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Sabah Shawkat Cabinet of Structural Engineering 2017
3.9 Concrete Foundations
A foundation is a integral part of the structure which transfer the load of the superstructure
to the soil without excessive settlement. A foundation is that member which provides support
for the structure and it's loads.
It also provides a means by which forces or movements within the ground can be resisted by
the building. In some cases, foundation elements can perform a number of functions: for
example, a diaphragm wall forming part of a basement will usually be designed to carry loading
from the superstructure.
If new foundations are placed close to those of an existing building, the loading on the ground
will increase and movements to the existing building may occur. When an excavation is made,
the stability of adjacent buildings may be threatened unless the excavation is adequately
supported. This is particularly important with sands and gravels which derive their support from
lateral restraint.
The choice of foundation type or the type of foundation selected for a particular structure
is influenced by the following factors:
1. The imposed loads or deformations, the magnitude of the external loads
2. Ground conditions, the strength and compressibility of the various soil data
3. The position of the water table
4. Economics
5. Buildability, and the depth of foundations of adjacent structures
6. Durability.
Figure: 3.9-1 Foundations of tall building
Sabah Shawkat Cabinet of Structural Engineering 2017
An essential requirement in foundations is the evaluation of the load which a structure can
safely bear. The types of foundation generally adopted for building and structures are spread
(pad), strip, balanced and cantilever or combined footings, raft and pile foundations.
For example, strip footings are usually chosen for buildings in which relatively small loads
are carried mainly on walls. When the spread footings occupy more than half the area covered
by the structure and where differential settlement on poor soil is likely to occur a raft foundation
is found to be more economical. Pad footings, piles or pile groups are more appropriate when
the structural loads are carried by columns. If differential settlements must be tightly controlled,
shallow strip or pad footings (except on rock or dense sand) will probably be inadequate so
stiffer surface rafts or deeper foundations may have to be considered as alternatives.
This type of foundation viewed as the inverse of a one-storey beam, slab and column
system. The slab rests on soil carrying the load from the beam/column system which itself
transmits the loads from the superstructure.
Figure: 3.9-2
Types of foundations
These are generally supporting columns and may be square or rectangular in plan and
in section, they may be of the slab, stepped or sloping type. The stepped footing results in
a better distribution of load than a slab footing. A sloped footing is more economical although
constructional problems are associated with the sloping surface. The isolated spread footing in
plan concrete has the advantage that the column load is transferred to the soil through dispersion
in the footing. In reinforced concrete footings, i.e. pads, the slab is treated as an inverted
cantilever bearing the soil pressure and supported by the column. Where a two-way footing is
provided it must be reinforced in two directions of the bending with bars of steel placed in the
bottom of the pad parallel to its sides.
Sabah Shawkat Cabinet of Structural Engineering 2017
Foundations under walls or under closely spaced rows of columns sometimes require a specific
type of foundation, such as cantilever and balanced footings and strip footings.
Pad footing
Square or rectangular footing supporting a single column.
Strip footing
Long footing supporting a continuous wall.
Combined footing
Footing supporting two or more columns.
Balanced footing
Footing supporting two columns, one of which lies at or near one end.
Raft
Foundation supporting a number of columns or loadbearing walls so as to transmit
approximately uniform loading to the soil.
Pile cap
Foundation in the form of a pad, strip, combined or balanced footing in which the forces
are transmitted to the soil through a system of piles.
The plan area of the foundation should be proportioned on the following assumptions:
a. All forces are transmitted to the soil without exceeding the allowable bearing pressure
b. When the foundation is axially loaded, the reactions to design loads are uniformly
distributed per unit area or per pile. A foundation may be treated as axially loaded if the
eccentricity does not exceed 0.02 times the length in that direction
c. When the foundation is eccentrically loaded, the reactions vary linearly across the
footing or across the pile system. Footings should generally be so proportioned that zero
pressure occurs only at one edge. It should be noted that eccentricity of load can arise
in two ways: the columns being located eccentrically on the foundation; and/or the
column transmitting a moment to the foundation. Both should be taken into account and
combined to give the maximum eccentricity.
d. All parts of a footing in contact with the soil should be included in the assessment of
contact pressure
e. It is preferable to maintain a reasonably similar pressure under all foundations to avoid
significant differential settlement.
Sabah Shawkat Cabinet of Structural Engineering 2017
3.9.1 Shallow Foundations
A shallow foundation distributes loads from the building into the upper layers of the
ground. Shallow foundations are susceptible to any seismic effect that changes the ground
contour, such as settlement or lateral movement. Such foundations are suitable when these
upper soil layers have sufficient strength (‘bearing capacity’) to carry the load with an
acceptable margin of safety and tolerable settlement over the design life.
The different types of shallow foundation are:
a) Strip footing
b) Spread or isolated footing
c) Combined footing Strap or cantilever footing
d) Mat or raft Foundation.
Sabah Shawkat Cabinet of Structural Engineering 2017
Punching in Spread Footing
Figure: 3.9.1-1
Shallow Foundations
Figure: 3.9.1-2
Design of Reinforcement
Spread Footing
Sabah Shawkat Cabinet of Structural Engineering 2017
Figure: 3.9.1-3
Figure: 3.9.1-4
Sabah Shawkat Cabinet of Structural Engineering 2017
Figure: 3.9.1-5
Figure: 3.9.1-6
3.9.2 Strap Footing
It consists of two isolated footings connected with a structural strap or a lever, as shown
in figure 3.9.2-1. The strap connects the footing such that they behave as one unit. The strap
simply acts as a connecting beam. A strap footing is more economical than a combined footing
when the allowable soil pressure is relatively high and distance between the columns is large.
Sabah Shawkat Cabinet of Structural Engineering 2017
Figure: 3.9.2-1
Figure: 3.9.2-2
Figure: 3.9.2-3
Sabah Shawkat Cabinet of Structural Engineering 2017
3.9.3 Combined Footing
It supports two columns as shown in fig. 3.9.3-1. It is used when the two columns are
so close to each other that their individual footings would overlap. A combine footing may be
rectangular or trapezoidal in plan. Trapezoidal footing is provided when the load on one of the
columns is larger than the other column.
Figure: 3.9.3-1 Combined Footing
3.9.4 Strip/continuous footings
A strip footing is another type of spread footing which is provided for a load bearing
wall. A strip footing can also be provided for a row of columns which are so closely spaced that
their spread footings overlap or nearly touch each other. In such a cases, it is more economical
to provide a strip footing than to provide a number of spread footings in one line. A strip footing
is also known as “continuous footing”.
Sabah Shawkat Cabinet of Structural Engineering 2017
Figure: 3.9.4-1
A traditional strip foundation consists of a minimum thickness of 150 mm of concrete
placed in a trench, typically 0.8–1 m wide. Reinforcement can be added if a wider strip is
required to bridge over soft spots at movement joints or changes in founding strata.
Figure: 3.9.4-2
3.9.5 Mat or Raft footings
It is a large slab supporting a number of columns and walls under entire structure or a
large part of the structure. A mat is required when the allowable soil pressure is low or where
the columns and walls are so close that individual footings would overlap or nearly touch each
other. Mat foundations are useful in reducing the differential settlements on non-homogeneous
soils or where there is large variation in the loads on individual columns.
Figure: 3.9.5-1
Sabah Shawkat Cabinet of Structural Engineering 2017
Figure: 3.9.5-2
3.9.6 Pile foundations
Deep foundations are used when the soil at foundation level is inadequate to support the
imposed loads with the required settlement criterion. Where the bearing capacity of the soil is
poor or the imposed load are very heavy, piles, which may be square, circular or other shapes
are used for foundations. If no soil layer is available, the pile is driven to a depth such that the
load is supported through the surface friction of the pile. The piles can be precast or cast in situ.
Deep foundations act by transferring loads down to competent soil at depth and/or by
carrying loading by frictional forces acting on the vertical face of the pile. Diaphragm walls,
contiguous bored piles and secant piling methods are covered later in this chapter.
Short-bored piles have been used on difficult ground for low-rise construction for many years.
They can be designed to carry loads with limited settlements, or to reduce total or differential
settlements. They can have bases that are flat, pointed or bulbous, and shafts that are vertical or
raked. In some circumstances, piles can be constructed of other materials, such as timber or
plastics.
Piled walls or sheet piles are used to resist lateral movements, such as in forming a basement.
Sabah Shawkat Cabinet of Structural Engineering 2017
The piling technique used to install the piles will be determined by the ground conditions,
loading requirements for the final pile as well as other factors such as access or proximity to
other buildings and the need for noise reduction.
Pile types
There are two basic types of piles:
● cast-in-place (or replacement) piles and
● driven (or displacement) piles.
Figure: 3.9.6-1
Figure: 3.9.6-2
Piles are individual columns, generally constructed of concrete or steel, that support
loading through a combination of friction on the pile shaft and end-bearing on the pile toe. The
distribution of load carried by each mechanism is a function of soil type, pile type and
settlement. They can also be used to resist imposed loading caused by the movement of the
surrounding soil, such as vertical movements of shrinking and swelling soils. Piles can be
installed vertically or may be raked to support different loading configurations.
Sabah Shawkat Cabinet of Structural Engineering 2017
Figure: 3.9.6-3
All pile caps should generally be reinforced in two orthogonal directions on the top and
bottom faces and the amount of reinforcement should not be less than 0.0015bh in each
direction. The bending moments and the reinforcement should be calculated on critical sections
at the column faces, assuming that the pile loads are concentrated at the pile centres. This
reinforcement should be continued past the piles and bent up vertically to provide full
anchorage past the centreline of each pile.
Figure: 3.9.6-4
Sabah Shawkat Cabinet of Structural Engineering 2017
Figure: 3.9.6-5
Figure: 3.9.6-6
Sabah Shawkat Cabinet of Structural Engineering 2017
Figure 3.9.6-3: collapse of unbearable soil
Sabah Shawkat Cabinet of Structural Engineering 2017
Figure 3.9.6-4: Main reinforcement in slab foundation
Example 3.9-1: Assessment of slab foundation to punching
Depth of the reinforced slab foundation:
Tensile strength of concrete:
Width of column:
Height of column:
Design strength of reinforcement:
Figure: 3.9.1-1
Perimeter of critical cross-section:
hd 80 cm
fctm 0.9 MPa
bs 50 cm
hs 40 cm
fyd 375 MPa
ucr bs hd hs hd 2 ucr 5m
Sabah Shawkat Cabinet of Structural Engineering 2017
Shearing force carrying by concrete:
Required surface area of reinforcement to punching:
Reinforcement diameter:
Number of profiles:
Figure: 3.9.1-2
Data of rolled I profiles:
Qbu 0.42 hd fctm ucr Qbu 1512kN P1 2700 kN
P P1 Qbu P 1188kN
AsbP
0.86 fyd Asb 0.00368372m
2
25 mm As1
2
4 As1 0.00049087m
2
nAsb
As1 n 7.504 Q 0.42 hd fctm ucr Q 1512kN
I28
A1 6.10 103 mm
2 J1y 75.8 106 mm
4 h1 280 mm
Sabah Shawkat Cabinet of Structural Engineering 2017
The total moment of inertia of composite section:
b1 119 mm b2 119 mm
b3 119 mm
I34
A1 8.67 103 mm
2 J1y 157 106 mm
4
h1 340 mm
b1 137 mm b2 137 mm
b3 137 mm
I38
A1 10.7 103 mm
2 J1y 240 106 mm
4 h1 380 mm
b1 149 mm b2 149 mm b3 149 mm h2 20 mm h3 20 mm
L 1.45 m p1P
8 p1 148.5kN M p1 0.75 L M 161.49375m kN
A2 b2 h2 A3 b3 h3 J2b2 h2
3
12 J3
b3 h33
12
e2
A1h1
2h2
A2h2
2
A3h3
2h1 h2
A1 A2 A3 e2 0.21m
H h1 h2 h3 H 0.42m e1 H e2
e1 0.21m
a1h1
2h2 e2 a2 e2
h2
2 a3 H
h3
2 e2 a1 0m
J J1y J2 J3 A1 a12 A2 a2
2 A3 a32 J 0.0004786m
4
J
J1y1.99416111
Sabah Shawkat Cabinet of Structural Engineering 2017
Figure: 3.9.1-3
Section modulus:
Stress control:
WdJ
e1 Wd 0.00227904m
3 WhJ
e2 Wh 0.00227904m
3
dM
Wd d 70.86 MPa s 210 MPa
hM
Wh h 70.8604 MPa s 210 MPa
Sabah Shawkat Cabinet of Structural Engineering 2017
Example 3.9-2: Determination of the design bearing capacity of the soil at depth
dp =1,5 m
Soli classification F6:
Bearing coefficient of the soil:
Base area of the footing: Width: bp = 1 m Length: Lp = 6 m
Coefficient of the shape of the footing:
Design bearing capacity of the soil:
cef 16 kPa ef 21 deg cdcef
2d
ef
m
Nd tan 45 degd
2
2
e tan d
Nb 1.5 Nd 1 tan d Nc 2
mef
ef 4 deg 1 21kN
m3
2 1
sc 1 0.2bp
lp sd 1
bp
lpsin d sb 1 0.3
bp
lp dc 1 0.1
dp
bp
dd 1 0.1dp
bpsin 2 d id 1 ic 1 ib 1
Rd cd Nc sc dc ic 1 dp Nd sd dd id 2bp
2 Nb sb db ib Rd 243.077 k