V Semester Civil CE2306 DRC by M.Alaguraj AP / Civil Page 1

MAHALAKSHMI

ENGINEERING COLLEGE

TIRUCHIRAPALLI - 621213.

QUESTION BANK

DEPARTMENT: CIVIL SEMESTER: V

SUBJECTCODE/NAME: CE2306/ DESIGN OF RC ELEMENTS

UNIT V– LIMIT STATE DESIGN OF FOOTING AND DETAILING

PART - A (2 marks)

1. Define safe bearing capacity of soil? (may/June 07) Bearing capacity of soil after allowing a certain factor of safety over the ultimate bearing capacity against failure (collapse) is termed as safe bearing capacity of soil. 2. What are the governing factors to decide the depth of location R.C footings? (Nov/Dec 07)

Footing should be carried well below the top soil, miscellaneous fill, abandoned

foundation, debris or muck.

Footing should be carried below the depth of weathering.

Footing in sloping ground should have sufficient edge distance as protection

against erosion.

Difference in elevation off foundation, should not be ss great as to introduce

undesirable overlapping of stresses in soil.

3. What are the sections at which moments are to be computed for the design of a footing?

As per (IS:456-2000)the bending moment to be taken for design of footing is the moment of the reaction forces due to be applied(excluding the self weight of the footing) at the following sections. At the face of the column, pedestal ort wall, for footing supporting a concrete column, pedestal or wall. Half way between the centre line and the edge of the wall, for footing under masonry wall, and Half way between the face of the column or pedestal and the edge of the gusseted base for the footings under gusseted base. 4. What are the factors govern the depth of footing? Depth should be adequate in one way and two way shear without shear reinforcement.

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Depth should be adequate for bending moment without compression reinforcement. Depth should be adequate to transfer bond length by the main bars of dowel bars. 5. How is the main steel distributed in wall footings and two way rectangular footing?

In one way reinforced footing as in wall footing the main steel is distributed uniformly over the full width of the footing. In two way reinforced footings as in rectangular footings the main steel in the long direction is distributed uniformly across the full width of the footing. 6. What is mean by eccentrically loading on a footing?

A footing subjected to a concentric loading with a moment or a load applied off the centre, causes the loading to be eccentric and such a footing is called eccentrically loaded footing. The analysis of an eccentrically loaded footing involves the evaluation of contact pressure beneath the footing and the ultimate bearing capacity. 7. What are the situations in which combined footings are preferred to isolated footings? (April/may 12, 08)

A combined footing is preferred when equal or unequal columns are positioned so close4ly that the individual footing are not practicable. These footing are usually rectangular in shape. In order to accommodate unequal column loads or columns close to property lines the rectangular shape is modified to a trapezoidal shape.sometimes, a strap is provided to combine two columns which have a wide spacing or if one of them is close to property line. 8. What are the causes of failure of foundation?

Unequal settlement of the subsoil. Horizontal movement of the soil adjoining the structure. Shrinkage due to withdrawel of moisture from the soil below the foundation. Lateral pressure tending to overturn the structure. Action of atmosphere.

9. What are assumptions made in the design of footing?

Foundation is rigid. The distribution of pressure from soil on the base is uniform.

10. Name the common types of foundation?

Spread footings Combined footings Raft or mat footing Pile foundation

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11. Under what circumstances combined footing are provided? When the columns are very near to each other so that their footing overlap. When the bearing capacity of soil is less.requireing more area under individual bars. When the end column is near a property line, so that its footing cannot be spread in that direction. 12. Define combined footing. A footing consists of a common footing provided to two columns. These footings are formed by combining two or more equally or unequally loaded columns into one footing. This arrangements average out and provides a more or less uniform load distribution in the supporting soil. These footings are usually rectangular in shape. 13. What are the common shapes of reinforcement? Reinforcements used in structural concrete members may be in different shapes as straight or cranked bars, single or double legged stirrups or bundled bars.

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16 MARKS

Problem no: 01

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Problem no: 02

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Problem no: 03

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Problem no: 04

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Problem no: 05

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Problem no: 06

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(16 MARKS)THEORY QUESTIONS 1. Write down the different types of footing and their suitability. (April may 11) a) Spread footings These footings are the most common of all types of footings with minimum cost and complexity of construction. It is necessarily provides the function of distributing the column load over a wide area taking care of the strength and deformation characteristics of the soil. These types of footing are also known as isolated footings (or) square, rectangular footings.

b)Combined footings These footings are formed by combining two or more equally or unequally loaded columns into one footing. This arrangements average out and provides a more or less uniform load distribution in the supporting soil. These footings are usually rectangular in shape. It may be modified to a trapezoidal shape so as to accommodate unequal column loadings or columns closed to property line. If may be provided with a strap to accommodate wide column spacing or columns close to property.

c) Continuous footings These footings carry closely spaced columns or a continuous wall such that the load distribution is uniform and load intensity low on the supporting soil. In such footing for design purpose the load per unit length is taken. Load intensity is given in force per unit length of the footing. These footings are also named as strip footing or wall footing.

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d) Mat foundation These are characteristic by the feature that column frame into the footing in two directions. Any number of columns can be accommodated with as low as four columns. In the majority of the cases mat foundation are used where the soil has a low bearing capacity76.mat foundation are used where the soil has low bearing capacity.

2. Write the procedure for design the combined footing with two columns only. (April may 11) The following are the procedure for design the combined rectangular reinforced footing two columns only.

The following are the procedure for design the combined footing.

The footing is infinitely rigid and does not have any bearing on the pressure

distribution.

The soil pressure beneath the footing is linearly distributed such that the center

of soil pressure coincides with the line of action of the resultant force of all the

loads.

Find the total column load which is to be positioned in the footing and determined

the line of action of the resultant.

Obtain the soil pressure distribution (stress per the unit length of the footing).

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Find the width of the footing.

Draw the shear force diagram along the length of the footing.

Draw the bending moment diagram along the length of the footing.

Design the footing for transverse bending also in the same manner as for spread

footing.

Draw the reinforcement details and also the curtailment lengths.

The footing will be designed with strap beam also, the strap beam will be

designed based on the upward soil pressure on the area.

The depth of the footing is taken by the consideration of shear stress

consideration.

The one way shear is located on half of the effective depth of the footing.

3. Sketch the standard detailing for the following a) Two span one way continuous slab with curtailment details.

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b) Curtailment details in a trapped cantilever beam.