X direction and Y direction with the help of the αx and αy values which are considered from the page number 91 of IS 456 2000 code standards.
The maximum bending moment values are determined by using below specified expression with the help of the page number 91 values of αx and αy.
Mu = αxwxl2
Where w = factored load
Mu Bending moment
l = length of the slab member
Check for the depth
The check for the depth is the fifth step in the two way slab design in this we will calculate the depth by using below expression
Mu = 0.138xfckxbxd2
By using above expression depth d is determined if the calculated depth is less than calculated effective depth hence it is safe. Otherwise we have to assume one more effective thickness value.
Calculation of area of steel
The area of the steel is determined in this step by using Ast formulae which is specified in the IS 456 code standards. The values of grade of concrete, grade of steel, factored load, effective depth values are assumed in this step.
The area of the steel in both X direction and Y direction are calculated by using the maximum amount of the bending moment values which is determined in the step 4. After determining the area of the steel we will calculate the spacing of the reinforced bars in the main reinforcement as well as the distribution reinforcement is determined for the slab member.
Check for the deflection in two way slab design
This is the last step in the design of the two way slab as per the considerations of IS code. After determining the area of the steel and spacing values we will check for the deflection as per the standards which is provided in the IS 456 2000 code standards. The ratios of the Lprovided/d and Lmax/d are calculated by considering the specifications of the modification factor from the page number 38 of IS 456 2000 standards.
If the factor of the Lprovided/d is less than Lmax/d then the design is safe otherwise we have to repeat the procedure by assuming other values
Watch the complete details of RCC slab design as per IS 456 2000 in my YouTube channel Civil Engineering by shravn in below video.
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Conclusions of various steps used in the two way slab design as per IS 456 2000
Slab design is the process of determining the amount of the steel that required as per the considerations of the load which is comes to the structure. The various types of the steps considered for the two way slab design are discussed in this article which are related to the calculation of the ration of longer span to shorter span, effective depth, load calculations, bending moment calculations, check for the depth, area of the steel and lastly check for the deflection.
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