12- footing design [compatibility mode]

8/7/2019 12- Footing Design [Compatibility Mode]

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Footing Design Considerations

Footing should be placed below frost line

The centroid of footing must coincide withthe centroid of the applied loads

Allowable soil ressure should not be exceeded

Below grade footing should be equipped with drainage system.Water causes deterioration of the soil bearing pressure

oo ngs s ou e p ace n excava ons ma e n compac e . oo ngs canno e p ace pr orto soil compaction.

For sizing the footing we need to take into account all the loads which produce soilpressure- These are UNFACTORED loads and include:

Dead Load and Live load on the footing

Weight of the footing itself

Weight of the slab on grade (if any)

Weight of surcharge (if any)

When we determine the re uired thickness and a ro riate steel reinforcement we must use

1

FACTORED loads.

Initial estimates of 18-24 inches for footing thickness are appropriate for start up calculations.


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Isolated Footings

The thickness of a square pad or isolated footing is governed by the punching shear which is the

ten ency o t e co umn to pus t roug t e oot ng or t e so pus ng upwar mov ng t e paupward.

)(4 +== dWidthcolumnPerimeterbo

75.04

'

==

dbfv occ

The effective depth d is determined such that the shearforce resulting from the factored loads does not exceedthe equation above:

uc VV =nce e c ness s e erm ne ase on e s ear

value, the bending reinforcement will be calculated verysimilar to a beam.

.

2


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Isolated Footings- Design Procedure

Steps for designing a footing can be summarized as the following:

1) Determine the effective depth. There is usually some information to start with anintelligent estimate an effective depth (d) value of 18-20 inches is a good startingpoint

2) Calculate the effective earth pressure( net earth pressure)

qe = qallowable weigh of the footing and weight of the soil

=e

qallowable = allowable soil pressure orsoil bearing capacity (ksf)

a) Sandy soil: pressures are larger under the center of footing and-

3

good lateral support and can easily be movedb) Clayey soil

c) Design assumption


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3 Based on effective stress find the re uired area.

ifVc > Vu Assumed thickness is adequate

Vu = Shearforce from thefactoredloads

=

af

u

Critical Area = af2

(bc + d)2

Vu

= qu af2

(bc + d)2[ ]

af

d/

d/2

bc

orce

4) Check punching shear to see if the assumed d is adequate

Vc = fc/

bo d bcd/2 d/2

Effective area

qe =area

=Ar

Ar =Load(unfactored)

qe

Critical perimeter

4

Ar = af af

af = sqauresided/2 d/2

d


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5 Select rebars to carr the moment. Assume the footin is made u of beams eachprojected beyond the face of the column acting as an inverted cantilever beam- the criticalsection is at the face of the column

cf baL =

af

Cantilever beam

u beamcantileveraformomentLwM

=

= )(2

2

af

action

fuu

bc//2

af/2

ACI code provisions for footing design and analysis are:

-piles the minimum depth is 12 inches

Minimum cover distance for steel reinforcement is 3 inches

5Reinforcing bars can be in contact and there is nor distance required between thebar layers

12- footing design [compatibility mode]

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