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Footing design

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Shubham .
Shubham .

Footings transfer structural loads from a building to the ground. This document discusses various types of footings and their design procedures. Spread footings are the most common type and are proportioned to have an area large enough that soil and building settlement will be minimized. The general design process involves checking that factored loads are less than the soil's allowable bearing capacity and footing thickness is sufficient to resist punching and beam shear. Reinforcement is calculated and placed to resist bending stresses. Combined and strap footings are also discussed along with their unique design considerations. Brick footings can be used for small residential loads.

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Topic : Footing design Roll: 1012800099 Name: SHUBHAM Section: B Year & Semester: 7
What is footing?? It is the lower part of the foundation which is constructed below the Ground level in solid surface.
Purpose of footing Transfer the live and dead loads of the structure to the soil over a large enough area so that neither the soil nor the building will move. Resist settlement & lateral load.
Types of footing Footing Spread Footing Isolated Footing Strip/ Continous footing Strap Footing Combine footing
Data/information Required for footing design 1. Allowable Bearing capacity of soil(Building codes of various organizations in different countries gives the allowable bearing capacity that can be used for proportioning footings) 2. Total load(live load + dead load) 3. Length and width of column
General Design procedure of Spread footing Method USD or WSD Economical Step:(USD) Area=Total load / Factored load()=(1.2*DL + 1.6*LL) Net under pressure ( ) = Factored load/Area
Punching shear : The column rested on the footing tends to punch through the footing due to the shear stress that act around the footing, the fracture forms a truncated pyramid shaped failure section. truncated pyramid
Punching shear : Assume thickness (t)of footing d= t-3 Nominal punching-shear strength,=- + (+) 144 ( ) Allowable shear strength, = 4 0d If > , Increase t.
Beam shear Shear failure can also occur, as in a beam or one way slab at a section a distance d from the face of column.
Beam shear Nominal Beam-shear strength,= 汲 2 *( ) Allowable Beam-shear strength, = 2 bd If > , Increase t. Critical section
Reinforcement calculation For square footing: Same re-bar in both direction. == ゐ2 2 基=基= 0.9( 2 ) a= .85 ()=200bd/
Reinforcement calculation For rectangular footing: M= ゐ2 2 As= 0.9( 2 ) a= .85 L should be changed for Long and short direction. In case of short direction calculate As(band),which is provided along the band-width length. Band width is always shortest dimension. As(bend)= 2 瑞≠ ＄ ≠ ＄ +1 ＄＄ (short) Rest (T () ()) of the re-bar is provided at remaining portion.
Reinforcement placement Fig: Transverse and longitudinal section
Combined footing If two columns are so close to each other that their individual footing overlaps then they are combined to form one. Combined footing is also provided if one footing goes beyond the property line. The load is evenly distributed. A combine footing may be rectangular or trapezoidal in plan
Design of combined footing Punching is to be checked for both interior and exterior footing Punching shear: =load from column- area of critical section
Beam shear: Design of combined footing
Design of combined footing
Reinforcement calculation` Have to provide both top and bottom reinforcement Rebar for long direction: Bottom -M=( ) *B*distance As= 0.9( 2 ) a= .85 ()=200bd/ B
Top rebar: M=(M+)-(M-) As= 0.9( 2 ) a= .85 ()=200bd/ Rebar for short direction: Have to calculate both for interior & exterior column = 汲(+1.5) As= 0.9( 2 ) a= .85 , ()=200bd/ M+ M-
Fig: Reinforcement placement
Strap footing Strap footing consists of two isolated footings connected with a structural strap or a lever.
Design of strap footing Beam design: Mn = 0.85fc ba (d a/2) or Mn = As fy (d a/2) = bd fy [ d (dfyb / 1.7fc) ] = fc [ 1 0.59 ] bd2 = As / bd, = fy / fc, Mn = Kn bd2, Kn = fc [ 1 0.59 ] Mu = Mn = Kn bd2 As= 0.9( 2 ) a= .85 ()= 200bd
Brick footing Used in case of small load. Fig: Brick Foundation
Design steps: 1. Calculate dead load from slab,floor,wall,beam etc.(P) 2. Stress on brick= 駒 3. Assume self wt. of foundation(10 to 20% of P) 4. Total load=P+ SW 5. Width of footing= ＄ 基 6. Check: (volume of foundation*unit wt. of brick)< self wt. (ok),Otherwise increase width. Brick footing
Footing design
Footing design

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Footing design

  • 1. Topic : Footing design Roll: 1012800099 Name: SHUBHAM Section: B Year & Semester: 7
  • 2. What is footing?? It is the lower part of the foundation which is constructed below the Ground level in solid surface.
  • 3. Purpose of footing Transfer the live and dead loads of the structure to the soil over a large enough area so that neither the soil nor the building will move. Resist settlement & lateral load.
  • 5. Data/information Required for footing design 1. Allowable Bearing capacity of soil(Building codes of various organizations in different countries gives the allowable bearing capacity that can be used for proportioning footings) 2. Total load(live load + dead load) 3. Length and width of column
  • 6. General Design procedure of Spread footing Method USD or WSD Economical Step:(USD) Area=Total load / Factored load()=(1.2*DL + 1.6*LL) Net under pressure ( ) = Factored load/Area
  • 7. Punching shear : The column rested on the footing tends to punch through the footing due to the shear stress that act around the footing, the fracture forms a truncated pyramid shaped failure section. truncated pyramid
  • 8. Punching shear : Assume thickness (t)of footing d= t-3 Nominal punching-shear strength,=- + (+) 144 ( ) Allowable shear strength, = 4 0d If > , Increase t.
  • 9. Beam shear Shear failure can also occur, as in a beam or one way slab at a section a distance d from the face of column.
  • 10. Beam shear Nominal Beam-shear strength,= 汲 2 *( ) Allowable Beam-shear strength, = 2 bd If > , Increase t. Critical section
  • 11. Reinforcement calculation For square footing: Same re-bar in both direction. == ゐ2 2 基=基= 0.9( 2 ) a= .85 ()=200bd/
  • 12. Reinforcement calculation For rectangular footing: M= ゐ2 2 As= 0.9( 2 ) a= .85 L should be changed for Long and short direction. In case of short direction calculate As(band),which is provided along the band-width length. Band width is always shortest dimension. As(bend)= 2 瑞≠ ＄ ≠ ＄ +1 ＄＄ (short) Rest (T () ()) of the re-bar is provided at remaining portion.
  • 13. Reinforcement placement Fig: Transverse and longitudinal section
  • 14. Combined footing If two columns are so close to each other that their individual footing overlaps then they are combined to form one. Combined footing is also provided if one footing goes beyond the property line. The load is evenly distributed. A combine footing may be rectangular or trapezoidal in plan
  • 15. Design of combined footing Punching is to be checked for both interior and exterior footing Punching shear: =load from column- area of critical section
  • 16. Beam shear: Design of combined footing
  • 18. Reinforcement calculation` Have to provide both top and bottom reinforcement Rebar for long direction: Bottom -M=( ) *B*distance As= 0.9( 2 ) a= .85 ()=200bd/ B
  • 19. Top rebar: M=(M+)-(M-) As= 0.9( 2 ) a= .85 ()=200bd/ Rebar for short direction: Have to calculate both for interior & exterior column = 汲(+1.5) As= 0.9( 2 ) a= .85 , ()=200bd/ M+ M-
  • 21. Strap footing Strap footing consists of two isolated footings connected with a structural strap or a lever.
  • 22. Design of strap footing Beam design: Mn = 0.85fc ba (d a/2) or Mn = As fy (d a/2) = bd fy [ d (dfyb / 1.7fc) ] = fc [ 1 0.59 ] bd2 = As / bd, = fy / fc, Mn = Kn bd2, Kn = fc [ 1 0.59 ] Mu = Mn = Kn bd2 As= 0.9( 2 ) a= .85 ()= 200bd
  • 23. Brick footing Used in case of small load. Fig: Brick Foundation
  • 24. Design steps: 1. Calculate dead load from slab,floor,wall,beam etc.(P) 2. Stress on brick= 駒 3. Assume self wt. of foundation(10 to 20% of P) 4. Total load=P+ SW 5. Width of footing= ＄ 基 6. Check: (volume of foundation*unit wt. of brick)< self wt. (ok),Otherwise increase width. Brick footing