05 chapter 6 mat foundations

Foundation Engineering
CVL 4319
Chapter 6
Dr. Sari Abusharar
University of Palestine
Faculty of Applied Engineering and Urban Planning
Civil Engineering Department
1st Semester 2015-2016 1
Mat Foundations

Outline of Presentation
Introduction
Combined Footings
Common Types of Mat Foundations
Bearing Capacity of Mat Foundations
Differential Settlement of MatsDifferential Settlement of Mats
Field Settlement Observations for Mat Foundations
Compensated Foundation
Structural Design of Mat Foundations
2

Introduction
Under normal conditions, square and rectangular footings are
economical for supporting columns and walls. However, under
certain circumstances, it may be desirable to construct a footing that
supports a line of two or more columns. These footings are referred
to as combined footings. When more than one line of columns is
supported by a concrete slab, it is called a mat foundation.
Combined footings can be classified generally under the followingCombined footings can be classified generally under the following
categories:
a. Rectangular combined footing
b. Trapezoidal combined footing
c. Strap footing
Mat foundations are generally used with soil that has a low bearing
capacity.
3

Rectangular combined footing
Trapezoidal combined footing
Strap footing
4

Rectangular Combined Footing
In several instances, the load to be carried by a column and the soil bearing
capacity are such that the standard spread footing design will require
extension of the column foundation beyond the property line. In such a case,
two or more columns can be supported on a single rectangular foundation.
5

If the net allowable soil pressure is known, the size of the foundation (LxB)
can be determined in the following manner:
Step (1): Determine the area of the foundation
Step (2): Determine the location of the resultant of the column loads.
Step (3): For a uniform distribution of soil pressure under the foundation, the
resultant of the column loads should pass through the centroid of
the foundation. Thus,
6

Step (4): Once the length L is determined, the value of L1 can be obtained as
follows:
Note that the magnitude of L2 will be known and depends on the location
of the property line.
Step (5): The width of the foundation is thenStep (5): The width of the foundation is then
7

8

Trapezoidal Combined Footing
Trapezoidal combined footing (see Figure 6.2) is sometimes used as an
isolated spread foundation of columns carrying large loads where space is
tight.
9

Trapezoidal Combined Footing
The size of the foundation that will uniformly distribute pressure on the soil
can be obtained in the following manner:
Step (1): If the net allowable soil pressure is known, determine the area of
the foundation:
From Figure 6.2,
Step (2): Determine the location of the resultant for the column loads:
Step (3): From the property of a trapezoid,
Solve Eqs. (6.6) and (6.7) to obtain
B1 and B2 Note that, for a trapezoid, 10
centroid of the
foundation

Cantilever Footing
Cantilever footing construction uses a strap beam to connect an eccentrically
loaded column foundation to the foundation of an interior column. (See
Figure 6.3). 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.
11

Their purpose is to redistribute Excesses stresses, and possible
differential settlements between adjacent spread footings.
Cantilever Footing
12

Find the Dimensions of the combined footing for the columns A and B
that spaced 6.0 m center to center, column A is 40 cm x 40 cm carrying
dead loads of 50 tons and 30 tons live load and column B is 40 cm x 40
cm carrying 70 tons dead load and 50 tons live loads.
Example 1
13

Example 1
1- Find the required area:
2- Find the resultant force location (Xr):
3- To ensure uniform soil pressure, the resultant force (R) should be in
the center of rectangular footing:
14

Find the Dimensions of the trapezoidal combined footing for the
columns A and B that spaced 4.0 m center to center, column A is 40 cm
x 40 cm carrying dead loads of 80 tons and 40 tons live load and
column B is 30 cm x 30 cm carrying 50 tons dead load and 25 tons live
loads.
Example 2
15

Example 2
1- Find the required area:
2- Determine the resultant force
16

Example 2
3- Put the resultant force location at the centroid of trapezoid to
achieve uniform soil pressure.
For uniform soil pressure:
17

Example 3
Design a strap footing to support two columns, that spaced 4.0 m
center to center exterior column is 80cm x 80cm carrying 1500 KN
and interior column is 80cm x 80cm carrying 2500 KN
18

Example 3
1- Find the resultant force location:
2- Assume the length of any foot, let we assume L1=2m.
19

Example 3
3- Find the distance a:
20

Example 3
4- Find the resultant of each soil pressure:
5- Find the required area for each foot:
21

The mat foundation, which is sometimes referred to as a raft foundation,
is a combined footing that may cover the entire area under a structure
supporting several columns and walls.
Mat foundations are sometimes preferred for soils that have low load-
bearing capacities, but that will have to support high column or wall
loads.
Under some conditions, spread footings would have to cover more thanUnder some conditions, spread footings would have to cover more than
half the building area, and mat foundations might be more economical.
Several types of mat foundations are used currently.
Some of the common ones are shown schematically in Figure 6.4 and
include the following:
22

1. Flat plate (Figure 6.4a). The mat is of uniform thickness.
2. Flat plate thickened under columns (Figure 6.4b).
3. Beams and slab (Figure 6.4c). The beams run both ways, and the columns
are located at the intersection of the beams.
23

4. Flat plates with pedestals (Figure 6.4d).
5. Slab with basement walls as a part of the mat (Figure 6.4e). The walls act
as stiffeners for the mat. 24

25

The gross ultimate bearing capacity of a mat foundation can be determined
by the same equation used for shallow foundations
The net ultimate capacity of a mat foundation is
A suitable factor of safety should be used to calculate the net allowableA 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.
For mats constructed over sand, a factor of safety of 3 should
normally be used. Under most working conditions, the factor of safety
against bearing capacity failure of mats on sand is very large. 26

For saturated clays with ɸ = 0 and a vertical loading condition, Eq.
(3.19) gives
From Table 3.4, for ɸ = 0
Substitution of the preceding shape and depth factors into Eq. (6.8) yields
27

Hence, the net ultimate bearing capacity is
For FS = 3, the net allowable soil bearing capacity becomes
The net allowable bearing capacity for mats constructed over granular soilThe net allowable bearing capacity for mats constructed over granular soil
deposits can be adequately determined from the standard penetration
resistance numbers. From Eq. (5.64), for shallow foundations,
28

When the width B is large, the preceding equation can be approximated as
In English units, Eq. (6.12) may be expressed as
29

The net allowable pressure applied on a foundation (see Figure 6.7) may be
expressed as
30

Figure 6.7 and Eq. (6.15) indicate that the net pressure increase in the soil
under a mat foundation can be reduced by increasing the depth Df of the
mat. This approach is generally referred to as the compensated foundation
design and is extremely useful when structures are to be built on very soft
clays. In this design, a deeper basement is made below the higher portion of
the superstructure, so that the net pressure increase in soil at any depth is
relatively uniform. (See Figure 6.8.)
32

From Eq. (6.15) and Figure 6.7, the net average applied pressure on soil is
For no increase in the net pressure on soil below a mat foundation, q should
be zero. Thus,
The factor of safety against bearing capacity failure for partially compensated
foundations may be given as
33

For saturated clays, the factor of safety against bearing capacity failure can
thus be obtained by substituting Eq. (6.10) into Eq. (6.20):
34

Structural Design of Mat Foundations
The structural design of mat foundations can be carried out by two
conventional methods:
conventional rigid method
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.
39

Conventional Rigid Method
The conventional rigid method of mat foundation design can be explained
step by step with reference to Figure 6.10:
40

Step 1. Figure 6.10a shows mat dimensions of L x B and column loads of Q1,
Q2, Q3 …. Calculate the total column load as
Step 2. Determine the pressure on the soil, q, below the mat at points A, B, C,
D, ……, by using the equation
42

Step 3. Compare the values of the soil pressures determined in Step 2 with
the net allowable soil pressure to determine whether
Step 4. Divide the mat into several strips in the x and y directions. (See Figure
6.10). Let the width of any strip be B1 .
Step 5. Draw the shear, V, and the moment, M, diagrams for each individual
strip (in the x and y directions). For example, the average soil pressurestrip (in the x and y directions). For example, the average soil pressure
of the bottom strip in the x direction of Figure 6.10a is
44

The total soil reaction is equal to Now obtain the total column load
on the strip as The sum of the column loads on the strip
will not equal because the shear between the adjacent strips has
not been taken into account. For this reason, the soil reaction and the
column loads need to be adjusted, or
Now, the modified average soil reaction becomes
and the column load modification factor is
45

So the modified column loads are This modified
loading on the strip under consideration is shown in Figure 6.10b. The shear
and the moment diagram for this strip can now be drawn, and the procedure
is repeated in the x and y directions for all strips.
Step 6. Determine the effective depth d of the mat by checking for diagonal
tension shear near various columns. According to ACI Code 318-95
(Section 11.12.2.1c, American Concrete Institute, 1995), for the
critical section,critical section,
46

Step 7. From the moment diagrams of all strips in one direction (x or y),
obtain the maximum positive and negative moments per unit width
Step 8. Determine the areas of steel per unit width for positive and negative
reinforcement in the x and y directions. We have
47

Approximate Flexible Method
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. (See Figure 6.11a.) In the
approximate flexible method of design, the soil is assumed to be
equivalent to an infinite number of elastic springs, as shown in Figure
6.11b. This assumption is sometimes referred to as the Winkler6.11b. This assumption is sometimes referred to as the Winkler
foundation. The elastic constant of these assumed springs is referred
to as the coefficient of subgrade reaction, k.
65

To understand the fundamental concepts behind flexible foundation design,
consider a beam of width B1 having infinite length, as shown in Figure 6.11c.
The beam is subjected to a single concentrated load Q. From the
fundamentals of mechanics of materials,
66

67

Combining Eqs. (6.35) and (6.36) yields
However, the soil reaction is
So,
68

Solving Eq. (6.38) yields
69
The unit of the term as defined by the preceding equation, is (length-1). This
parameter is very important in determining whether a mat foundation
should be designed by the conventional rigid method or the approximate
flexible method. According to the American Concrete Institute Committee
336 (1988), mats should be designed by the conventional rigid method if the
spacing of columns in a strip is less than 1.75/β. If the spacing of columns is
larger than 1.75/β the approximate flexible method may be used.

To perform the analysis for the structural design of a flexible mat, one must
know the principles involved in evaluating the coefficient of subgrade
reaction, k. Before proceeding with the discussion of the approximate
flexible design method, let us discuss this coefficient in more detail.
If a foundation of width B (see Figure 6.12) is subjected to a load per unit
area of q, it will undergo a settlement Δ. The coefficient of subgrade modulus
can be defined as
70
can be defined as
The unit of k is kN/m3.

The value of k can be related to large foundations measuring in the following
ways:
Foundations on Sandy Soils
Foundations on Clays

For rectangular foundations having dimensions of BxL (for similar soil and q),

For long beams,Vesic (1961) proposed an equation for estimating subgrade
reaction, namely,
For most practical purposes, Eq. (6.46) can be approximated as

The approximate flexible method of designing mat foundations, as proposed
by the American Concrete Institute Committee 336 (1988), is described step
by step.
Step 1. Assume a thickness h for the mat, according to Step 6 of the
conventional rigid method. (Note: h is the total thickness of the mat.)
Step 2. Determine the flexural ridigity R of the mat as given by the formula
Step 3. Determine the radius of effective stiffness—that is,

Step 4. Determine the moment (in polar coordinates at a point) caused
by a column load (see Figure 6.13a). The formulas to use are

Step 5. For the unit width of the mat, determine the shear force V caused byStep 5. For the unit width of the mat, determine the shear force V caused by
a column load:
Step 6. If the edge of the mat is located in the zone of influence of a column,
determine the moment and shear along the edge. (Assume that the
mat is continuous.) Moment and shear opposite in sign to those
determined are applied at the edges to satisfy the known conditions.

Step 7. The deflection at any point is given by

End of Chapter 6End of Chapter 6
79

HW # 4
Solve problems:
6.2, 6.4, 6.6, 6.8 and 6.10
Due to Sunday, 3/11/2013
6.2, 6.4, 6.6, 6.8 and 6.10

05 chapter 6 mat foundations

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05 chapter 6 mat foundations