Foundations types- Shallow foundations.
In the last article (Foundations types and selection of suitable foundation) we explained foundations types. In this article we will speak about one most common foundation types which is Sallow Foundations.
- Shallow Foundation.
It the most common foundations and include:
- Isolated footing.
Foundations with separate footings are more common know
types of foundations and are often used as a substrate or support for a single
column, which is either square or rectangular.
Accordingly, when column is rested on a square or
rectangular footing, this footing receives a backlash from the bottom to the
top from the soil as a result of carrying the column from top to bottom, and
accordingly, the dimensions of this footing will fit with both from the value
of the column load down and value of the maximum permissible stress of the soil
( the permissible vertical reaction of the soil) so, the effect of soil
reaction upward on the footing causes it to form and bend in the form of a dish
and therefore, it will generate in this footing bending moment and shearing
forces at the critical sectors corresponding to these forces.
Therefore, any isolated footing must be have two layers of
rebars in two perpendicular directions near the bottom surface of the footing
and parallel to the edge of the footing in order to resist the bending moment in
addition to choosing an appropriate depth corresponding to the amount of steel
reinforcement used in both directions to resisting other types of stress that
will be generated from both shear forces such as vertical shear stress, maximum
punching shear, bond stress, etc.
The figure below show square or rectangular isolated
footing.
- Wall footing.
This type is often used as a substrate for loads located on
load-bearing walls, which consist of a long narrow strip. Therefore, under the
influence of soil pressure and reaction from the bottom to the top of the base,
it will bend and form only in the short direction of this segment and
perpendicular to the wall, which requires the necessity of placing and arranging
the main rebar in the short direction of this segment near to its bottom .
In
addition, to this reinforcement must be placed secondary steel reinforcement in
the longitudinal direction of this slide for resistance to high stress
requirements generated both shrinkage and temperature stresses with the benefit
of this secondary steel for tie and adjust main steel reinforcement spacing.
- Combined Footing.
Combined footing used as a substrate for the transfer of
loads on two Contiguous columns where there is congruence and overlap between
each footing if they are separated or when the flap of one of them exceeds the
boundaries of the neighbor and the ownership with respect to the site.
This type of foundations can be rectangular in shape or in
the form of a trapezoid.
It is worth noting that the rules grouped in the form of a
rectangle in common use for most engineers due to the ease of design and
implementation, and this should be noted that the choice of assembled
rectangular rules may be impractical in some circumstance where the restriction
and the possible limitation of the dimensions of the base and its extension
from the two sides of the two columns of this base besides the big difference
in the loads of these two columns, where in this case it is required to use a
base grouped in the form of a trapezoid.
The figure below show
rectangular and trapezoid combined footing.
This must be noted and
pointed out that the calculation of the properties and area of the base in the
form of a trapezoid, in addition to the calculations of the internal forces
generated in it from bending moments and shear forces of the critical sectors
are often longitude. In addition to the fact that the details of the rebar, its
distribution and arrangement in this type of footings often relatively complex.
Such type of combined
bases often exhibit structural behavior
corresponding to an similar to the behavior of inverted beams that are affected
by the pressure and stress of the soil from the bottom and are centered on the
columns as pillars and the distance between the columns is the span for these
footings.so, the steel reinforcement of such type of footing is usually in the
longitudinal direction and placed near the bottom base and near to the surface
which face the tension force.
In addition to this
longitudinal steel reinforcement, secondary steel reinforcement is often placed
and distributed in the transverse direction of these bases, provide that this
steel reinforcement is concentrated below the columns in order to reduce the
localized bending moment which is generated around these columns and is similar
to the case of the columns in the isolated footing.
- Strap Footing.
Strap footing used in
case of adjacent footing or adjacent columns to the neighbor borders, which
does not accept any extension of the isolated footing from borders of the
neighbor. In addition, in that case the distance between the outer column that
in the border and nearest internal column is too large that makes the use of
combined footing between these two columns very skinny and zero width.
In this type of
foundations consist of two separate isolated footing, one under each column
separately linked by a beam called ‘strap beam’. The purpose of the strap beam
is to avoid the inversion and rotation of the external footing of the neighbor
due to the non-centrality of the column bearing in it and subject to vertical
forces and bending moment, which exposes the footing to unequal stresses below
it from the soil. Accordingly, this strap beam is often subjected to a negative
bending moment from the external neighbor’s footing bearing and therefore its
main steel reinforcement will be on the side and the upper surface of this beam
due to the rules of tensile stresses on this surface
- Strip footing.
Strip footing divided
into two types:
- Slab strip footing.
It is used in the case
of the footing used as a substrate for a group of columns on one row and the
distance between these columns is approximately equal, and the loads on these columns are also equal
to some extent.
- Strip footing with
strengthening beams.
It is used when the
distance between the columns is relatively large to increase the stiffness of
the slab and reduce its required thickness by strengthening the slab with a
beam on the upper surface of the slab (upstanding beam) or on the
lower surface of the slab (down standing beam).
- Raft footing.
It is only one base
with large dimensions and a large size on which it is based and contains all
the columns and walls likely to be present in the building as a whole and in
general
Often this type of
foundations is used in the event that the separate or combined bases of the
columns overlap with each other and also in the case of the increased flatness
of the separate or combined bases more than 50% of the horizontal area of the
building, as a result of either the increase in the values of the loads on the
columns and their density in addition to twice the capacity Soil bears below
these bases
There are two common
structural systems possible for this type of foundation:
- Flat slab system
- Slab-beam system