3.2.9-spread footing design
DESCRIPTION
design aciTRANSCRIPT
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Spread Footing Design for 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)
SPREAD FOOTING (Concrete)
COLUMN
Spread (Column) Footing
• The widened part of a foundation that spreads a column load over a broader area of soil
• Design based on– Soil Bearing Capacity– Column Load
LOAD
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Column Loads
•Roof Loads
•Supported Floor Loads
•Weight of the Column
•Weight of the Foundation
Note: If the ground floor is a slab-on-grade, the loads are not included.
Roof Loads
2nd Floor Loads
1st 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 due to the total load must be less than or equal to the allowable bearing capacity
SOIL BEARING PRESSURE
LOAD
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Soil Bearing Pressure
Where q = Soil bearing pressure
P = 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 PoorGRAVELS 3,000 GoodGRAVELS w/ FINES 3,000 Good SAND 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
qallowable ≥ q
allowableP
qA
allowable
PA
q
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Net Allowable Bearing Pressure
t footing
1 ft2
3lb
footing footing ftp t 150
net allowable footingq q p
column
net
PA
q
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Required Footing Area Using Net Allowable Soil Bearing Pressure
column
net
PA
q
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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 the building shown below. The footing thickness is 1ft 9in. Assume that the footings bear on silty sand with an allowable soil bearing pressure of 3000 psf. COLUMN B-3
Example
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FootingColumn
Girder
Beam
ExamplePartial 2nd Floor Framing 3D View
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Column B-3
Gird
erG
irder
BeamBeam
ExamplePartial 2nd Floor Framing
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Column Loads
Assume that an engineer has performed analyses 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
1st Floor Slab
Roof
Interior Beam End Reaction x 2
= (5,000 lb) 2 = 10,000 lb
Interior 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 Soil Bearing 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
7'-6''
Say 3ft 9in. radius