FOOTING

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FOOTING FOOTING

description

FOOTING. Footing -  A base (in or on the ground) that will support the structure. A masonry section, usually concrete, in a rectangular form wider than the bottom of the foundation wall or pier it supports. - PowerPoint PPT Presentation

Transcript of FOOTING

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Footing -  A base (in or on the ground) that will support the structure.

- A masonry section, usually concrete, in a rectangular form wider than the bottom of the foundation wall or pier it supports.

- The base or bottom of a foundation pier, wall, or column that is usually wider than the upper portion of the foundation. The added width at the bottom spreads the load over a wider area.

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TYPES OF FOOTINGS

Footing requirements are generally covered in the building code and sized in accordance with the bearing capacity of the soil and the weight of the building. In areas subject to seasonal frost, the bottom of the footing must be placed below the frost line to prevent damage to the footing and structure due to frost heave. Typical footing types include:

•spot footings

•continuous spread footing

•grade beam footing

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Problem 1:

A 150 mm wall is carrying a total load of 150 kN per meter length of wall. Design a wall footing using the following:

fc’= 20 Mpafs = 140 Mpan = 9R = 1.45 Mpaj = 0.878v = 0.50 Mpau = 2.2 MpaAllowable soil pressure = 75 kPa

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Solution: Consider 1m length of wall;

Assume wt. of footing = 0.09(150) = 13.5 kN

Total load = 150 + 13.5 = 163.5 kN

Req’d. Area = 163.5 = 2.18 m² 75

L(1) = 2.18

Net soil pressure = 150 = 68.2 kN 2.2 m

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Determine depth of footing:

1) from bending

M = 68.2 (1.025) (1.025) 2

M = 35.83 kN · m

d = 157 mm

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A reinforcement;

u = V jd

V = 68.2 (1.025) = 69.9 kN

= (20) (1000) = 370 mm 170

u = 69.9 (1000) = 1.34 MPa < 2.2 MPa (ok) 370 (0.878) (160)

Deck bond stress:

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temp. bars

Ast = 0.0020 (2200) (235)

Ast = 1034 mm²

For 12 mm bars

A = (12) ² = 113 mm ²

Use 10 – 12mm bars

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Problem 2:

Design a square footing for a spiral column having a diameter of 678 mm carrying an axial load of 1780 kN.

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(678)² = D²

D = 600 mm M = 223899 (2.82) (1.11) (1.11) 2

d = 316 mm (Add 100- 200 mm approx.) Try d = 490 mm

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Check for beam shear: V = q (c-d) L V = 223899 (1.11- 0.490) (2.82) V = 385151 N ν = 385151 2820 (490) ν = 0.28 MPa va = 0.09  va = 0.09  va = 0.38 MPa > 0.28 (safe)  

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Compute the steel requirements:

As = 6650 mm²  Using 28 mm Ø bars:  (28)² n = 6650 n = 10.81 say 13 bars (to compensate for bond stress)

Check for Bond stress:

= (28) (13)  = 1144 mm

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V = 223899 (1.11) (2.82) 

V = 694535 N  u = 1.43 MPa   ua = 1.51 MPa > 1.43 (safe)

check wt. of footing: Total depth = 490 + 28 (1.5) + 75

= 607 mm  wt. = 0.607 (2.82) (2400) (9.81) (2.82)   wt. = 113694 N  check the area required: 

A =  

A = 7.89 m² < 7.95 m² (our trial area is bigger than the required area)

 

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Foundation – the interfacing element between the superstructure and the underlying soil or rock. Foundation Engineering – art and science of applying engineering judgment and the principles of soil mechanics to solve the interfacing problem Retaining Structure – structure whose purpose is to retain a soil or other similar mass in a geometric shape other than occurring naturally under the influence of gravity S = total ultimate settlement S = Si + Sc + Ss Si = immediate settlement resulting from the constant volume distortion of the loaded soil mass Sc = consolidation settlement resulting from the time dependent flow of water from the loaded area under the influence of the load generated excess pore pressure which is itself dissipated by the flow Ss = secondary settlement or creep which is also the dependent but may occur at essentially constant effective stress 

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Typical Foundation Types are:

•Foundation for building (shallow/deep)

•Foundations for smoke stacks, radio and television towers, etc. (s or d)

•Foundations for port or Marine structures (maybe s or d, w/ extensive low or deep)

•Foundation elements support open cuts or retain earth masses or bridge abutments (retaining structures)

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Problems encountered in foundation designs

1. What constitutes satisfactory and tolerable settlements?

2. How variable is the soil Profile, and has client seen willing to authorize an adequate exploration program?

3. Can be the building be supported by the underlying soil on spread footing, mats, piles?

4. What is the likehood of a lawsuit if the foundation does not perform adequately?

5. Is money available for the foundation portion of the construction?

6. What is the ability of the local construction force?

7. What is the engineering ability of the foundation engineer? 

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General Requirements of Foundations

1. Depth must be adequate to avoid lateral expulsion of materials from beneath the foundation, particularly footing and mats.

2. Depth must be below seasonal volume changes such as freezing and thawing or the zone of organic materials

3. System must be safe against overturning, rotation, sliding, or soil rupture

4. System must be safe due to harmful materials present in soil

5. System should be adequate to sustain some changes be major in scope

6. The foundation should be economical in terms of the method of installation

7. Total earth movements and differential movements should be tolerable for the foundation element and or any superstructure elements

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Soil Mechanics Engineering study of soil to obtain properties such as: 1.Strength Parameters2.Compressibility Indexes3.Permeability4.Gravimetric – volumetric data (unit, weight, specific gravity, void ratio)  This makes possible engineering Predictions and estimates of: 1.Bearing Capacity2.Settlements-amount-rate3.Earth Pressures4.Pore Pressures and Dewatering quantities The foundations engineer is concerned with the construction of some type of engineering structure of the earth great effort is required to separate the particles

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Earth – composed of rock and soil Rock – naturally occurring materials composed of mineral, particles so firmly bonded together that relatively Soil – naturally occurring materials minerals particles which are fairly readily separated into relatively small pieces and in which the mass may certain air, water, or organic materials in varying accounts Attenberg Limits – laboratory tests for arbitrary moisture contents to determine when the soil is on the verge of being viscious liquid (liquid limit) or nonplastic Plastic Index – water content beyond which no further reduction of mass, volume takes place with additional moisture loss Specific Gravity – may be determined in a laboratory test with moderate difficulty  

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Soil Classification Terms 1.Bedrock – rock its native location, usually extending greatly both horizontally and vertically

2.Boulders – smaller pieces of materials which have broken away from bedrock (10-12 in)

3.Gravel – common term used to describe pieces of rock from about 6 in. max. to less than ¼ in min. size

4.Sand – mineral particles ranging size from 0.05 to 0.074 mm max to 0.002 to 0.006 mm

5.Clay – mineral particles smaller than silt size

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SPT N value Relative density 

0-4 very loose

4-10 loose

10-30 medium dense

30-50 dense

50 very dense