Presentation on theme: "Summary Sheet Session Number : Date : Subject Expert : 2 13.03.2007 Dr. M.C. Nataraja Professor Department of Civil Engineering, S.J. College of Engineering,"— Presentation transcript:
Summary Sheet Session Number : Date : Subject Expert : Dr. M.C. Nataraja Professor Department of Civil Engineering, S.J. College of Engineering, Mysore – Phone: ,
Learning Outcomes: After this students will be able to do the detailing of different types of isolated footings through drawing and bar bending schedule.After this students will be able to do the detailing of different types of isolated footings through drawing and bar bending schedule. Detailing of Steel in Footings
Footings Footings The function of a footing or a foundation is to transmit the load form the structure to the underlying soil. The choice of suitable type of footing depends on the depth at which the bearing strata lies, the soil condition and the type of superstructure. Detailing of Steel in Footings
Types of footing Following are the different types of footing used for concrete structure Isolated footing Combined footings Strap footing Mat or raft foundation Pile foundation
Isolated footings Isolated footings are provided under each column and may be square, rectangular, or circular in plan. Footing may be flat or tapered.
Combined footings COMBINED FOOTING PROPERTY LINE RECTANGULAR AND TRAPEZOIDAL TYPES
Combined footings Combined footings are provided to support two or more column loads. These may be continuous with rectangular or trapezoidal in plan as shown. Combined footings become necessary under the following circumstances: when the isolated footings overlap. when the exterior column is close to the property line with the result symmetrical isolated footing can not be provided.
Strap footing Strap footing is one of the types of combined types of combined. It consists of an isolated footing of two columns connected by a beam called strap beam. The strap beam does not remain in contact with the soil and thus does not transfer any load to the soil. This is provided when one of the columns is on the property line. STRAP FOOTING STRAP BEAM PROPERTY LINE
Mat foundation The is provided when the soil is having very low bearing capacity and or when columns loads are heavy, the required footing area becomes very large and uneconomical. MAT FOUNDATION
Detailing Size of footing Depth at footing Depth of footing edge Nominal and effective cover Development length Minimum and maximum steel Spacing of bars and stirrups PLAN BM1 OWS TWS d/2 d BM2
Detailing of steel in rectangular footing as per IS: Reinforcement Long and short direction In short direction larger steel area is needed in the central portion and is given by Reminder of the steel in end bands Development length= L dt = L dt = 47φ s for M20 concrete and Fe 415 steel
PROBLEM An isolated footing is to be provided for a column of section 400 mm x 400 mm. The following details re given: Height of the column =3m Main reinforcement in column = 4 Nos. 16 mm diameter. Transverse reinforcement = 6 mm at 220 mm c/c Plan size of footing = 2.7 m x 2.7 m Depth of footing at column face = 500 mm Depth of footing at edge = 150 mm Depth of foundation = 1000 mm
Isolated footing Contd.. Footing reinforcement = a mesh of 20 mm diameter steel at 250 mm c/c. Grade of concrete= M20 Grade of steel = Fe415 Draw to a suitable scale the following: Plan Sectional Elevation Prepare bar bending schedule20 Marks
Solution: Development length in tension = 47φ = 47 x 16= 752 mm Depth at junction = 500 mm Leg length available for column bars = L dt – 500 = 252 mm < 300 mm. Provide 300 mm minimum. Development length of 20 mm bars in footing = 47 x 20 = 940 mm This is provided by the horizontal projection of bars. Bars need not be bent at ends in to the depth of footing.
Number of bars in footing = 11 Numbers at 200 mm c/c with a side cover of 100 mm. Width of footing= 10 x x 2 =2700 mm Follow all specification of SP 34 and prepare the drawing
Detailing with dowel bars Pu/bD < 0.45fck √ A1/A2 2:1 A1=bxD A2= [b+4Df] x [D+4Df} Df 2Df
SECTIONAL ELEVATION LEVELLING COURSE DEPTH OF FOUNDATION ≥ 500 mm L dt L dc L dt 300 min Detailing with dowel bars LAP Dowel bars or starter bars
Bar bending schedule For exercise problem For footing only MemberMark No. of bars Diameter in mm Length in m/piece Total length in m Bar detailsRemarks #20 Footing A B Total length in m/diameter56.10 Weight in kg/m2.45 Total weight in kg/diameter137.45
Estimation of quantity of concrete Volume of concrete = L x B x D L= length in m B=Breadth in m D=Thickness in m Apply for rectangular and trapezoidal part Weight of concrete, kg = Volume x density