Mat Foundations / about kind of foundation

Rectangular Combined Footing
If the net allowable soil pressure is known, the size of the foundation (B * L) can be determined in the following manner:

1. 1 Mat Foundations Chapter 10 Arash Mobaraki 1401 - 1402 DR. Ardakani

2. 2 When more than one line of columns is supported by a concrete slab, it is called a mat or raft foundation. Combined footings can be classified generally under the following categories: a. Rectangular combined footing b. Trapezoidal combined footing c. Strap footing Introduction

3. 3 Combined Footings Rectangular Combined Footing If the net allowable soil pressure is known, the size of the foundation (B * L) can be determined in the following manner:

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5. 5 Trapezoidal Combined Footing The size of the foundation that will uniformly distribute pressure on the soil can be obtained in the following manner:

6. Cantilever Footing 6 Cantilever footings may be used in place of trapezoidal or rectangular combined footings when the allowable soil bearing capacity is high and the distances between the columns are large.

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9. 9 Common Types of Mat Foundations Some of the common ones are shown schematically in Figure 10.4 and include the following:

10. Bearing Capacity of Mat Foundations 10 The gross ultimate bearing capacity of a mat foundation can be determined by the same equation used for shallow foundations. The term B in Eq. (6.28) is the smallest dimension of the mat. The net ultimate capacity of a mat foundation is: A suitable factor of safety should be used to calculate the net allowable bearing capacity. For mats on clay, the factor of safety should not be less than 3 under dead load or maximum live load. However, under the most extreme conditions, the factor of safety should be at least 1.75 to 2.

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12. 12 definition of net pressure on soil caused by a mat foundation the depth of significant stress increase in the soil below a foundation depends on the width of the foundation. Hence, for a raft foundation, the depth of the zone of influence is likely to be much larger than that of a spread footing. Thus, the loose soil pockets under a raft may be more evenly distributed with further stiffening of the soil taking place, resulting in a smaller differential settlement. Based on this concept we can respectively approximate :

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14. 14 Differential Settlement of Mats

15. Compensated Foundation 15

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19. Structural Design of Mat Foundations 19 The structural design of mat foundations can be carried out by two conventional methods: the conventional rigid method and the approximate flexible method . Finite difference and finite-element methods can also be used, but this section covers only the basic concepts of the first two design methods.

20. 20 Conventional Rigid Method Each beam is analyzed as a structural member, and reinforcements are provided from the structural design considerations based on the bending moment and shear force diagrams

21. 21 Figure 10.10 Conventional rigid mat foundation design

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23. Approximate Flexible Method 23 In the conventional rigid method of design, the mat is assumed to be infinitely rigid. Also , the soil pressure is distributed in a straight line, and the centroid of the soil pressure is coincident with the line of action of the resultant column loads.

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31. 31 Load and Resistance Factor Design (LRFD) CHapter 11

32. 32 Load and Resistance Factor Design (LRFD)

33. 33 Design Philosophy

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35. 35 Allowable Stress Design (ASD)

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37. 37 Load and Resistance Factor Design (LRFD)

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