When analyzing the stability of structures, sliding, tipping and lifting are mainly taken into account. Sliding and pitching are due to lateral loads and liftoff is due to upward pressure created by the fluids.
When designing a simple retaining wall, you may not need to follow the process outlined in this article.
The basic method shown in another article on this site can be consulted.
Depending on the occurrence of an event or taking into account the return period of an event, stability analyzes are carried out.
The safety factor varies with the return period or probability of occurrence.
By extending the payback period, the safety factor also increases.
Furthermore, the type/use of the structure is also taken into account when determining the safety factor. Depending on the importance of the structure, superior factors are used in analyzing the stability of structures.
This discussion is based on information provided in EM 1110-2-2100:2005.
Based on the probability of occurrence/recurrence period of an event, three main categories of stress conditions are identified.
They are common, uncommon and extreme.
Explanations in EM 1110-2-2100 are given below, in which the limits of the individual conditions are clearly explained.
USUAL
Common loads are loads and loading conditions related to the primary function of the structure and occur frequently during the service life of the structure.
A common event is a common occurrence and the structure is assumed to operate in the linear elastic regime.
Example: Applying a load of earth to a retaining wall.
UNUSUAL
Unusual loads are operating loads and loading conditions that occur rarely.
Construction and maintenance loads are also classified as unusual loads because risks can be controlled by determining the sequence or duration of activities and/or monitoring their execution.
Loads on temporary structures used to support project construction are also considered unusual.
In the case of an unusual event, some minor nonlinear behavior is acceptable, but the repairs required will likely be minor.
EXTREME
Extreme stress refers to events that are highly unlikely and can be considered emergency conditions.
Such events may involve serious accidents such as impacts or explosions, as well as natural disasters caused by earthquakes or floods, the frequency of which far exceeds the economic life of the structure.
Extreme stress can also arise from a combination of unusual stressful events.
The structure is expected to withstand extreme loads without suffering catastrophic failures. However, structural damage is expected, which may partially impair operational functions and require extensive renovation or replacement of the structure.
The variation of charge state probabilities is shown in the following table.
SAFETY FACTOR AGAINST SLIPING
The safety factor against sliding is defined depending on local conditions and depending on the type of loading, as explained previously in the structure stability analysis.
Sliding resistance is generally calculated from the sum of the friction force and the resistance caused by the cohesion of the layers.
The following equation can be used for the calculated safety factor against slippage.
FS = (N tanΦ + cL) / T
Where,
N – Force acting perpendicular to the sliding fracture plane under the structural wedge
Φ – Angle of internal friction of the foundation material under the structural wedge
c – Cohesive strength of the foundation material under the structural wedge
L – Length of the structural wedge in contact with the foundation
T – Shear force acting parallel to the base of the structural wedge
The safety factor for sliding can be calculated using the values indicated in EM 1110-2-2100. Factors of safety for critical structures can be calculated using the following table.
*For preliminary seismic analysis without detailed site-specific ground motion, use FS = 1.7 for unusual and FS = 1.3 for extreme.
**Limited location information is not permitted for critical structures.
Slip safety factors for normal structures can be found in the following table in accordance with EN 1110-2-2100.
As discussed, depending on the load state and site conditions, the slip safety factor can be calculated.
SAFETY FACTORS FOR FLOTATION IN STRUCTURE STABILITY ANALYSIS
Most underground structures require buoyancy testing as they can be lifted by the pressure of water acting on them.
Additionally, some structures need to be checked for possible uplift during construction.
First, the total downward vertical force is calculated.
Weight of the structure, any liquid contained in the structure (if any), and any additional load applied to the structure.
When considering the critical load case, the minimum vertical load must be taken into account. Application of additional load may be considered depending on the type or continuous load.
FS = total vertical load / buoyancy force
The values indicated in the following table can be used as safety factors against slipping.
Tilt for structure stability analysis
It is common practice to determine the safety factor against tipping by calculating the relationship between the floor moment and the tipping moment.
However, as explained in EM 1110-2-2100, checking whether the base is compressed and whether the resultant is present with the base is more relevant.
To ensure that the slip is within limits, the following conditions are checked.
The location where the resultant force hits the base can be used to determine whether or not it is acceptable for the applied loading conditions.
The article Stability Calculation of Retaining Walls Calculation methods can be used for some of the above tests.
