Footing

Failures

1.Bearing failure

2.Serviceability failure

3.Excessive total settlementq

FS

qq ultA

P

MCentroidal Axis

P/A

My/I

L/6 L/6

Kern

L

B

6

LeK

I

My

A

Pq

Pc=0

540 lb/ft2

300 lb/ft2

240 lb/ft2

Pc

540 lb/ft2

300 lb/ft2

240 lb/ft2

qn=Pc/A

2 ft of soil @120 lb/ft3

2 ft of concrete @150 lb/ft3

Gross soil pressure =

540+qn lb/ft3

qn=Pc/ANet soil pressure

M

P

Area of footing

aq

LesurchfootingStructureDA

arg,,

Only Dead load and Live load

Including Wind load also

Area of footing

aq

WLesurchfootingStructureDA

33.1

arg,,

A

LstructureDqnu

7.14.1

or

A

WLstructureDqnu

]7.17.14.1[75.0

Isolated footing with axial load and moment

Design Input

•Grade of concrete•Grade of steel•Allowable soil pressure•Column load•Moment•Condition about overburden of soil

Design procedure

•Assume column size•Assume footing size•Total load=wt of footing+column load•Average soil pressure q= total load / footing area

Allowable soil pressure > Average soil pressure

then footing are is ok

Checking the footing area

Finding factored loads

•Factored column load=1.4xdead load+1.7xlive load

•Average factored soil pressure qnet=Factored column load/footing area

•Average dv=total thick – cover - bar dia

•Eccentricity e=Mu/Pu

Solving two equations to get q1 and q2

•Service load equation• Vertical equilibrium

Check for One way shear

•Take the critical section as ‘d’ distance from the

face of the column (as per ACI 318 )

1.find pressure at critical section

2.find Vu

3.find Vu

If Vu>Vu

Check for Two way shear• Take the critical section as ‘d/2’ distance all around

the face of the column (as per ACI 318 )

1. find pressure at critical section

2. Find the value of bo

2.find Vu

3.find Vu is smallest of these

dbfV occ

c'4

2

dbfb

dV oc

o

sc

'2

dbfV occ'4

cu VV Safe against two way shear

Otherwise increase the depth

Check for flexural reinforcement

Max moment occurs at the face of the column

•Find average pressure

•Find required moment Mn=Mu/

•Find required Ast

•Min Ast=0.0018bh

•Max spacing=18 in

Development length

'

04.0

c

yd

f

Afl

Thank You