Footing design(09.02.03.096)

A short description of footing design.

1. Topic : Footing design Roll:09.02.03.096 Name: Jawad khalil Section: B Year & Semester:4.2

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 T ransfer 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 & l ateral load.

4. Types of footing

5. Data/information Required for footing design 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 ) Total load(live load + dead load) 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, = - Allowable shear strength, = 4Ø d 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, = * Allowable Beam-shear strength, = 2Ø bd If , Increase t . Critical section  

11. Reinforcement calculation For square footing: Same re-bar in both direction. = = = = a= =200bd /  

12. Reinforcement calculation For rectangular footing: M = As = a = 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)= (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- Allowable shear strength, = 4Ø d If , Increase t.  

16. Beam shear: Design of combined footing

17. Design of combined footing

18. Reinforcement calculation` Have to provide both top and bottom reinforcement Rebar for long direction: Bottom -M= *B*distance As = a = =200bd /   B

19. Top rebar: M=(M+)-(M-) As = a = =200bd / Rebar for short direction: Have to calculate both for interior & exterior column = As= a= =200bd /   M + M -

20. Fig: Reinforcement placement

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 ] bd 2 ρ = As / bd ,  =  fy / fc’, Mn = Kn bd 2 , Kn =  fc’ [ 1 – 0.59 ] Mu =  Mn =  Kn bd 2 As = a = =  

23. Brick footing Used in case of small load. Fig : Brick Foundation

24. Design steps: Calculate dead load from slab,floor,wall,beam etc.(P) Stress on brick= Assume self wt. of foundation(10 to 20% of P) Total load=P+ SW Width of footing= Check: (volume of foundation*unit wt. of brick)< self wt. (ok),Otherwise increase width.   Brick footing