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FOUNDATION DESIGN

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Introduction The primary function of all structural elements is to transfer the applied dead and imposed loading, from whichever source, to the foundations and subsequently to the ground. The type of foundation required in any particular circumstance is dependent on a number of factors such 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 foundation Pad foundation Combined foundation Strip footing**

Raft Foundation 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 the underside of the base and the ground − horizontal forces are not usually of sufficient magnitude to affect the size of foundations, vertical forces due to columns and/or walls and the bearing pressure from the ground underneath the base, moments due to loading from columns and/or walls etc. 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 Design Calculate the plan size of the foundation using the permissible bearing pressure and the critical loading arrangement for the serviceability limit state. Calculate the bearing pressure associated with the critical loading arrangement at the ultimate limit state. Assume a suitable value of thickness (h) and determine the effective depth (d).

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Pad Foundation Design Determine the reinforcement required to resist bending. Check that the shear stress at the column face is less than 5 N/mm2 or 0.8 cu f ,whichever is the smaller. Check that the direct shear stress at a section 1.0d from the column face is less than vc from Table 3.8. Check that the punching shear stress on a perimeter 1.5d from the column face is less than vc from Table 3.8.

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**Example 5.31: Axially Loaded Pad Foundation**

A pad foundation is required to support a single square column transferring an axial load only. Using the data provided: determine a suitable base size, check the base with respect to: bending, direct shear, and punching shear, designing suitable reinforcement where necessary.

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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 = 200 kN/m2 Column dimensions 375 mm × 375 mm Exposure condition severe

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Solution

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Combined Foundation Designed when there are 2 column either when they are relatively close together or when one of the columns is adjacent 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 axial loads as shown in Figure Using the data provided, 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/mm2 Characteristic of reinforcement fy = 460 N/mm2 Net permissible ground bearing pressure pg = 175 kN/m2 Nominal cover to centre of main reinforcement 40 mm Column A dimensions 350 mm × 350 mm Column B dimensions 350 mm × 350 mm

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