Types of Foundations - Yolamahmouddahab.yolasite.com/resources/Shallow Foundations.pdf · Types of Foundations ... Design of Combined Footing. 20 Cairo University ... the face of

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Shallow Foundations

Types of Foundations

� Foundations can be classified to two major

categories:

� Shallow.

� Deep.

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Introduction

� If the soil stratum is suitable for supporting the

structural loads from the superstructure is

located at a relatively shallow depth, the

foundation is called shallow.

� If the upper layer soil strata are too weak to

carry the structural loads, the loads may be

transferred to more suitable deeper layers using

deep foundations.

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Requirements for Foundations

� The foundation must be placed at an adequate

depth.

� Stresses should not exceed the soil strength and

total/differential settlement should be within tolerable

limits.

� The foundations must be designed based on the

structural loads.

� Foundations should be constructed from materials

that withstand any harmful chemicals (specially

sulfates and chlorides) in the ground / groundwater.

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Requirements for Foundations

� Foundations should be placed below the top organic

soil, fill materials, old abandoned foundations, & debris.

� Foundations should be placed below surface layers

affected by seasonal temperature or moisture changes

or by erosion.

� Foundations should be placed sufficiently away from

the edge of a sloping ground.

� Foundations should be placed at an acceptable level

with respect to adjacent foundations. The difference in

levels between adjacent foundations should not cause

undesirable overlapping between stresses.

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Types of Shallow Foundations

� Wall (Strip) footing is provided to support a wall in

case the structure is bearing wall type.

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� Isolated footing is used to support one column as in

skeleton type buildings.

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� Isolated footing is used to support two columns.

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� Strap footing is used when isolated footings are

subjected to large eccentric loading as in case of an

edge or exterior footing where the property line limits

the extension if the footing needed to make the

footing concentric with the column it supports.

� The strap beam footing consists of a rigid beam

connecting the exterior footing to an interior footing

in order to transmit the unbalanced shear and

moment.

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� The mat or raft covers the entire area of the

superstructure as it supports all the columns of the

structure.

Flat Plate Flat Plate thickened

under columns

Slab and beam

Flat Plate with pedestals Slabs with basement wall

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Strip Footing

� In the design of footings, the soil pressure is

assumed to be uniform.

� Compute width of footing (B) using the allowable

bearing pressure:

B = Pt / qall(gross)

Or

B = Pnet / qall(net)

Pnet = load at ground surface

Pt = Pnet + weight of footing + soil above foundation

level = approximately 1.1-1.15 Pnet

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Strip (wall) footing

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Strip (wall) footing

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Strip Footing

� Thickness of plain concrete base t (optional) is

typically taken as 20 – 40 cm.

� Determine the projection (x) of the plain concrete

footing. Typically, (x) ranges between (t) and (0.8t).

� Carry out the structural design of the footing i.e.

determine the depth and reinforcement.

� The cover for the reinforcing bars ranges between

50 to 70 mm to protect reinforcement against

chemical attack.

� The minimum diameter of reinforcing bare used in

foundations is 12 mm.

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Strip Footing� Maximum bending moment: (Section I-I : for a

footing with a wall beam)

Where pn

is the net contact stress between the

reinforced concrete footing and the plain concrete base

and is equal to Pn/B1.

If a masonry wall rests directly on the reinforced

concrete footing, the critical section lies at a distance

(b/4) beyond the face of the wall, i.e., midway between

the center of the wall and the face of the wall

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Strip Footing

� Check of shear: (Section II-II)

The shear stress should not exceed the allowable shear

stress of concrete (typically 6 kg/cm2).

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Centrically Loaded Isolated Footing

� Determine the footing area AxB using the allowable

bearing pressure .

A x B = Pt / qall(gross)

Or

A x B = Pnet / qall(net)

Pnet = load at ground surface

Pt = Pnet + weight of footing + soil above foundation

level = approximately 1.1-1.15 Pnet

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� Determine the footing dimensions A and B following

preferably the recommendations given below.

(i) If the column is square or circular in section,

take A equal to B.

(ii) If the column is rectangular in section (a x b),

determine A and B such that the projections

from the column faces are equal in both directions

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� Select the thickness of the plain concrete base

(typically between 0.25-m and 0.80-m).

� Determine the projection of the plain concrete

footing which is typically taken in the range between

(0.8 t) and (t).

� Carry out the structural design of the footing i.e.

determine the depth and reinforcement of the

reinforced concrete base.

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Maximum Bending Moment

pn

= Net contact pressure

� = reduction factor equal to

0.85 which takes into

account the effect of

including the pressure acting

over the common area aekg

in calculating both moments

Mm1

and Mn1

.

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Check of Shear

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Check of Punching

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Eccentrically Loaded Isolated Footing

� Determine the footing dimensions A and B following

preferably the recommendations given below.

(i) If the column is square or circular in section,

take A equal to B.

(ii) If the column is rectangular in section (a x b),

determine A and B such that the projections

from the column faces are equal in both directions

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� Consider a footing subjected to a vertical force Pn, a

horizontal force Q and a moment M. At the foundation

level, the forces acting are :

(i) Vertical force Ptwhich includes the force P

nand

the weight of the footing and the soil above it W.

(ii) Horizontal force Q.

(iii) Moment = W + Q.h = M + Q (t+t1)

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In order to determine the stress under the footing, the

moment may be removed by shifting the vertical load to

a fictitious location with an eccentricity e, where

In the design of an eccentrically loaded footing, the

stress distribution is assumed to be linear.

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1- e < A/6 (i.e. resultant inside middle third), under such

condition the pressure distribution will trapezoidal with

the maximum and minimum intensities of pressure

obtained as follows

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2- e = A/6 (i.e. resultant on the edge of core) the

pressure distribution will be triangular .

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3- e > A/6 (resultant outside middle third)

The entire base of the footing is not considered effective

because soil cannot resist tension. The maximum

pressure is, in this case, given by:

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Eccentrically Loaded Isolated FootingSpecial Considerations

� The maximum soil pressure qmax for all load

combinations ( dead and live loads and moments)

must not exceed 1.2 qall where qall is the allowable

pressure under axial load only.

� If the column loads include crane loads, the entire

footing area should be effective and the pressure

distribution should satisfy the following condition:

qmin ≥ 0.25 qmax

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� For footings subjected to permanent moments, column

could be placed off the center such that the resultant

passes through the centroid of the footing (i.e. e = 0)

and the stress distribution becomes uniform.

� If there are no cranes, a triangular pressure

distribution is permissible, but at least three quarters

of the footing area is effective,

y ≥ 0.75 A

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Eccentrically Loaded Isolated FootingPermanent Eccentricity

Uniform

Stresses

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� Safety against sliding should be checked as follows:

Pt� ≥ 2 Q

Where:

� = coefficient of friction between the footing and soil.

Sliding may be restricted by connecting the footings with

ground beams.

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Design of Combined Footing

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1- Determine the value and position of the resultant R (Rnet

= P1 + P2), Rt = P1 + P2 +W)

Where W = own weight of footing and soil above it which

can be assumed 10-15% of R.

2- Determine the area of the footing using the allowable

bearing capacity

A.B = Rnet/qallnet

OR

A.B = Rt/qallgross

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3- Determine the footing dimensions A and B such that

the centroid of the footing and the center of gravity of the

column loads coincide.

4- Select the thickness (t) of the plain concrete base. A

value ranging between 0.50 m and 1.0 m is usually

chosen. The projection (x) of the plain concrete base can

be taken in the range between (0.8t) and (t).

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5- Draw shear and moment

diagrams for the footing in

the longitudinal direction.

The column loads may be

taken as concentrated loads

for computing shear and

moment diagrams or more

accurately as distributed

loads. In the former case,

the moments at the columns

are calculated.

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6- The maximum positive

moment takes place at the

point of zero shear at a

distance (xm) which can be

calculated as follows:

pn

= net contact stress

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7- Determine the depth of the reinforced concrete footing

necessary to resist the maximum bending moment and

compute the area of steel Asl ; As2 and As3) required to

satisfy bending in the longitudinal direction.

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8-Check punching shear for each of the two columns as

for isolated footings.

If the column is located at the property line, the punching

shear force and stresses will be calculated us follows:

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Column located at the property line

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Design of Combined Footing9- Determine the area of steel in

the short direction considering

each column to be supported by

hidden beams of widths W1

and

W2

respectively;

If the column is located at the

property line,

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Design of Combined Footing9- Calculate the bending

moments at the faces of the

columns

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10- Calculate the required areas of main steel for the five

moments. Use 5 � 12 /m as secondary steel in the form

of upper and lower meshes. Distribute the hidden beam

main reinforcement over the beam width W1

and W2.

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Strap Footings

Frequently, isolated footings cannot be extended beyond

the face of the supported columns as for example when

columns are close to property line. In this case the

isolated footing will be subjected to a uneven eccentricity

which would most probably lead to excessive tilting. In

order to avoid such situation, two alternatives may be

used as illustrated in:

� Combined footing

� Strap footing

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Strap Footings

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Strap Footings

� When the distance between the edge column and the

adjacent interior column is large, it will be more

economical to use a strap footing.

� The strap footing may be regarded as two isolated

footings connected by a member termed a strap beam.

� Its function is to transmit the unbalanced moment from

the unbalanced exterior footing to the interior footing in

order to obtain uniform pressure distribution beneath

both footings.

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Strap Footings

� In design, the strap beam is considered to be a pure

flexural member and does not take soil reaction. The

strap must be sufficiently rigid for the solution to be valid.

� Its inertia should be equal to or greater than twice the

footing inertia to avoid exterior footing rotation.

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Design of Strap Footing

1- Assume a reasonable value for Ae

and determine the

corresponding eccentricity.

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2- Sum moments about the center of the interior column

and obtain the soil reaction beneath the exterior footing.


Where We

= Own weight of footing and soil above it

(approximately 10-15% of Pe).

The other dimension Be

of the footing can thus be determined,

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If B is too large or too small compared to A, steps 1 and 2

can be repeated until satisfactory dimensions are obtained.

Be

should not be greater than 2Ae.

3-Sum moments about the center of the exterior footing

and obtain the soil reaction beneath the other footing.


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The footing dimensions Aiand B

ican then be selected to

satisfy the following equation:


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In order to design the strap beam and the individual

footings, determine the net contact reactions and pressure

between the reinforced concrete footings and the plain

concrete bases.


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� The bending moment and shearing force diagrams can

be drawn, for the strap beam and the structural design

can then be performed. It may be noted that the own

weight of the strap can be included in the calculations.

� The footings are assumed to act as double cantilevers

and can be designed in the same way an for wall

footings.


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Types of Foundations - Yolamahmouddahab.yolasite.com/resources/Shallow Foundations.pdf · Types of Foundations ... Design of Combined Footing. 20 Cairo University ... the face of