3.9 Concrete Foundations - vsvu.sk · PDF filerectangular or trapezoidal in plan. Trapezoidal footing is provided when the load on one of the columns is larger than the other column

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


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.


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.


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.


Punching in Spread Footing

Figure: 3.9.1-1

Shallow Foundations

Figure: 3.9.1-2

Design of Reinforcement

Spread Footing


Figure: 3.9.1-3

Figure: 3.9.1-4


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.


Figure: 3.9.2-1

Figure: 3.9.2-2

Figure: 3.9.2-3


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”.


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


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.


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.


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


Figure: 3.9.6-5

Figure: 3.9.6-6


Figure 3.9.6-3: collapse of unbearable soil


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


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


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


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


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

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3.9 Concrete Foundations - vsvu.sk · PDF filerectangular or trapezoidal in plan. Trapezoidal footing is provided when the load on one of the columns is larger than the other column