7. Bearing Capacity

Chapter 1) What Is Bearing Capacity 

• Introduction to Bearing Capacity 
• Design requirements for foundations 
• Method to calculate the bearing capacity of soil by theoretical and field tests.  

Chapter 2) Example 

• Modelling, boundary condition setting, applying load using Midas GTS NX software to simulate a simple shallow foundation (strip footing) for calculating the bearing capacity of the soil. 
• Analysis and see the results for the same. 

Chapter 3) Comparison of results.  

• Compare the results with those obtained manually and using the program.  

Summary

1. Concept of Bearing Capacity

The foundation must be able to support the upper structure safely.
The load per unit area that the ground can support through the foundation is called the Ultimate Bearing Capacity
 And if the load acting on the structure exceeds the support force, support force destruction may occur.

The foundation must have sufficient bearing capacity to ensure stability and usability.

Stability: prevent structural collapse due to loads
Usability: design to ensure that there is no damage to the function during use due to deformation or cracking

The type and geometry of the foundation are very important factors in the bearing capacity analysis.
The foundation should be planned economically and mechanically advantageous by comprehensively considering load characteristics, ground conditions, and structure geometry.
Depending on the depth, shallow foundations as shown in Fig. 7.2 and deep foundations as shown in Fig. 7.3 have been proposed to effectively utilize the bearing capacity of the ground.

In addition, a special type of foundation has also been proposed, as shown in Fig. 7.4.

2. Design requirements for the foundation

The foundation must be safe against shear failure of the ground and the settlement of the foundation should not damage the structure
The load-bearing capacity of the foundation per unit area that causes soil failure is called Bearing capacity, and the maximum allowable settlement of the foundation is called Allowable settlement.

Ultimate bearing capacity: q_ult=Q_ult/B
Allowable bearing capacity: q_allow=q_ult/FS
Allowable settlement: S_allow

3. Method of calculating support force

There are three methods for calculating bearing capacity:

1. Bearing capacity formula
2. Plate load test
3. Numerical analysis

The first two methods are commonly used in traditional design methods.
However, the application has many limitations as it assumes simple geometric shapes and isotropic ground materials with two-dimensional conditions.
Many researchers have proposed theoretical formulas considering various conditions, but their applicability to complex problems is limited.

In the case of the Plate load test, field conditions can be considered realistically.
But there is a problem that the stress impact range is different from the actual foundation due to the size of the load plate.

The third method, Numerical analysis, requires the use of numerical analysis tools (commercial software like midas GTS NX).
The principle of numerical analysis for bearing capacity is similar to that of plate load tests, but it can consider ground behaviour more realistically since it targets actual-sized foundations.

4. Calculation of Bearing Capacity based on Bearing Capacity Theory

The bearing capacity theory can be obtained by using the Limit Equilibrium method or Limit theorem.
The limit equilibrium method assumes the ground's bearing capacity failure mechanism and considers the ultimate bearing capacity as the maximum load at the point of failure.
Many researchers have presented various theories of bearing capacity for different cases through various studies.

Let's take a look at Terzaghi's theory of bearing capacity. The basic assumptions are as follows:

• 2D Plane strain condition
• Shallow footing with rough bedrock, D_f=2.5B
• Continuous strip footing
• Full shear failure mode (Fig. 7.6)
• The effect of the depth of insertion of the foundation, q=γD_f