Post on 05-Jul-2018
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DESIGN OF SPREAD FOOTINGS
(USD)
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Probable pressure distribution beneath a rigid footing.
On a cohesionless soil.
Generally for cohesive soils.
Usual assumed linear distribution.
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Strength design requires converting working design dead ( D) and live ( L) loads toultimate loads through the use of load factors:
0.19.0
6.19.0
0.10.12.1
0.16.12.1
6.12.1
E D
W D
L E D
LW D
L D P u (a)
(b)
(c)
(d)
(e)
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ELEMENTS OF USD
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Minimum length of embedment L d for reinforcing bars in tension :
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Development length for bond for compression bars is the largest of thefollowing:
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Section for two-way shear (Punching shear)
Section for wide-beam shear
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Method of computing A 2 for allowable column bearing stress
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STRUCTURAL DESIGN OF SPREAD FOOTINGS
PUNCHING SHEAR (Two – way action)
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BENDING MOMENT
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Sections for computing bending moment.
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ccol comp f A P
compu P P
LL DL P u 6.12.1
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ADDITIONAL ACI CODE REQUIREMENTS FOR FOOTINGS:
ART. 7.6 SPACING OF REINFORCEMENT Not less than (clear) • d b ; d b = (bar diameter)
• 25 mm• 1.33 × maximum aggregate size
But not more than • 3 × Dc ; D c = thickness of concrete• 450 mm
7.7.1a MINIMUM REINFORCEMENT COVER - 75 mm Clear against earth
7.12 & 10.5 MINIMUM REINFORCEMENT
For footings of variable thickness
For footings of uniform thickness
y f
40.1min
002.0min
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10.33 MAXIMUM REINFORCEMENT
15.7 & 15.11.4 MINIMUM FOOTING DEPTH
Not less than • 150 mm above bottom of reinforcement for footings on soil• 200 mm for plain concrete footings• 300 mm for footings on piles
15.8 DOWELS• Area
- Not less than .005A c ; A c = Area of column- Minimum of 4 bars for rectangular columns- Minimum of 6 bars for circular columns
• Diameter
- Not greater than 4 mm than the longitudinal
y y
cb
b
f f f
600
600'85.0
75.0
1
max
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12.2 DEVELOPMENT OF REINFORCEMENT IN TENSION
D36
or smaller →
But not less than
or 300 mm
12.3 DEVELOPMENT OF REINFORCEMENT IN COMPRESSION
But not less than
or 200 mm
'
02.0
c
yb
f
f A
yb f d 058.0
'
24.0
c
b yd
f
d f l
b yd f 044.0
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Illustrative Problem:
Design a spread footing for the following data:kPaqa 200
DL = 350 kN LL = 450 kN f c’ = 21 MPa
Use grade 400 rebars ( f y = 400 MPa)
Column data: 0.35 m × 0.35 m4 – 30 mm diameter bars
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Seat Work:
Design a spread footing for the following data:kPaqa 200
DL = 350 kN LL = 550 kN f c’ = 21 MPa
Use grade 400 rebars ( f y = 400 MPa)
Column data: 0.30 m × 0.30 m4 – 32 mm diameter bars
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END