Footings MCQ Quiz - Objective Question with Answer for Footings - Download Free PDF

Explanation:

• An eccentric footing is used when the column cannot be placed at the center of the footing, often due to space constraints like property boundaries.

• This results in uneven load distribution, and special design considerations (like connecting beams or reinforcements) are needed to prevent tilting or rotation.

Additional Information

Shared by multiple columns (Combined Footing):

• Used when two or more columns are close together and their isolated footings overlap.

• Distributes loads evenly across multiple columns.

• Can be rectangular or trapezoidal in shape.

Used for machine foundations:

• Refers to foundations designed for machinery to resist vibrations and dynamic loads.

• Usually requires special designs like block-type or slab foundations.

• Found in industrial settings, such as factories or power plants.

Having a circular shape:

• Refers to a circular isolated footing, which is used for centrally loaded columns.

• Common in lightweight structures or specific column types like flagpoles or small towers.

• Provides uniform load distribution around the column.

Explanation:

• Strap footing is used when a column is close to a property line, and its footing cannot be extended symmetrically on one side.

• It connects the eccentrically loaded footing to an interior footing using a strap beam, allowing the load to be distributed evenly and avoiding tilting.

Additional Information

Isolated Footing

• Definition: A footing that supports a single column.

• Shape: Usually square, rectangular, or circular.

• Use Case: When columns are well spaced and loads are moderate.

• Limitation: Not suitable near property lines if the footing can't be placed symmetrically around the column.

Combined Footing

• Definition: A single footing supporting two or more columns.

• Shape: Usually rectangular or trapezoidal, depending on column spacing and loads.

• Use Case:

  • When one column is near a boundary and another is close enough to share the footing.

  • Useful when isolated footings would overlap.

• Advantage: Balances load and prevents eccentricity.

Mat Footing (Raft Foundation)

• Definition: A large, continuous slab supporting multiple columns or an entire structure.

• Use Case:

  • When soil bearing capacity is low.

  • When building loads are heavy and closely spaced.

• Advantage: Distributes load evenly over a large area.

• Limitation: Expensive; not typically used just for boundary restrictions.

Explanation:

• When columns are closely spaced in a row, a continuous footing is preferred as it can efficiently support multiple columns by distributing the load along a continuous strip of concrete beneath them. Th
• is setup is economical and structurally effective when individual footings would overlap due to close spacing.

 Additional InformationCombined footing

• Used when two columns are close, and their isolated footings would overlap.

• Can be rectangular or trapezoidal depending on load and spacing.

• Suitable when one column is near a property line and cannot have a symmetrical footing.

Strap footing

• Consists of two isolated footings connected by a beam (strap).

• Strap helps transfer load from an eccentrically loaded column to a centrally loaded one.

• Used when columns are far apart but one is near a property line.

Isolated footing

• Separate footing under each column.

• Economical when columns are spaced far apart and loads are moderate.

• Not suitable when footings would overlap due to close spacing.

Explanation:

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.

Explanation:

As per IS 456: 2000, the nominal reinforcement under different environmental conditions is as shown in the table below:

Explanation:

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.

1. Gravity retaining wall.
2. Cantilever retaining wall.
3. 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.

For footing:

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, 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 τc = 0.25√fck and τc = 0.16√fck according to working stress method.

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.

Concept:

The area of footing (A_f) is given by

\(\rm A_f = \rm\frac{{Total~ load}}{{{Safe ~bearing ~capacity}}}\)

Calculation:

Given:

Load = 330 kN, Self weight = 10% of Given Load = 0.1 × 330 = 33 kN

Total Load = 330 + 33 = 363 kN

Safe bearing capacity = 150 kN/m²

\(\rm A_f = \rm\frac{{Total~ load}}{{{Safe ~bearing ~capacity}}}\)

\(A_f = \frac{{363}}{{150}} = 2.42\;{m^2} \)

Reinforcement that will be required in central width band is:

\(\rm{\frac{{Reinforcement\;in\;central\;band\;width}}{{Total\;reinforcement\;in\;short\;direction}} = \frac{2}{{\beta + 1}}}\)

where,

β is the ratio of the longer side to the shorter side of the footing.

\(\beta = \frac{3}{2} = 1.5\)

Longer side of footing = 3 m

The shorter side of footing = 2 m

\(\rm{\frac{{Reinforcement\;in\;central\;band\;width}}{x} = \frac{2}{{1.5 + 1}} = 0.8}\)

Reinforcement in central bandwidth = 0.8x.

Explanation:

The ratio of Reinforcement that will be required in the central width band  and total reinforcement in the short direction is given as:

\(R = \frac{2}{\beta + 1}\)

Where,

β is the ratio of the longer side to the shorter side of the footing

It is given that the footing ratio of its large side to the small side = β = 5.2

The ratio is calculated as:

\(R = \frac{2}{5.2 + 1}\)

R = 0.322 

Concept:

 Design Load = 1.1 x Unfactored load.

Area required for footing = \(\frac{{{\rm{Design\;axial\;load\;on\;footing\;including\;self\;weight\;of\;footing}}}}{{{\rm{Safe\;bearing\;capacity\;of\;soil\;}}}}\)

Calculation:

Area required for footing = \(\frac{{{\rm{Design\;axial\;load\;on\;footing\;including\;self\;weight\;of\;footing}}}}{{{\rm{Safe\;bearing\;capacity\;of\;soil\;}}}}\)

UnFactored load, P = 1000/1.5 = 666.67 kN

 For safe , self-weight is considered as 10 % of load

 Adding 10 % of load as self weight = 0.1 ×666.66= 66.66kN

Design load = 666.66 + 66.66 = 733.32 kN

Safe bearing capacity of Soil, q = 150 kN/m2

For weight 'W', stress (load per unit area) (σ) is given by = W/a²

Critical section for punching shear occurs at d/2 from the face of the column.

∴ Punching shear resistance (Force) is given by:

\(P=\frac{W}{a^2}\times (a^2-(b+d)^2) \approx\frac{W}{a^2}\times \frac{a^2-b^2}{4\times d\times b}\)

∴ Punching shear stress per unit area is given by: \(\sigma'=\frac{W}{a^2}\times \frac{a^2-b^2}{4\times d\times b}\)

From the given data, punching shear resistance is denoted by q

\(\sigma'=\frac{W}{a^2}\times \frac{a^2-b^2}{4\times d\times b}=q\\\therefore q = \frac{W}{a^2}\times \frac{a^2-b^2}{4\times d\times b}\\\therefore d = \frac{W(a^2-b^2)}{4a^2bq}\)

Explanation:

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:

1. Stem
2. Toe slab 
3. 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:

1. Self-weight of the wall
2. 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.

Concept:

The recommendations of IS 456: 2000 related to different critical section in an isolated footing are listed in following table:

The recommendations in a pictorial view shown below: