Transcript CommercialFoundations

Commercial Foundations
• Purpose
• Considerations
• Types of Foundations
– Shallow Foundations
• Spread Footings
• Strip Foundations
• Slab-on-Grade and Thickened Slabs
• Mat Foundation
– Deep Foundations
• Piles
• Cast-in-Situ Piles
• Why Do Foundations Fail?
Purpose of Foundations
• Provide a level, stable surface to safely
support a building
• Transfer building loads to soil
• Anchor the building from wind, flood, and
seismic loads
Design Considerations
• Loads from the structure
• Allowable soil bearing pressure
• Frost depth
• Flood elevation
• Drainage
• Costs
Loads from the Structure
Foundations must resist
•Dead Load
•Live Load
•Lateral Loads
-- Wind
-- Seismic activity
-- Flood
SOIL REACTIONS
Allowable Soil Bearing Pressure
• Indicates the maximum pressure that a
soil may be designed to support
• Includes a factor of safety
• Dictates the size, depth, and type of
foundation
• Typically presented in pounds per square
foot (psf)
• Different types of soils have different
allowable soil bearing pressures
Soil Information
• Local Building Department, Codes and
Regulations
• Preliminary info: USDA Web Soil Survey
http://websoilsurvey.nrcs.usda.gov
• Local or State Building Codes
• Soil testing/analysis
– Site inspection and simple soil testing
– Soil borings taken at proposed foundation
locations
Estimated Allowable Soil Bearing
Pressure
Soil Type
BEDROCK
GRAVELS
GRAVELS w/ FINES
SAND
SAND W/ FINES
SILT
CLAYS
ORGANICS
Allowable Bearing
(lb/ft 2)
4,000 to 12,000
3,000
3,000
2,000
2,000
1,500
1,500
0 to 400
Drainage
Poor
Good
Good
Good
Good
Medium
Medium
Poor
Frost Depth
• Freezing of soil can cause heaving of
foundations
• Silt or clay soils with a high water table are
highly susceptible to frost
Defense
• Build base of foundation below frost depth
or
• Provide frost protection for foundation
Frost Heave
Flood Elevation
• Inundation by flood
waters should be
avoided
– Damage to structure
– Damage to contents
• Height of floors and/or
flood proofing is
dictated by building
codes
Courtesy Federal Emergency Management Agency. Photographer
Dave Saville.
Types of Foundations
• Shallow Foundation: Transfers loads to
the soil very near the surface
– Spread footing or strip footing
– Mat or raft foundation
– Slab-on-grade
• Deep Foundation: Transfers loads to
deeper soil layers
– Piles
– Cast-in-Situ Piles
Shallow Foundations
Shallow Foundation
• The load from the
footing spreads out
so that the soil
bearing pressure
diminishes with
depth.
• The soil directly
under the footing
takes the greatest
load.
LOAD
Critical
Load Area
Bearing
Pressure
(decreases
with depth)
Spread (Column) Footing
COLUMN
PIER
(Concrete or
Masonry)
SPREAD
FOOTING
(Concrete)
LOAD
A footing that spreads
the load over a broad
area which supports one
(or a few) load(s)
USES
Usually used in low-rise
buildings
Continuous (Strip) Foundation
A wide strip of reinforced
concrete that supports
loads from a bearing wall
USES
•
Light frame construction
•
Under foundation walls
FOUNDATION
WALL
(Concrete or
Masonry)
STRIP
FOOTING
(Concrete)
Slab-on-Grade
and Thickened Slab
WALL
Slab-on-Grade – Reinforced concrete
floor supported by soil
Thickened Slab – A slab on grade with
an integral footing created by
thickening the slab
USES
• Residential or light commercial
construction
• Shallow frost depth or when frost
protection is used (instead of strip
footing)
SLAB-ONGRADE
THICKENED
SLAB
Mat Foundation
A large, heavily reinforced
concrete slab placed under
the entire building to
support loads from several
points
MAT FOUNDATION
USES
Heavy loads on weak soil
CONCRETE PIER
Deep Foundations
Deep Foundation
LOAD
Top
Soil
PILE CAP
PILES
Friction Force
(Resisting Force)
Weak
Soil
Bearing Force
(Resisting Force)
Strong
Soil
The building
LOAD is
transferred
through friction on
the sides of the
piles and/or
bearing on the
end of the piles
Pile Foundation
PIER
(CONCRETE)
PILE CAP
(CONCRETE)
LOAD
Pile – Vertical structural
member that is driven,
jetted, or drilled into the
ground in order to gain
support from deeper soil
layers
USES
Weak shallow soil with
deep satisfactory soils
PILE
Cast-in-Situ Piles
A large diameter cast-in-place concrete pile
GRADE BEAM
CAST-IN-SITU
PILE
USES
Weak shallow soil
with satisfactory
soils at intermediate
depth
BELL can improve
bearing capacity
LOAD
Why Do Foundations Fail?
Foundation Failure
Bending Failure – Foundation
fractures due to bending moment
Shear Failure – Foundation breaks
due to excessive shear
Punch Through – Structural member
“punches through” concrete
foundation
Bending
Failure
Punch
Through
SECTION
PLAN
Why Do Foundations Fail?
Soil Failure
Settlement – Foundation
moves
• Weak or compressible soil
• Expansion/contraction of soil
(moisture)
• Frost heave
Illustrations courtesy U. S. Marine Corp.