footing design as per astm

8/10/2019 footing design as per astm

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This Powerpoint presentation was prepared byDr. Terry Weigel, University of Louisville.This work and other contributions to the

text by Dr. Weigel are gratefullyacknowledged.

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Design for load transfer to soil uses

unfactored loads

Support structural members and transferloads to the soil

Structural members are usually columns orwalls

Structural design of footing is done withfactoredloads


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Typically, bottom of footing must be locatedbelow frost line

Footings must be designed to prevent bearingfailure, sliding and overturning

Footings must be designed to preventexcessive settlement or tilting

Excavation may be required to reach a depthwhere satisfactory bearing material islocated


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Wall footings enlargement of the bottom ofthe wall


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Isolated or single column square footing loads relatively light and columns notclosely spaced


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Combined footings support two or morecolumns heavily loaded columns; closelyspaced columns; columns near property line


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Mat or raft foundation continuous concreteslab supporting many columns; soil strengthrelatively low; large column loads; isolated

spread footings would cover more than 50percent of area; reduce differentialsettlement


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Pile caps distribute column loads to groupsof piles


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Soil pressure is assumed to be uniformlydistributed beneath footing if column loadis applied at the center of gravity of thefooting

Footings supported by sandy soils

Footings supported by clayey soils

Footings supported eccentric loads


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Actual soil pressure is based on unfactoredloads

Allowable soil pressure may be determined bya geotechnical engineer

When soil exploration is not feasible, values

provided by building codes may be usedFactor of safety is typically 3


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Maximum Allowable Soil Pressure

Material Allowable Pressure,ksf

Rock 20% of ultimate

strengthCompact coarse or fine sand,hard clay or sand clay

8

Medium stiff clay or sandy clay 6

Compact inorganic sand and siltmixtures

4

Loose sand 3

Soft sand clay or clay 2

Loose inorganic sand-siltmixtures

1

Loose organic sand-silt mixtures,muck or bay mud 0


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Generally, beam design theory is used

Shear strength almost always controlsfooting depth

Compute moment at the face of the wall

(concrete wall) or halfway between wallface and its centerline (masonry walls)


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Shear may be calculated at distance d fromface of the wall

Use of stirrups is not economical set d sothat concrete carries all the shear

'

'

2

2

c c w

u

c w

V f b d

Vdf b


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Design a 12-in wide strip

Section 15.7 of ACI Code:

Depth of footing above bottomreinforcement not less than 6 infor footings on soil and not lessthan 12 in for footings on piles

Minimum practical depth of footing is 10 inand 16 in for pile caps


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Design a wall footing to support a 12-in. widereinforced concrete wall with a dead loadof 20 k/ft and a live load of 15 k/ft. The

bottom of the footing is to be 4 footbelow final grade, the soil weighs 100lb/ft3the allowable soil pressure is 4 ksf.The concrete strength is 3,000 psi and thesteel is Grade 60.


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Assume a footing thickness of 12 in. With aminimum cover of 3 in., this gives a d valueof about 8.5 in. Compute the footing

weight andsoil weight:

Footing weight

12 in150 150 psf

12 in/ft

Soil weight

36 in100 300 psf

12 in/ft


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Effective soil pressure and required width offooting:

4000 psf 150 psf 300 psf 3550 psf

Width of footing required

20 k/ft 15 k/ft9.86 ft

3.55 ksf

Use 10 ft

eq


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Factored bearing pressure for design ofconcrete:

1.2 20 k/ft 1.6 15 k/ft4.80 ksf

10 ftuq


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Compute design shear (at distance d fromface of wall):

10 ft 6 in 8.5 in

4.80 ksf 18.2 k 2 12 in/ft 12 in/ft

18,200 lb18.46 in

0.75(1.0) 2 3000 ksi 12 in

Much larger than orginal assumption

Try a thicker footing - say 20 in thick

16.5 in

uV

d

d


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20 in4000 psf 150 psf

12 in/ft

28 in 100 psf 3517 psf 12 in/ft

Width of footing required

20 k/ft 15 k/ft

9.95 ft3.517 ksf

Use 10 ft

eq


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10 ft 6 in 16.5 in4.80 ksf 15.0 k

2 12 in/ft 12 in/ft15,000 lb

15.21 in0.75 2 3000 ksi 12 in/ft

15.21 in 3.5 in 18.71 in

Use a 20 in thick footing

uV

d

h


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22

22

10 ft 6 in4.5 ft

2 12 in/ft

Compute moment on a one-foot-long strip

4.80 k/ft 4.5 ft48.6 k-ft/ft

2 2

12 in/ft 48,600 lb-ft/ft 198.3 psi0.9 12 in 16.5 in

u

u

wLM

Mbd


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Appendix Table 4.12,r = 0.00345 < 0.0136,section is tension controlled; = 0.9

2

in0.00345 12 in 16.5 in 0.68ft

sA

Use No 7 at 10 in (As = 0.72 in2 / ft from

Table A.6)


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Development length:

1

5 in side cover

0.875c 3 3 3.4375 3.5

2 2

10 in5 in one-half c-c spacing of bars

23.5 in 0

4.0 Use 2.50.875 in

t e s

b

bb b

b

b tr

b

c

duse c in

c

c K

d


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'

2,

2

,

3

40

3 60,000 psi 1 32.86 diameters

40 2.53000 psi

0.68 in /ft32.86 31.03 diameters

0.72 in /ft

31.03 0.875 in 27.15 in

yd t e s

b trb c

b

s requiredd

b s provided

d

f

c Kd fd

A

d A


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10 ft 12 in/ft6 in 3 in 51 in 27.15 in

2

Available length for development


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2

0.0018 12 in 20 in 0.432 in / ftsA

Temperature and shrinkage steel

Use No 5 at 8 in (As = 0.465 in2 / ft)


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Most isolated square footings have a constantthickness

For very thick footings, it may be economicalto step or taper footing

Two types of shear must be considered one-

way shear and two-way shear


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Constant thickness


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Stepped


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Tapered


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'2

u

c w

Vd

f b

Same as for wall footings


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ACI Code Section 11.11.1.2 states that criticalsection is at a distance d/2 from face ofsupport


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s= 40 for interior columns

s= 30 for exterior columns

s= 20 for corner columns


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Flexural reinforcement is required in twodirections

The values of d for the layers of steel in

the two directions will be different

For square footings, design using the value ofd for the upper layer is typical

For square footings supporting non-squarecolumns, moments are larger in theshorter direction of the column


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Reinforcing steel areas required to resistmoment are often less than minimumrequired steel:

Code Section 10.5.4 states that minimumarea and maximum spacing need only beequal to values required for temperatureand shrinkage steel

,min

'

,min

200

3

s w

y

c

s w

y

A b d

f

fA b d

f


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Maximum steel spacing may not exceed threetimes the footing thickness or 18 in.


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All forces at the base of the column must betransferred to the footing

Compressive forces must be transferred bybearing

Tensile forces may be transferred byreinforcement or mechanical connectors


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Columns transfer loads directly over the areaof the column

Load transfer into the footing may byassumed to occur over an effective areawhich may be larger than the column area

For the same strength of concrete, thefooting can support more bearing loadthan can the column


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Bearing strength permitted at the base ofthe column ->

Bearing strength permitted on the footing isthe same value multiplied by ->

See ACI Code Section 10.14.1

'

10.85 cf A

2

1

2A

A


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A2 is the area of footing geometrically similarto and concentric with the column

A1 is the area of the column


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Development length of dowels must besufficient to transfer column force tofooting

Development length of dowels may not be lessthan the length required if bearing stresswas not exceeded


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ACI Code does not permit splicing of No 14 orNo 18 bars

ACI Code Section 15.8.2.3 does permit No 14

or No 18 bars to be spliced to No 11 (orlarger) dowels in footings

These dowels must extend into the columnnot less than the development length forthe No 14 or No 18 bar, or thecompression lap splice length for thedowels, whichever is larger


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These dowels must extend into the footingfor a distance not less than thedevelopment length for dowels


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Use a larger number of smaller dowels

Use a deeper footing

Add a cap or pedestal to the footing


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Development length must be those fortension

Splice requirements are those found in ACICode Section 12.17


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Square footings are more econonical thanrectangular footings

Long direction steel is uniformly distributedalong short direction

Short direction steel is non uniformlydistributed along long direction


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ACI Code Section 15.4.4.2

Reinforcement in band width 2

Reinforcement in short direction 1

is the ratio of the length of the footing inthe long direction to the length in theshort direction

Remaining steel is distributed uniformlythroughout the two portions of thefooting outside the band


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Design a square column footing for a 16-in.square tied interior column that supportsloads of D = 200 k and L = 160 k. The

column is reinforced with eight No 8 bars,the bottom of the footing is 5 foot belowfinal grade, the soil weighs 100 lb/ft3theallowable soil pressure is 5 ksf. The

concrete strength is 3,000 psi and thesteel is Grade 60.


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Assume a footing thickness of 24in. with aminimum cover of 3 in., this gives a d valueof about 19.5 in. Compute the footing

weight andsoil weight:

Footing weight

24 in150 300 psf

12 in/ft

Soil weight36 in

100 300 psf 12 in/ft


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Effective soil pressure and required area offooting:

2

5000 psf 300 psf 300 psf 4400 psf 200 k 160 k

81.82 ft4.40 ksf

Use 9 ft x 9 ft

eq

A


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Factored bearing pressure for design ofconcrete:

2

1.2 200 k 1.6 160 k 6.12 ksf

81 ftuq


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Depth required to resist punching shear:

22

2

4(16 19.5) 142 in

81.0 ft 2.96 ft 6.12 442.09 k

442,090 lb18.95 in 19.5 in Ok

0.75 4 3000 psi 142 in

442,090 lb

40 19.5 in0.75 2 3000 psi 142 in

142 in

10.12 in 19.5 in Ok

o

u

b

V

d

d


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Depth required to resist one-way shear:

1 9 ft 2.208 ft 6.12 ksf 121.62 k

121,620 lb13.71 in 19.5 in Ok

0.75 2 3000 psi 108 in

uV

d


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Appendix Table 4.12,r = 0.00225


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Development length:

1

bottom cover 3.5 inone-half center-to-center bar spacing 6 in

3.5 in 03.5 Use 2.5

1.0 in

t e s

b

b

b tr

b

cc

c K

d


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'

2,

2,

3

40

3 60,000 1 32.86 diameters

40 2.53000

6.95 in

32.86 32.30 diameters7.07 in

32.30 1.0 in 32.30 in

yd t e s

b trb c

b

s requiredd

b s provided

d

f

c Kd fd

A

d A


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9 ft 12 in/ft 16 in

3 in 43 in 32.30 in2 2

Available length for development


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Design for load transfer for the column andfooting in Example 12.2. The strength ofthe sand-lightweight concrete (different

from Example 12.2) in the column is 4 ksi.


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Bearing force at the column base:

1.2 200 k 1.6 160 k 496 k

Design bearing force at the column base:

2'

10.85 0.65 0.85 4 ksi 16 in

566 k 496 k Ok

cf A


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Design bearingforce in thefooting

concrete:

2

2

' 21

1

2

108 in6.75 Use 2

16 in

0.85

0.65 0.85 3 ksi 16 in 2

848.6 k 496 k Ok

cAf AA

Minimum dowel area:

2 20.005 16 in 1.28 in


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'0.02 0.02 0.75 in 60,000 psi

16.74 in0.85 4000 psi

b y

d

c

d f

f

Dowel development length into the column

'

0.02 0.02 0.75 in 60,000 psi16.43 in

1.0 3000 psi

b y

d

c

d f

f

Dowel development length into the footing


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0.0003 0.0003 0.75 in 60,000 ksi

13.50 in

8.0 in

d b y

d

d f

Development length must not be less than:


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Design for load transfer for a 14-in. squarecolumn to a 13 ft square footing if Pu =800 k. Normal weight concrete is used in

both the column and the footing. Theconcrete in the column is 5 ksi and in thefooting is 3 ksi. The column is reinforcedwith eight No 8 bars.


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Design bearing force in the footing concrete:

2

2

2

1

' 21

1

2

156 in11.14 Use 2

14 in

0.85

0.65 0.85 3 ksi 14 in 2

649.7 k 800 k No good

c

A

A

Af A

A


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Design dowels to resist excess bearing force:

2

2 2

800 k 541.5 k 258.5 k

258.5 k 4.79 in0.9 60 k

0.005 14 in 0.98 in

sA

Use eight No 7 bars (As = 4.80 in2)


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'0.02 0.02 0.875 in 60,000 psi 14.85 in

1 5000 psi

0.0003 0.0003 0.875 in 60,000 ksi

15.75 in8.0 in

b yd

c

d b y

d

d ff

d f

Dowel development length into the column


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'

0.02 0.02 0.875 in 60,000 psi

19.42 in1.0 3000 psi

0.0003 0.0003 0.875 in 60,000 ksi

15.75 in

8.0 in

b y

d

c

d b y

d

d f

f

d f

Dowel development length into the footing


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Design a rectangular footing for an 18-in.interior square column for D = 185 k andL = 150 k. The long side of the footing

should be twice the length of the shortside. The normal weight concretestrength for both the column and thefooting is 4 ksi. The allowable soil

pressure is 4000 psf and the bottom ofthe footing is 5 ft below grade.


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Effective soil pressure and required area offooting:

2

2

2

4000 psf 300 psf 300 psf 3400 psf

185 k 150 k 98.5 ft

3.40 ksf

Use a footing 7'-0" x 14'-0" 98.0 ft

1.2 185 k 1.6 150 k 4.71 ksf

98.0 ft

e

u

q

A

A

q


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Depth required to resist one-way shear. Takeb = 7 ft.

1 7 ft 4.625 ft 4.71 ksf 152.49 k

152,490 lb19.14 in

0.75 1 2 4000 psi 84 in

19.14 4.5 in 23.64 in

uV

d

h


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Depth required to resist punching shear:

22

2

4 18 19.5 in 150 in

98.0 ft 3.125 ft 4.71 ksf 415.58 k

415,580 lb14.60 in 19.5 in Ok

0.75 1 4 4000 psi 150 in

415,580 lb

40 19.5 in0.75 2 4000 psi 150 in

150 in

8.11 in 19.5 in Ok

o

u

b

V

d

d


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22

14 ft 9 in

6.25 ft2 12 in/ft

6.25 ft6.25 ft 7 ft 4.71 ksf 643.9 k-ft

2

12 in/ft 643,900 lb-ft 268.8 psi0.9 84 in 19.5 in

u

u

M

Mbd

Flexural design (steel in long direction)


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Appendix Table 4.13,r = 0.00467

20.00467 84 in 19.5 in 7.65 insA

Use ten No 8 (As = 7.85 in2)


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2

2000.0033

60,000 psi

3 4000 psi

0.0031660,000 psi

0.0033 168 in 19.5 in 10.81 insA

r

r

Use 18 No 7 (As = 10.82 in2)


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Reinforcement in band width 2 2 2

Reinforcement in short direction 1 2 1 3

Use 2/3 x 18 = 12 bars in band width


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footing design as per astm