Foundation design.ppt

7/27/2019 Foundation design.ppt

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Introduction The primary function of all structural elements is to

transfer the applied dead and imposed loading, fromwhichever source, to the foundations and

subsequently to the ground. The type of foundation required in any particular

circumstance is dependent on a number of factorssuch as: the magnitude and type of applied loading,

the pressure which the ground can safely support,

the acceptable levels of settlement,

the location and proximity of adjacent structures.


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Types of foundation1. Pad foundation

2. Combined foundation

3. Strip footing

4. Raft Foundation

5. Piled Foundation


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Pad foundation


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Combined foundation


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Strip footing


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Raft Foundation


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Piled Foundation


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Loading The loading effects which occur in foundations, are

generally one or more of the following three types:

horizontal forces due to lateral loading such as wind

on the supported structure or friction between theunderside of the base and the ground horizontalforces are not usually of sufficient magnitude to affectthe size of foundations,

vertical forces due to columns and/or walls and thebearing pressure from the ground underneath the base,

moments due to loading from columns and/or wallsetc. which are eccentric to the centroid of the base.


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Base Pressures The magnitude of the pressure in each

case is determined using elastic

analysis. Case 1: Uniform Pressure (compression

throughout)


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Base Pressures Case 2: Varying Pressure (compression

throughout)


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Base Pressures Case 3:Varying Pressure

(compression over part of the base)


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Pad Foundation Design1. Calculate the plan size of the foundation using

the permissible bearing pressure and the criticalloading arrangement for the serviceability limitstate.

2. Calculate the bearing pressure associated withthe critical loading arrangement at the ultimate

limit state.3. Assume a suitable value of thickness (h) and

determine the effective depth (d).


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Pad Foundation Design4. Determine the reinforcement required to resist

bending.

5. Check that the shear stress at the column face isless than 5 N/mm2 or 0.8 cu f,whichever is thesmaller.

6. Check that the direct shear stress at a section

1.0dfrom the column face is less than vc fromTable 3.8.

7. Check that the punching shear stress on aperimeter 1.5dfrom the column face is less than

vc from Table 3.8.


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Design Data:

Characteristic dead load on column 800 kN

Characteristic imposed load on column 300 kN

Characteristic concrete strength fcu = 40 N/mm2

Characteristic of reinforcement fy = 460 N/mm2

Net permissible ground bearing pressure pg = 200kN/m2

Column dimensions 375 mm 375 mm

Exposure condition severe


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Solution


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Solution


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Solution


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Solution


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Solution


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Solution


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Solution


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Solution


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Combined Foundation

Designed when there are 2 columneither when they are relatively close

together or when one of the columns isadjacent to an existing structure.

The shape can be rectangular,

trapezoidal or T-shape.


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Example 5.33:Combined Foundation An inverted T-beam combined foundation is required

to support two square columns transferring axialloads as shown in Figure 5.125. Using the dataprovided, design suitable reinforcement for the base.

Design Data: Characteristic dead load on column A 450 kN Characteristic imposed load on column A 450 kN Characteristic dead load on column B 750 kN Characteristic imposed load on column B 750 kN


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Characteristic concrete strength fcu = 40 N/mm2Characteristic of reinforcement fy = 460 N/mm2

Net permissible ground bearing pressure pg = 175 kN/m2Nominal cover to centre of main reinforcement 40 mmColumn A dimensions 350 mm 350 mmColumn B dimensions 350 mm 350 mm


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Foundation design.ppt

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