unit v - limit state design of footing

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CE6505 – DESIGN OF REINFORCED CONCRETE ELEMENTS

Compiled by: Mr. S. MANIKANDAN, Assistant Professor, Department of Civil Engineering, Shanmuganathan Engineering College.

UNIT – V

LIMIT STATE DESIGN OF FOOTING

Design of wall footing

Design of axially and eccentrically loaded rectangular pad and sloped footings

Design of combined rectangular footing for two columns only



2 CE6505 – DESIGN OF REINFORCED CONCRETE ELEMENTS

S.M.K./A.P./CIVIL/S.E.C.

2 Marks [Questions & Answers]

01) State the governing factors to decide the design of R.C. footing.

Ans.: The following factors to decide the design of R.C. footing:

(i) Soil strata

(ii) Bearing capacity and standard penetration test value

(iii) Type of structure & Type of loads

(iv) Permissible differential settlement, and

(v) Economy.

02) Define safe bearing capacity of soil.

Ans.: The maximum pressure which the soil can carry safely without risk of shear failure is called the

safe bearing capacity of soil.

03) State the rankine’s equation to determine the minimum depth of foundation?

Ans.:

p0 1 – sin 2 Where,

Depth of foundation (h) = p0 = Axial load on the column

1 + sin = Unit weight of soil

= Angle of repose

04) Define wall footing.

Ans.: The wall footings (as shown in fig.) provided under masonry or

concrete walls carrying direct vertical loads may be designed either

of plain concrete or reinforced concrete.

A wall footing essentially deflects upward in transverse

direction as cantilever (one-way action) and a strip of unit width

along its length is considered for its design.

05) How do you calculate the base area in the case of axially loaded footings?

Ans.:

(Axial load on the column + 10 % of axial load on the

column as self weight)

Area for axially loaded footing =

Safe bearing capacity of the soil



UNIT V – LIMIT STATE DESIGN OF FOOTING 3

TWO MARK Q & A and BIG QUESTIONS

06) How is the main steel distributed in wall footings and two-way rectangular footings?

Ans.: In wall footings, the main steel is distributed uniformly over the full width of the footing.

In two-way rectangular footing, the main steel in the longer direction is distributed uniformly

across the full width of the footing.

07) Write down the Codal provisions for tensile reinforcement for two way R.C. rectangular

footing.

Ans.: As per clause 34.3.1 of IS 456: 2000, In two-way reinforced

rectangular footing, the reinforcement in the longer direction shall

be distributed uniformly across the full width of the footing. For

reinforcement in short direction, a central band equal to the width

of the footing shall be marked along the length of the footing and

portion of the reinforcement as shown in fig.

08) What is one way and two way shear in footing?

Ans.: One way shear is rate of change of bending moment. It’s typically called beam shear. However

two way shear is punching shear and its effect is two way. It's also called flat slab shear.

09) What is punching shear in RCC footing?

Ans.: The shear action of footing slab is in two directions, it is known as punching shear.

10) Under what situations trapezoidal footings are preferred?

Ans.: An isolated footing for column on property line would extend beyond the property line which

can be avoided by combining such footing with that of interior column. In that situation trapezoidal

footing should be preferred.

11) When is the combined footing provided?

Ans.: The combined footing should be provided in following circumstances:

(i) when the columns are very near to each other so that their footings overlap,

(ii) when the bearing capacity of soil is less, requiring more area under individual footing,

(iii)

when the end column is near a property line, so that its footing cannot be spread in that

direction.





12) What is the necessity to provide combined rectangular footing?

Ans.: Combined footings are formed by combining two or more equally or unequally loaded columns

into one footing. This arrangement averages out and provides a more or less uniform load distribution

in the supporting soil. These footings are usually rectangular in shape.

13) Give the general steps involved in the design of combined footing.

Ans.: The following steps are essential to design the combined footing:

(i)

Proportioning of footing

(ii) Bending pattern

(iii) One way shear

(iv)

Two way shear

(v)

Development length, etc.

14) Name any four loads you would consider in the design of masonry walls.

Ans.: Design load is decided based on the following consideration:

(i) Dead load + Live load

(ii) Dead load + Appropriate live load + Seismic load

(iii) Dead load + Appropriate live load + Wind load

(iv)

Dead load + Seismic load.





16 Mark [Questions]

CE2306 (R 2008) - April / May 2015 (A.U. Chennai) – Q.P. Code: 71258

(Or)

CE2306 (R 2008) – November / December 2014 (A.U. Chennai) – Q.P. Code: 91243

(Or)

CE2306 (R 2008) – May / June 2014 (A.U. Chennai) – Q.P. Code: 51242

(Or)






(Or)

CE2306 (R 2008) - May / June 2013 (A.U. Chennai) – Q.P. Code: 21210

(a) Write down the design steps in detail of rectangular combined footing. (16)

(Or)

(b) Design a RC combined rectangular footing for two columns located 3.6 m apart. The overall

sizes of the columns are 400 mm X 400 mm and 600 mm X 600 mm and the loads are 1000 kN

and 1500 kN respectively. The space available for width of the footing restricted to 1800 mm.

The safe bearing capacity of soil is 280 kN/m2. Use M 15 concrete and Fe 415 steel. (16)


(Or)





CE2306 (R 2008) - May / June 2012 (A.U. Chennai) – Q.P. Code: 10229

(Or)

CE1354 (R 2004) - April / May 2011 (A.U. Chennai) – Q.P. Code: 55232

(Or)

Compiled By:

Mr. S. MANIKANDAN M.E. (Structural Engg.)

Assistant Professor, Department of Civil Engineering,

Shanmuganathan Engineering College, Arasampatti, Pudukkottai – 622507.

Email: [email protected]

unit v - limit state design of footing

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