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Spread Footing Designfor Columns
Architects and engineers have, to my mind,honorable roles in
building things. Life-threatening, property-threatening forces of
compression, thrust, and the like need serious countermeasures.
- AutoCAD Grandad
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PIER(Concrete or Masonry)
SPREADFOOTING(Concrete)
COLUMN
Spread (Column) Footing
The widened part of afoundation that spreads acolumn load over
a
broader area of soil
Design based on Soil Bearing Capacity Column Load
LOAD
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Column Loads
Roof LoadsSupported Floor Loads
Weight of the Column
Weight of the Foundation
Note: If the ground floor isa slab-on-grade, the loadsare not
included.
Roof Loads
2nd Floor Loads
1 st Floor/ Slab Loads
Soil Bearing Pressure
COLUMN
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Sizing a Spread Footing
Size based on: The total load
applied to the soil The allowable soil
bearing capacity
The pressure applied dueto the total load must beless than or
equal to theallowable bearing capacity
SOIL BEARING PRESSURE
LOAD
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Soil Bearing Pressure
Where q = Soil bearing pressureP = Load applied
A = Area of the footing
Pq A
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6
Soil Bearing CapacitiesSoil Type Allowable Soil Bearing
(lb/ft 2)Drainage
BEDROCK 4,000 to 12,000 Poor GRAVELS 3,000 GoodGRAVELS w/ FINES
3,000 GoodSAND 2,000 GoodSAND W/ FINES 2,000 GoodSILT 1,500
MediumCLAYS 1,500 MediumORGANICS 0 to 400 Poor
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Required Footing Area
Where qallowable = Allowable soil bearing pressure
q allowable q
allowable
Pq A
allowable
P Aq
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Net Allowable Bearing Pressure
t footing
1 ft 2
3
lbfooting footing ft
p t 150
net allowable footingq q p
column
net
P Aq
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Required Footing AreaUsing Net Allowable Soil Bearing
Pressure
column
net
P Aq
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3D View of Retail Building
Steel framing and 1st floor slab shown
Size a spread footing for Column B-3 in thebuilding shown below.
The footing thickness is 1ft9in. Assume that the footings bear on
silty sandwith an allowable soil bearing pressure of 3000
psf. COLUMN B-3
Example
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Footing
Column
Girder
Beam
ExamplePartial 2nd Floor Framing 3D View
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G i r d e r
G i r d e r
BeamBeam
ExamplePartial 2 nd Floor Framing
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Column Loads
Assume that an engineer has performedanalyses for the floor and
roof members
with the following results:Roof Beam Reaction = 5,000 lbRoof
Girder Reaction = 10,000 lb
2nd Floor Beam Reaction = 13, 500 lb2nd Floor Girder Reaction =
27,000 lb
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Column Loads
Roof
2nd Floor
1 st Floor Slab
Roof
Interior Beam End Reaction x 2
= (5,000 lb) 2 = 10,000 lbInterior Girder End Reaction x 2
= (10,000 lb) 2 = 20,000 lb
2nd Floor Interior Beam End Reaction x 2
= (13,500 lb) 2 = 27,000 lb
Interior Girder End Reaction x 2= (27,000 lb) 2 = 54,000 lb
Column Weight = 1,500 lb
TOTAL COLUMN LOAD = 112,500 lb
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Calculate the Net Allowable SoilBearing Pressure
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Required Footing Area
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Footing Shape
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Square Footing Size
Use 6'- 6'' x 6'- 6'' x 1'- 9'' thick
6 -6
6 - 6
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Round Footing Size
Use 7'- 6 diameter x 1' - 9'' thick
Say 3ft 9in. radius
