commercial foundations he who has not first laid his foundations may be able with great ability to...
TRANSCRIPT
Commercial Foundations
He who has not first laid his foundations may be able with great ability to lay them afterwards, but they will be laid with trouble to the architect and danger to the building. Niccolo Machiavelli (1469 - 1527), The Prince
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 Allowable 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
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.
Flood Zones
• A Zone – Areas with a 1% annual chance of flooding
• V Zone – Coastal areas with a 1% or greater chance of flooding and an additional hazard associated with storm waves
• BFE (Base Flood Elevation) – The elevation shown on the Flood Insurance Rate Map that indicates the water surface elevation resulting from a flood with a 1% chance of equaling or exceeding that level in any given year
Flo
od In
sura
nce
Rat
e M
aps
Note: V29 indicates V-zone and BFE = 29 ft
A Zone• Residential buildings - lowest floor (including basement)
elevated at or above the BFE • Non-residential buildings
– Lowest floor (including basement) elevated at or above the BFE
OR
– Dry-floodproofed to the BFE
Dry floodproofing: Building must be designed and constructed to be watertight to floodwaters
V Zone
All Buildings (residential and non-residential)
• Elevated on piles and columns • Lowest horizontal structural member of lowest floor at
or above the BFE• Anchored to resist flotation, collapse, and lateral
movement• The area below the lowest floor must be
– Used only for the parking of vehicles, building access, or storage,
– Free of obstruction, OR– Any enclosure must be constructed of breakaway
walls
V ZoneBreak-away walls: non-supporting and non-load bearing walls that easily break away from the structure when subjected to lateral flood forces.
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
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
PIER (Concrete or Masonry)
SPREAD FOOTING (Concrete)
COLUMNLOAD
Continuous (Strip) Footing
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)
LOAD
THICKENED SLAB
WALL
SLAB-ON-GRADE
Slab-on-Grade and Thickened Slab
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)
Mat Foundation
A large, heavily reinforced concrete slab placed under the entire building to support loads from several points
USES
Heavy loads on weak soil
MAT FOUNDATION
CONCRETE PIER
Deep Foundations
Deep Foundation
PILES
LOAD
Friction Force (Resisting Force)
Bearing Force (Resisting Force)
PILE CAP
The building LOAD is transferred through friction on the sides of the piles and/or bearing on the end of the piles
Top Soil
Weak Soil
Strong Soil
Pile Foundation
Pile – Vertical structural member that is driven, jetted, or drilled into the ground in order to gain support from deeper soil layers
PILE
PILE CAP (CONCRETE)
USESWeak shallow soil withdeep satisfactory soils
PIER (CONCRETE) LOAD
Cast-in-Situ Piles
A large diameter cast-in-place concrete pile
USES
Weak shallow soil with satisfactory soils at intermediate depth
CAST-IN-SITU PILE
BELL can improve bearing capacity
GRADE BEAM
LOAD
Why Do Foundations Fail?
Bending Failure – Foundation fractures due to bending moment
Shear Failure – Foundation breaks due to excessive shear
Punch Through – Structural member “punches through” concrete foundation
Foundation Failure
SECTION
PLAN
Bending Failure
Punch Through
Why Do Foundations Fail?
Settlement – Foundation moves
• Weak or compressible soil• Expansion/contraction of soil
(moisture)• Frost heave
Soil Failure
Illustrations courtesy U. S. Marine Corp.
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?