Topic : Footing design

Roll:09.02.03.096Name: Jawad khalilSection: BYear & Semester:4.2

What is footing??

It is the lower part of the foundation which is constructedbelow 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

SpreadFooting

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

EconomicalStep:(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,=- • Allowable shear strength, = 4Ød• 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,=*• Allowable Beam-shear strength, = 2Øbd• If , Increase t.

Critical section

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

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.

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- Allowable shear strength, = 4Ød If , Increase t.

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=• a=• =200bd/ B

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-

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

ρ= As / bd, = fy / fc’, Mn = Kn bd2, Kn = fc’ [ 1 – 0.59 ] Mu = Mn = Kn bd2

• As=• a=• =

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=3. Assume self wt. of foundation(10 to 20% of P)4. Total load=P+ SW5. Width of footing=6. Check: (volume of foundation*unit wt. of brick)< self wt.

(ok),Otherwise increase width.

Brick footing