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Foundation Engineering CE Combined foundation

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CONTENTS – Introduction – Design of combined foundation – References CE Foundation Engineering - 6. Combined foundations 2

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Introduction CE Foundation Engineering - 6. Combined foundations 3 Definition Types of combined foundations Why do we use combined foundation?

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CE Foundation Engineering - 6. Combined foundations 4 Spread footings such as square and rectangular footings are economical for support columns under normal condition. Introduction Definition SPREAD FOOTING Combined footing is a system of foundation that support more than one columns (usually 2 columns).

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CE Foundation Engineering - 6. Combined foundations 5

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7 Combined foundation may have several types.

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8 Introduction Types of combined foundations CE Foundation Engineering - 6. Combined foundations Column Plan view Cross section This is the simplest type. Rectangular

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9 Introduction Types of combined foundations CE Foundation Engineering - 6. Combined foundations Column View plan of Trapezoidal combined Footing Rectangular Trapezoidal

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10 Introduction Types of combined foundations Rectangular Trapezoidal Cantilever (Strap) –Consist of two single column footings connected with a cantilever beam or a strap. –This type is used to connect an eccentrically loaded column footing to an interior column so that a uniform soil pressure is generated beneath both footings. Column Footing Plan view Cross section Column Footing Strap Strap beam –Strap is used instead of other types if distance between columns is large and/ or the allowable soil pressure is relatively large so that the additional footing area is not needed.

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11 Introduction Types of combined foundations CE Foundation Engineering - 6. Combined foundations Rectangular Trapezoidal Cantilever Tee-shaped Plan view Column

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CE Foundation Engineering - 6. Combined foundations 12 Introduction Why do we use combined foundation? Property boarder Combined footing is usually used when: 1.one column is located at or near the property boarder line. 2.two columns are so close that single footings cannot be used. 3.supporting two columns of unequal loads

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Design of combined foundation CE Foundation Engineering - 6. Combined foundations 13 Forces Types of design Rectangular combined foundation Trapezoidal combined foundation

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14 Property boarder line Flexural reinforcement Design of combined foundation Forces Tensile

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15 Forces Design of combined foundation Longitudinally, the footing acts as an upward loaded beam spanning between columns and cantilevering beyond. Using statics, the shear force and bending moment diagrams in the longitudinal direction are drawn. The footing is also subjected to transverse bending

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16 Types of design Design of combined foundation The structural design will include: -Determining the thickness of the foundation -Reinforcement of the concrete (for shear and bending) The geotechnical design, will include: -Determining of allowable bearing capacity of the soil -Determine the horizontal geometries of the combined foundation such as width, length and total area A.

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17 Geometric Design of Rectangular combined foundation

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18 Design of Rectangular combined foundation Design of combined foundation Let’s consider the rectangular combined foundation shown in the figure. The allowable bearing capacity q all is estimated from the bearing capacity equation (see previous chapter). The geotechnical design of this foundation will require to determine the design area A, and dimensions L, B (keeping uniformly distributed pressure). The following steps are used: area A

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19 Design of Rectangular combined foundation Design of combined foundation Step #5. The width B is then calculated from: (5)

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20 Geometric Design of Trapezoidal combined foundation

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21 Design of Trapezoidal combined foundation Design of combined foundation Let’s consider the Trapezoidal combined foundation shown in the figure. We will assume that the allowable bearing capacity q all is known for the soil. The geotechnical design of this foundation will require to determine the design area A, and dimensions B1, B2 (keeping uniformly distributed pressure as much as possible). The following steps are used: Note that A also equal to: is determined from: (6)

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22 Design of Trapezoidal combined foundation Design of combined foundation Note: Q1 and Q2 might be given as dead load DL and live load LL. Some design code recommends to use a factor of safety (1.4 for DL and 1.7 for LL). In this case the ultimate bearing capacity q ult should be used.

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23 Design of Trapezoidal combined foundation Design of combined foundation Class Example Design a trapezoid-shaped footing, with 2 columns at ends of footing as shown below, where q all = 120 kPa and L = 7 m.

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24 Class Example/ Solution Note. Because the FS is applied on the loads we should use the ultimate bearing capacity q ult

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25 Class Example/ Solution (cont..)

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Reference s 26 CE Foundation Engineering - 6. Combined foundations 1.Braja M Das, 2011, Principles of Foundation Engineering, 7th ed, Chapter Previous course materials and presentations at KSU. 3.Geotechnical on the web: 4.Andrew Bond and Andrew Harris, 2008, Decoding Eurocode 7, London. 5.The Institution of Structural Engineers library:

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