Test: Footing & Rafts - 1 - Civil Engineering (CE) MCQ

As per IS 456: 2000, Clause 34.1.2,
Thickness at the Edge of Footing
In reinforced and plain concrete footings, the thickness at the edge shall be not less than 150 mm for footings on soils. 
For footings on piles, the thickness at the edge shall be not less than 300 mm above the tops of piles.

(i) Reinforced concrete wall is designed as a compression member. The reinforced concrete wall is used in cases where beam is not provided and load from the slab is heavy or when the masonry wall thickness is restricted.
Braced and Unbraced Concrete Walls:
When cross walls are provided for the walls such that they can take lateral load and 2.5% of vertical load, then the wall is braced. Otherwise, the wall is known as unbraced wall.
Detailing of Reinforcement [IS 456 Guidelines]:

• For plain concrete walls, the minimum vertical steel is 0.12% for HYSD bars and 0.15% for mild steel bar.
• For RC wall, minimum vertical reinforcement is 0.4% of cross-section.
• In plain concrete wall, transverse steel is not required
• In RC wall, transverse steel is not required (not less than 0.4%)
• Maximum spacing of bars is 450mm or 3t, whichever lesser
• The thickness of wall in no case should be less than 100mm
• If thickness is greater than 200mm, double grid reinforcement is provided along both the faces.

For Bending Moment:
The critical section for the bending moment is at the face of the column.

For Shear stress:
a) One Way shear
The critical section for one-way shear is at a distance d from the face of the column.

b) Two-way shear or punching shear
The critical section for two-way shear is at a distance d/2 from the face of the column.

So, the correct answer is A – 1, B – 3, C - 2.

According to I.S. 456-1978, the thickness of reinforced concrete footing on piles at its edges is kept less than ​15 cm.
However, from the updated IS Module of 456:2000:
As per IS 456: 2000, Clause 34.1.2,
Thickness at the Edge of Footing
In reinforced and plain concrete footings, the thickness at the edge shall be not less than 150 mm for footings on soils.
For footings on piles, the thickness at the edge shall be not less than 300 mm (30 cm) above the tops of piles.

Specification for footing as per IS 456:2000 are as follows:
i) The thickness at the edge shall not be less than 150 mm for footings on soil and not less than 300 mm for footing on piles.
ii) The depth of the foundation should be a minimum of 500 mm.
iii) For reinforcement, the footing is treated as an inverted slab. As per IS:456-2000, the minimum percentage of reinforcement of steel is 0.12% of the gross sectional area with HYSD bar and 0.15% of the gross area with plain bars of mild steel.
iv) Minimum clear cover should be 50 mm.
v) Permissible shear stress for footing, according to limit state method is τ_c  = 0.25√f_ck and τ_c = 0.16√f_ck according to working stress method.

Concept:
To calculate the area of foundation required for the given load, we need the weight of the foundation also, so in columns and walls, we assumed the weight of the foundation to be 10% of the column load or wall load for column and wall respectively.
Additional Information
The critical section for bending moment under different situation is given below:

• If RCC foundation is provided for a RCC column/wall, then critical section for BM is at the face of column or wall as it behaves as cantilever bending as shown in figure:
• If RCC foundation is provided over brick masonry wall, then critical section for BM is at middle of wall as shown in figure:

Retaining wall:

• The retaining wall is used to retain earth and resist the lateral pressure of soil at a place with a sudden change in elevation.
• Cantilever type retaining wall is used for heights up to 6m.
• Cantilever type retaining wall has the following components:
  • Stem
  • Toe slab
  • Heel slab 
• The stability of a cantilever-type retaining wall against overturning can be obtained by taking moment about the toe of the retaining wall.
• Due to backfill pressure overturning moment is generated which causes the overturning of the retaining wall about the toe of the wall.
• Resisting moment against overturning is generated due to the:
  • Self-weight of the wall
  • Backfill over the heel slab

• Let M_r = Resisting moment against overturning and M_o = Overturning moment
• If M_r > M_O⇒ Retaining wall is safe against overturning
• If the Heel slab length increases, the Resisting moment also increases as more backfill over the heel slab tends to increase the resistance of the wall against overturning thus wall becomes safe against overturning.

Retaining wall:

• A retaining wall or retaining structure is used for maintaining the ground surfaces at different elevations on either side of it.
• Whenever embankments are involved in construction, retaining walls usually necessary.

Types of retaining wall:

• Depending upon the mechanisms used to carry the earth's pressure, These are classified into the following types.

(i) Gravity retaining wall.
(ii) Cantilever retaining wall.
(iii) Butters wall.

Gravity retaining wall:

• It is not used for heights of more than 3.0 m.
• In it, the resistance to the earth's pressure is generated by the weight of the structure.

Cantilever retaining wall:

• It is the most common type of retaining wall and its height ranges up to 10-25 feet (3 to 8m).
• Counterfort retaining walls are economical for height over about 6 m.
• A cantilever retaining wall resists the earth pressure horizontal & another, by the cantilever bending action.

As per IS 456: 2000, Clause 34.1.2,
Thickness at the Edge of Footing
In reinforced and plain concrete footings, the thickness at the edge shall be not less than 150 mm for footings on soils. 
For footings on piles, the thickness at the edge shall be not less than 300 mm above the tops of piles.

Reinforcement that will be required in central width band is:

where,
β is the ratio of the longer side to the shorter side of the footing.
β = 3/2 = 1.5
Longer side of footing = 3 m
The shorter side of footing = 2 m

Reinforcement in central bandwidth = 0.8x.