Topic : Footing design What is footing?

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.

4. Types of footing
Footing
Spread
Footing
Isolated
Footing
Strip/
Continous
footing
Strap Footing
Combine
footing

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 /𝑞 𝑎
• Factored load(𝑃𝑢)=(1.2*DL + 1.6*LL)
• Net under pressure (𝑞 𝑢) 𝑛𝑒𝑡
= 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,𝑉𝑐=𝑃𝑢-
𝑏+𝑑 ∗(𝑐+𝑑)
144
∗ (𝑞 𝑢) 𝑛𝑒𝑡
• Allowable shear strength, 𝑉𝑎= 4Ø 𝑓′ 𝑐 𝑏0d
• If 𝑉𝑐 > 𝑉𝑎, 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,𝑉𝑐=
𝐵−𝑐
2
− 𝑑 *(𝑞 𝑢) 𝑛𝑒𝑡
• Allowable Beam-shear strength, 𝑉𝑎= 2Ø 𝑓′ 𝑐bd
• If 𝑉𝑐 > 𝑉𝑎, Increase t.
Critical section

11. Reinforcement calculation
• For square footing: Same re-bar in both direction.
• 𝑀𝑙𝑜𝑛𝑔=𝑀𝑠ℎ𝑜𝑟𝑡=
𝑤𝑙2
2
• 𝐴𝑠𝑙𝑜𝑛𝑔=𝐴𝑠𝑠ℎ𝑜𝑟𝑡=
𝑀
0.9𝑓𝑦(𝑑−
𝑎
2
)
• a=
𝐴 𝑠 𝑓𝑦
.85𝑓′ 𝑐 𝑏
• 𝐴 𝑠(𝑚𝑖𝑛)=200bd/𝑓𝑦

12. Reinforcement calculation
• For rectangular footing:
• M=
𝑤𝐿2
2
• As=
𝑀
0.9𝑓𝑦(𝑑−
𝑎
2
)
• a=
𝐴 𝑠 𝑓𝑦
.85𝑓′ 𝑐 𝑏
• 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
𝐿𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑓𝑜𝑜𝑡𝑖𝑛𝑔
𝑊𝑖𝑑𝑡ℎ 𝑜𝑓 𝑓𝑜𝑜𝑡𝑖𝑛𝑔
+1
∗ 𝑡𝑜𝑡𝑎𝑙 𝐴 𝑠(short)
• Rest (T𝒐𝒕𝒂𝒍 𝑨 𝒔(𝒔𝒉𝒐𝒓𝒕) − 𝑨 𝒔(𝒃𝒂𝒏𝒅)) 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:
 𝑉𝑐=load from column- area of critical section ∗

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=(𝑞 𝑢) 𝑛𝑒𝑡
*B*distance
• As=
𝑀
0.9𝑓𝑦(𝑑−
𝑎
2
)
• a=
𝐴 𝑠 𝑓𝑦
.85𝑓′ 𝑐 𝑏
• 𝐴 𝑠(𝑚𝑖𝑛)=200bd/𝑓𝑦
B

19. Top rebar:
• M=(M+)-(M-)
• As=
𝑀
0.9𝑓𝑦(𝑑−
𝑎
2
)
• a=
𝐴 𝑠 𝑓𝑦
.85𝑓′ 𝑐 𝑏
• 𝐴 𝑠(𝑚𝑖𝑛)=200bd/𝑓𝑦
Rebar for short direction:
Have to calculate both for interior &
exterior column
 𝑞 𝑢=
𝑙𝑜𝑎𝑑
𝐵∗(𝑏+1.5𝑑)
 As=
𝑀
0.9𝑓𝑦(𝑑−
𝑎
2
)
 a=
𝐴 𝑠 𝑓𝑦
.85𝑓′ 𝑐 𝑏
, 𝐴 𝑠(𝑚𝑖𝑛)=200bd/𝑓𝑦
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 ] bd2
ρ= As / bd, =  fy / fc’, Mn = Kn bd2, Kn =  fc’ [ 1 – 0.59 ]
Mu =  Mn =  Kn bd2
• As=
𝑀𝑢
0.9𝑓𝑦(𝑑−
𝑎
2
)
• a=
𝐴 𝑠 𝑓𝑦
.85𝑓′ 𝑐 𝑏
• 𝐴 𝑠(𝑚𝑖𝑛)=
200bd
𝑓𝑦

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=
𝑙𝑜𝑎𝑑
𝐶𝑜𝑟𝑟𝑒𝑠𝑝𝑜𝑛𝑑𝑖𝑛𝑔 𝑎𝑟𝑒𝑎
3. Assume self wt. of foundation(10 to 20% of P)
4. Total load=P+ SW
5. Width of footing=
𝑇𝑜𝑡𝑎𝑙 𝑙𝑜𝑎𝑑
𝑞 𝑠𝑜𝑖𝑙∗𝐴𝑟𝑒𝑎
6. Check: (volume of foundation*unit wt. of brick)< self wt.
(ok),Otherwise increase width.
Brick footing