Summary Sheet

1
Summary Sheet
Session Number
2

Date

13.03.2007

Subject Expert
Dr. M.C. Nataraja Professor Department of Civil
Engineering, S.J. College of Engineering, Mysore
570 006. Phone[phone],
9880447742 E-mail nataraja96_at_yahoo.com


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Detailing of Steel in Footings
Learning Outcomes

• After this students will be able to do the
  detailing of different types of isolated footings
  through drawing and bar bending schedule.

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Detailing of Steel in 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.
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• 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

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Isolated footings

• Isolated footings are provided under each column
  and may be square, rectangular, or circular in
  plan. Footing may be flat or tapered.

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Combined footings
PROPERTY LINE
COMBINED FOOTING
COMBINED FOOTING
RECTANGULAR AND TRAPEZOIDAL TYPES
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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.

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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.

PROPERTY LINE
STRAP BEAM
STRAP FOOTING
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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
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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

BM1
OWS
d/2
BM2
TWS
d
PLAN
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Detailing of steel in rectangular footing as per
IS456-2000
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 Ldt
Ldt 47fs for M20 concrete and Fe 415 steel
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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

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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 schedule 20 Marks

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Solution Development length in tension 47f
47 x 16 752 mm Depth at junction 500 mm Leg
length available for column bars Ldt 500
252 mm lt 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.
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Number of bars in footing 11 Numbers at 200 mm
c/c with a side cover of 100 mm. Width of
footing 10 x 250 100 x 2 2700 mm Follow all
specification of SP 34 and prepare the drawing
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Square footing
400
17
Rectangular footing
400
PLAN
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Rectangular footing
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Column steel detailing
20
Column steel detailing
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Column steel detailing
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Detailing with dowel bars
A1bxD

• Pu/bD lt 0.45fck vA1/A2

2Df
21
Df
A2 b4Df x D4Df
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Detailing with dowel bars
Dowel bars or starter bars
LAP
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Bar bending schedule
For exercise problem For footing only
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Estimation of quantity of concrete

• Volume of concrete L x B x D
• L length in m
• BBreadth in m
• DThickness in m
• Apply for rectangular and trapezoidal part
• Weight of concrete, kg Volume x density