The basics and safety requirements for the design of foundations in general
The
foundations and safety requirements of foundations when designing require the
provision and achievement of several factors, in order to be safe and able to
withstand the permanent loads affecting the foundation resulting from the
normal daily use of the structure such as dead load (DL), live load ( LD) and
pressure water, soil pressure, regularly moving loads, as well as non-permanent
loads such as wind pressure, etc. whether, during the construction phase or during the life span of the
structure, and the foundation must carry a safety factor sufficient to cope
with the rare loads that result from operating accidents, earthquakes and
natural disasters. Among these factors are the following:
- Safety against weather factors and erosion (the minimum depth of foundation).
To ensure that the soil at the foundation level is not affected by
weather factors, the foundation depth should not be less than 0.8 meter below the permanent final level of
the ground surface around the foundation, and this depth may be less in the
case of intact rocky soil and temporary or small buildings such as kiosks,
gatekeepers, etc.
- Safety against overturning.
Shallow foundations should be safe against overturning by making the
total area for the footing foundation or part of it affected by pressure
stresses, by assuming that the loads on the foundation result in a liner
distribution of the stresses on the soil directly below the foundations. To
clarify it, in the event that the foundation is subjected to the dead load (DI)
only, the resultant forces acting at the foundation level must fall into the
core of the cross section, that is, it affects the area only compressive
forces.
In this case, the
amount of the decentralizing force of the sum of the influencing forces (e)
must not exceed the following:
In the case of
foundations with a circular section: e < r/4
In the case of foundation with a
rectangular section: e < b/6 or a/6
As per the below figure:
In the event that the foundations are
exposed to dead and live loads, it is permissible to allow a part of the total
area of the foundation to transfer and resist compressive stresses to the soil,
provided that the area of this part is not less than a 50 % of the total area in the case of a
symmetrical foundation, taking into account that the soil does not bear tensile
stresses and its limits are This part of the base area is sandwiched between
the edges of the foundation and a straight line that passes through the
geometric center of the foundation (centroid) as per the below figure:
To
satisfy this condition, the resultant of the point of influence of the
resultant force acting on the foundation must fall within the boundaries and
within the scattered areas in the below figure for both rectangular and
circular foundations.
It
should be noted that special attention must be given to some foundation’s
sensitive to a overturning such as foundations of double cantilever
installations and retaining walls, which must be taken into account the
accuracy in calculating the loads on these foundations and installations as
shown in the below figure.
- Safety against uplift forces.
In
some cases, in the event that the ground water level rises above the foundation
level of the foundations, the foundations will be exposed to the forces of water
pushing from below upward and this momentum depends on the height of the ground
water column above the foundation level (h), on the density of the water, and
also on the area of the foundation exposed to the thrust force from bottom to
top. Sufficient to meet this balance and this parameter is called the uplift
factor of safety and this parameter must not be less than the values specified
in the below table:
Load Conditions |
Dead load + live load |
Dead load + live load + wind
+ pressure |
Dead load + live load + wind
+ earthquake load |
Uplift Factor of Safety |
1.3 |
1.2 |
1.1 |
and according to the different loading
conditions and this coefficient is equivalent to:
Fu
(uplift factor of safety) = Q/U
Where,
Q=
the total vertical load held down at the foundation level in tonnes
U
= the total thrust effecting to top at the foundation level in tons, which is
equal to the base area (A) in square meter * height of the groundwater column
above the foundation level (h) in meters * groundwater density (γ) in tons / m3
as per below equation:
U
= A * γ *h tons.
It
should be noted that in the event that the values of the shear resistance
between the soil and the sides of the foundation are taken into account as an
additional value for the strength of the vertical impacting down (as is the
case in some fundamentals sensitive to flotation such as tanks buried under the
surface of the ground) it must in this case is to increase the values mentioned
in the previous table for the safety factor from the float by 0.2. In this
case, it must be confirmed and stipulated in to not soil removal throughout the
life of the structure.
In
next article we will continue explaining the remaining safety factors for the
design foundations.