View
232
Download
0
Category
Preview:
Citation preview
1
Cairo University
Shallow Foundations
Types of Foundations
� Foundations can be classified to two major
categories:
� Shallow.
� Deep.
Cairo University
2
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.
Cairo University
Cairo University
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.
3
Cairo University
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.
Cairo University
Types of Shallow Foundations
� Wall (Strip) footing is provided to support a wall in
case the structure is bearing wall type.
4
Cairo University
Types of Shallow Foundations
� Isolated footing is used to support one column as in
skeleton type buildings.
Cairo University
Types of Shallow Foundations
� Isolated footing is used to support two columns.
5
Cairo University
Types of Shallow Foundations
� 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.
Cairo University
Types of Shallow Foundations
6
Cairo University
Types of Shallow Foundations
� 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
Cairo University
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
7
Cairo University
Strip (wall) footing
Cairo University
Strip (wall) footing
8
Cairo University
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.
Cairo University
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
9
Cairo University
Strip Footing
� Check of shear: (Section II-II)
The shear stress should not exceed the allowable shear
stress of concrete (typically 6 kg/cm2).
Cairo University
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
10
Cairo University
Centrically Loaded Isolated Footing
Cairo University
Centrically Loaded Isolated Footing
11
Cairo University
Centrically 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
Cairo University
Centrically Loaded Isolated Footing
� 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.
12
Cairo University
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
.
Cairo University
Check of Shear
13
Cairo University
Check of Punching
Cairo University
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
14
Cairo University
Eccentrically Loaded Isolated Footing
Cairo University
Eccentrically Loaded Isolated Footing
� 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)
15
Cairo University
Eccentrically Loaded Isolated Footing
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.
Cairo University
Eccentrically Loaded Isolated Footing
16
Cairo University
Eccentrically Loaded Isolated Footing
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
Cairo University
Eccentrically Loaded Isolated Footing
2- e = A/6 (i.e. resultant on the edge of core) the
pressure distribution will be triangular .
17
Cairo University
Eccentrically Loaded Isolated Footing
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:
Cairo University
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
18
Cairo University
Eccentrically Loaded Isolated FootingSpecial Considerations
� 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
Cairo University
Eccentrically Loaded Isolated FootingPermanent Eccentricity
Uniform
Stresses
19
Cairo University
Eccentrically Loaded Isolated FootingSpecial Considerations
� 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.
Cairo University
Design of Combined Footing
20
Cairo University
Design of Combined Footing
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
Cairo University
Design of Combined Footing
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).
21
Cairo University
Design of Combined Footing
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.
Cairo University
Design of Combined Footing
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
22
Cairo University
Design of Combined Footing
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.
Cairo University
Design of Combined Footing
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:
23
Cairo University
Design of Combined Footing
Column located at the property line
Cairo University
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,
24
Cairo University
Design of Combined Footing9- Calculate the bending
moments at the faces of the
columns
Cairo University
Design of Combined Footing
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.
25
Cairo University
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
Cairo University
Strap Footings
26
Cairo University
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.
Cairo University
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.
27
Cairo University
Design of Strap Footing
1- Assume a reasonable value for Ae
and determine the
corresponding eccentricity.
Cairo University
Design of Strap Footing
28
Cairo University
2- Sum moments about the center of the interior column
and obtain the soil reaction beneath the exterior footing.
Design of Strap 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,
Cairo University
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.
Design of Strap Footing
29
Cairo University
The footing dimensions Aiand B
ican then be selected to
satisfy the following equation:
Design of Strap Footing
Cairo University
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.
Design of Strap Footing
30
Cairo University
� 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.
Design of Strap Footing
Cairo University
Design of Strap Footing
Recommended