2a - Design of Spread Footing-usd Method

8/16/2019 2a - Design of Spread Footing-usd Method

1/27

DESIGN OF SPREAD FOOTINGS

(USD)


2/27

Probable pressure distribution beneath a rigid footing.

On a cohesionless soil.

Generally for cohesive soils.

Usual assumed linear distribution.


3/27

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)


4/27

ELEMENTS OF USD


5/27


6/27


7/27


8/27

Minimum length of embedment L d for reinforcing bars in tension :


9/27

Development length for bond for compression bars is the largest of thefollowing:


10/27


11/27

Section for two-way shear (Punching shear)

Section for wide-beam shear


12/27


13/27

Method of computing A 2 for allowable column bearing stress


14/27

STRUCTURAL DESIGN OF SPREAD FOOTINGS

PUNCHING SHEAR (Two – way action)


15/27


16/27

BENDING MOMENT


17/27

Sections for computing bending moment.


18/27

ccol comp f A P

compu P P

LL DL P u 6.12.1


19/27

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


20/27

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


21/27

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


22/27


23/27


24/27


25/27

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


26/27

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


27/27

END

Documents

2a - Design of Spread Footing-usd Method