As per IS 486:2000, in reinforced and plain concrete footings, the thickness at the edge of the footing shall not be less than ________ for footings on soils.
150 mm
Understanding Minimum Footing Thickness per IS 486:2000
The question asks about the minimum thickness required at the edge of reinforced and plain concrete footings when they are placed on soil, as specified by the Indian Standard IS 486:2000.
Building foundations, especially footings, are critical structural elements that transfer loads from the structure above to the supporting soil. Their design and construction must follow specific codes and standards to ensure safety and durability. IS 486:2000 is a standard that provides guidelines for the design and construction of plain and reinforced concrete footings.
IS 486:2000 Provisions for Footing Thickness
According to IS 486:2000, there are specific requirements for the minimum thickness of footings to ensure adequate strength, stiffness, and cover for reinforcement (in the case of reinforced concrete footings). The standard specifies different minimum thicknesses based on whether the footing is placed on soil or on piles.
For footings resting on soil, the standard lays down a clear minimum thickness requirement for the edge. This is important because the edges are often subjected to bending stresses and need to be robust enough to prevent failure and provide sufficient cover to the reinforcement, preventing corrosion.
Minimum Thickness Requirement on Soil
As per clause 28.1.1 of IS 486:2000 (which covers General Design Requirements for footings), the thickness at the edge of the footing shall not be less than:
- 150 mm for footings on soils.
- 300 mm for footings on piles.
The question specifically refers to footings on soils. Therefore, the minimum thickness at the edge of the footing shall not be less than 150 mm.
Analyzing the Options
Let's look at the given options in the context of the IS 486:2000 requirement for footings on soils:
- 120 mm
- 125 mm
- 150 mm
- 100 mm
Comparing these options with the standard requirement of 150 mm, it is clear that only one option meets or exceeds this minimum value.
Conclusion on Minimum Edge Thickness
Based on the provisions of IS 486:2000 for reinforced and plain concrete footings on soils, the thickness at the edge must be at least 150 mm. Options less than 150 mm do not satisfy the standard's requirement.
Therefore, the minimum thickness at the edge of the footing for footings on soils, as per IS 486:2000, is 150 mm.
| Condition | Minimum Thickness at Edge (as per IS 486:2000) |
|---|---|
| Footings on Soils | 150 mm |
| Footings on Piles | 300 mm |
Revision Table: Key IS 486:2000 Footing Details
| Topic | Relevant IS 486:2000 Clause | Key Requirement |
|---|---|---|
| Minimum Edge Thickness (on soil) | 28.1.1 | 150 mm |
| Minimum Edge Thickness (on piles) | 28.1.1 | 300 mm |
| General Design Requirements | 28.1 | Consider soil pressure, settlement, overall stability |
Additional Information on Footing Design
While the minimum edge thickness is a specific requirement, the overall design of a footing involves several other considerations:
- Bearing Capacity of Soil: The footing size is primarily determined by the safe bearing capacity of the soil to ensure the load is distributed over a large enough area to prevent excessive settlement or shear failure of the soil.
- Punching Shear: Footings must be designed to resist punching shear stresses caused by the column or pedestal load concentrated over a small area. This often dictates the overall depth of the footing near the column.
- Bending Moment: The footing acts like a cantilever projecting from the column/pedestal. It must be designed to resist the bending moments caused by the upward soil pressure. This determines the required reinforcement in the footing.
- Development Length: The reinforcement bars from the column or pedestal must be anchored properly within the footing to transfer the load effectively. Adequate development length is crucial for this.
- Differential Settlement: Footings should be designed to minimize differential settlement between adjacent footings, which can cause cracking and damage to the structure.
- Cover to Reinforcement: Sufficient concrete cover must be provided to the reinforcement bars to protect them from corrosion, especially in aggressive environments. IS codes specify minimum cover requirements based on exposure conditions.
The minimum edge thickness specified in IS 486:2000 helps ensure basic structural integrity and durability at the periphery of the footing.
Important Questions from Footings
For a number of columns constructed in a row, the type of foundation provided is
If p is the net upward pressure on a square footing of side b for a square column of side a, the maximum bending moment is given by
A. \({\rm{B}}.{\rm{M}} = \frac{{{\rm{pb}}\left( {{\rm{c}} - {\rm{a}}} \right)}}{4}\)
B. \({\rm{B}}.{\rm{M}} = \frac{{{\rm{pb}}{{\left( {{\rm{b}} - {\rm{a}}} \right)}^2}}}{4}\)
C. \({\rm{B}}.{\rm{M}} = \frac{{{\rm{pb}}{{\left( {{\rm{b}} - {\rm{a}}} \right)}^2}}}{8}\)
D. \({\rm{B}}.{\rm{M}} = \frac{{{\rm{pb}}\left( {{\rm{b}} + {\rm{a}}} \right)}}{8}\)
_______ is provided to support an individual column. It is circular, square or rectangular slab of uniform thickness Sometimes it is stepped or haunched to spread the load over a large area.
The minimum nominal cover to reinforcement for R.C.C. footings as per IS: 456-2000 shall be
For a proposed building, raft foundation, isolated footings and combined footings are being considered. These foundations are to be listed in the decreasing order of preference in terms of performance. Which one of the following is the correct order of listing?
