LECT 5 UPDATE.pdf

BUILDING
SUB-STRUCTURE
BSR402 BUILDING CONSTRUCTION AND MATERIALS I
JULAIDA KALIWON,PhD

INTRODUCTION
Generally, there are two major
components of a building project which
are substructure and superstructure.
The substructure is the part of the building
that is built below the ground level
whereas superstructure is the part of the
structure that is constructed above the
ground level.

Superstructure
- The superstructure is the
portion of a building which
is constructed above the
ground level and it serves
the purpose of structure’s
intended use. It includes
columns, beams, slab
upwards including all
finishes, door and window
schedules, flooring, roofing,
lintels, and parapets.

Substructure
- The substructure is the lower part of a building which is constructed
below the ground level. The function of substructure is the transfer
of loads from the superstructure to the underlying soil. So, the
substructure is in direct contact with supporting soil. Substructure
involves footing and plinth of a building.
- An experienced structural engineer should generate plans and works
for the substructure of a building project. Added to that, structural
engineers are responsible for computing stresses and loads which are
required to be supported by the building under consideration. Lastly,
structural engineers need to comprehend how to incorporate support
beams, columns and foundations into the substructure plans.

SUBSTRUCTUREAND SUPERSTRUCTURE

DIFFERENCESBETWEENSUPERSTRUCTUREAND SUBSTRUCTURE
OF A BUILDING
SUPERSTRUCTURE SUBSTRUCTURE
Part of a building that constructed above
ground level
Portion of a building that constructed below
ground level
It serves the purpose of building’s intended
use
It transfers loads received from superstructure
to supporting soil
Superstructure elements include walls,
columns, beams, doors and windows, etc.
Elements of substructure include foundation
and plinth.

BUILDING
FOUNDATION
INTRODUCTION
- Foundation is the lowest part of the building or the civil structure that is
in direct contact with the soil which transfers loads from the structure
to the soil safely. Generally, the foundation can be classified into two,
namely shallow foundation and deep foundation.
- The foundation is a system comprised of foundation wall, footings and soil.
The prime purpose of an efficient structural foundation system is to
transmit the building loads directly to the soil without exceeding the
bearing capacity of the soil.

THE PRINCIPLES OF
FOUNDATIONS
The basic function of a foundation is to intercept the load exerted
by a building structure and transfer this load to the supporting soil
in such a way that the building will not sink into the ground
(subside) Structural stability is normally achieved in either of two
ways, or indeed a combination of both.
❑Spread the load exerted by the building over a sufficiently
wide area to prevent the supporting groundbeing
overstressed
❑Divert or transfer the load to a strata, deep in the ground,
which is capable of supporting the imposed load without
failure

TYPICAL SUBSOIL BEARING CAPACITIES

FUNCTION OF
FOUNDATION
Based on the purposes of foundation in
construction, the main functions of the
foundation can be enlisted as below:
➢ Provide overall lateral stability for the
structure - avoid differential settlement.
➢ Foundation serve the function of providing a
level surface for the constructionof
substructure
➢ Load Distribution is carried out evenly
➢ The load intensity is reduced to be within
the safe bearing capacity of the soil
➢ The soil movement effect is resisted and
prevented
➢ Scouring and the undermining issues are
solved by the construction of foundation
➢ To act as a platform for superstructure

Requirements of a Good Foundation
The design and the construction of a well-
performing foundation must possess some
basic requirements that must not be ignored.
They are:
The design and the construction
of the foundation is done such
that it can sustain as well as
transmit the dead and the
imposed loads to the soil. This
transfer has to be carried out
without resulting in any form of
settlement that can result in any
form of stability issues for the
structure.
Differential settlements can be
avoided by having a rigid base
for the foundation. These issues
are more pronounced in areas
where the superimposed loads
are not uniform in nature.
Based on the soil and area it is
recommended to have a deeper
foundation so that it can guard
any form of damage or distress.
These are mainly caused due to
the problem of shrinkage and
swelling because of
temperature changes.
The location of the foundation
chosen must be an area that is
not affected or influenced by
future works or factors.

Basic Design Procedure
This can
be
considered
as a series
of steps or
stages
1. Assessment of site conditions in the context of the site
and soil investigation report.
2. Calculation of anticipated structural loading(s).
3. Choosing the foundation type taking into consideration.
• Soil conditions.
• Type of structure.
• Structural loading(s).
• Economic factors.
• Time factors relative to the proposed contract period.
• Construction problems.
4. Sizing the chosen foundation in the context of loading(s),
ground bearing capacity and any likely future movements of
the building or structure.

WHAT TYPE OF LOADS MUST A
FOUNDATION SUPPORT ?
Dead load:
- Is the sum of all material loads from the
frame, the floors, roof, walls, electrical and
mechanical equipment and the foundation
itself.
Live load:
- Is the sum of the weight of the people,
furniture and equipment that people use. Live
loads also include snow ice and water that
are on the roof.
Wind load:
- Is the load form any direction; lateral,
downwardand uplift of the foundation
Pressure:
• Is the horizontal pressure of the earth
and water against the basement walls.
Thrust:
• In some buildings the horizontal thrusts
from arches, rigid frames, domes, vaults
or tensile structures.
Buoyancy:
• In some cases the buoyant uplift from
underground water.
Seismic:
• Horizontal and vertical forces caused by
the motion of the ground relative to the
building.

TYPES OF SOIL
Rock
The hardest rock is
igneous e.g. granite and
basalt. Normally they have
a high safe bearing
capacity, 2-3 times that of
sedimentary rocks and 25-
30 times that of clays and
sands. Generally bedrock
is an excellent base to
build on but unfortunately
the cost of levelling and
the cost of excavating
service trenches
outweighs the initial
advantage of a good
natural base.
Course grained
non-cohesivesoils
Gravels and sands come
under this heading. When
loaded they shear if
unconfined. The particles
slide over each other at an
angle known as the angle
of internal friction.
Fine grained
cohesive soils
These include clays and
silts. The major problem
with these types of soil is
that their nature changes
with the level of moisture
in the soil. When the soil
drys out they shrink, but
when the moisture content
is increased the soil
swells. When water
trapped in the soil freezes
it can cause vertical heave
Organicsoils
These include peat, loam
and mud. Generally
unsuitable for building on.
Normally 150-200 mm
thick. Such soil (top soil) is
usually removed before
building begins.
Made up soil
As the stock of quality
building land diminishes,
poorer ground is often
used. Today made up
ground is being utilised.
Extreme care should be
taken to ensure that such
land is properly
investigated.

PROBLEMS
POSED BY
THE NATURE
OF THE
GROUND

INTRODUCTION Foundations are classified as shallow and deep
foundations. The words shallow and deep refer to the
depth of soil in which the foundation is made. Shallow
foundations can be made in depths of as little as 3ft
(1m), while deep foundations can be made at depths of
60 - 200ft (20 - 65m). Shallow foundations are used for
small, light buildings, while deep ones are for large, heavy
buildings..

SHALLOW
FOUNDATION
Shallow foundations are also called spread footings or open footings. The
'open' refers to the fact that the foundations are made by first excavating all the
earth till the bottom of the footing, and then constructing the footing. During the
early stages of work, the entire footing is visible, and is therefore called an open
foundation. The idea is that each footing takes the concentrated load of the
column and spreads it out over a large area, so that the actual weight on the
soil does not exceed the safe bearing capacity of the soil.
There are several kinds of shallow footings:
❑ Individual footing or isolated footing
❑ Combined footing
❑ Strip foundation
❑ Raft or mat foundation

Individual Footing
or Isolated Footing
- Individual footing or an isolated footing is the most
common type of foundation used for building construction.
This foundation is constructed for a single column and
called a pad foundation.
- The shape of individual footing is square or rectangle and
is used when loads from the structure is carried by the
columns. Size is calculated based on the load on the
column and the safe bearing capacity of soil. Rectangular
isolated footing is selected when the foundation
experiences moments due to the eccentricity of loads or
due to horizontal forces.

Combined Footing
❑Combined footing is constructed when two or more columns are
close enough and their isolated footings overlap each other. It is a
combinationof isolated footings, but their structural design
differs.
❑The shape of this footing is a rectangle and is used when loads
from the structure is carried by the columns.

Strip
Foundation
- Known as spread footingsor Strip footingsand Wall
footings. Spread footingsare those whose base is
wider than a typical load-bearing wall foundations. The
wider base of this footing type spreads the weight
from the building structureover more area and
providesbetter stability.
- Spread footingsand wall footingsare used for
individual columns, walls and bridgepierswhere the
bearing soil layer is within 3m (10 feet) fromthe
ground surface. Soil bearing capacity must be
sufficient to supportthe weightof the structureover
the base area of the structure.
- These should not be used on soils where there is any
possibilityof a ground flow of water abovebearing
layer of soil which may result in scour or liquefaction.

Strip Foundation
- Simplest form of foundation is the strip foundation, used to support a load bearing wal.
Main types of strip foundations;

Raft or mat
foundation
❑ Raft or mat foundations are the types of foundation which
are spread across the entire area of the building to support
heavy structural loads from columns and walls.
❑ The use of mat foundation is for columns and walls
foundations where the loads from the structure on columns
and walls are very high. This is used to prevent differential
settlement of individual footings, thus designed as a single
mat (or combined footing) of all the load-bearing elements
of the structure.
❑ It is suitable for expansive soils whose bearing capacity is
less for the suitability of spread footings and wall footings.
Raft foundation is economical when one-half area of the
structure is covered with individual footings and wall
footings are provided.
❑ These foundations should not be used where the
groundwater table is above the bearing surface of the soil.
The use of foundation in such conditions may lead to scour
and liquefaction.

Raft or mat
foundation
- Raft Foundations, also called Mat Foundations, are most
oftenused whenbasementsare to be constructed. Ina
raft, the entire basement floor slab acts as the foundation;
the weight of the building is spread evenlyover the entire
footprint of the building. It is called araft because the
building is like a vessel that 'floats'ina seaof soil.
- Mat Foundations are used wherethe soil is week, and
therefore building loads have to be spreadover alarge
area, or where columns are closely spaced, whichmeans
that if individual footings were used, they wouldtouch
each other.
- A raft foundationis also very good for basements.
Foundations are created by excavating soil inorder to
find strong, compact, undisturbed natural soil that is at
least a fewfeetbelowground level.

Ground Beam
- What is a ground beam? A
concrete ground beam is often
used in place of other foundation
techniques such as concrete
footings. Instead of strip or
trench footings, a concrete slab,
called a beam, is laid on top of
piles or pad foundations.

THANK YOU
SEE NEXT LECTURE ON DEEP FOUNDATION

LECT 5 UPDATE.pdf

More Related Content

Similar to LECT 5 UPDATE.pdf

More from DRJULAIDAKALIWON

Recently uploaded

LECT 5 UPDATE.pdf