The basics and safety requirements for the design of foundations in general
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The basics and safety requirements for the design of foundations in general

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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.



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