contrasting design approaches for slab-on ... -...

Contrasting Design Approaches forContrasting Design Approaches forSlab-on-Ground and Raised FloorSlab-on-Ground and Raised Floor

Foundations on Expansive SoilsFoundations on Expansive Soils

Robert L. Lytton

Houston Foundation Performance AssociationHouston, Texas

December 9, 2009

OutlineOutline

• Elements of design• Site conditions• Slabs-on-ground• Raised floor foundations• Costs

1

Elements of DesignElements of Design

• Structural requirements Moments Shear Deflection

• Costs

2

As determined by site conditions

Site ConditionsSite Conditions

• Soils

• Site hazards Trees Slopes Drainage

• Building geometry

3

Slabs-on-Ground: SoilsSlabs-on-Ground: Soils

• ym: differential movement of soil Perimeter swelling Perimeter shrinking Center swelling Center shrinking

4


• em: edge moisture variation distance Not a cantilever distance A property of the soil mass Depends on soil activity and cracking

7


• zm: depth of the moisture active zone Depth of shrinkage cracks created by roots (log for root

fibers) No deeper than soil at the wilting point of plants

11


• zA: depth of the movement active zone Always shallower than zm

Large factor in ym

15

Raised Floor Foundations: SoilsRaised Floor Foundations: Soils

• ym: differential movement Center drying, total shrinkage Perimeter total swelling

• Drainage• Flower beds

Perimeter total shrinking: trees Downhill creep

18

NOTE

AM OUNT OF SHRINKAGE DEPENDS ON THE LEVEL OF RELATIVE HUM IDITYIN THE AIR BENEATH THE FLOOR. THE LOWER THE RELATIVE HUM IDITY, THEGREATER WILL BE THE AM OUNT OF SHRINKAGE. INITIALLY, THE SOIL WILLBE AT OR ABOVE 98% RELATIVE HUM IDITY. e m IS THE EDGE M OISTUREVARIATION DISTANCE WHICH VARIES TYPICALLY BETWEEN 3 AND 9 FEETDEPENDING ON THE TYPE OF SOIL.

NOTE


FLOORFLOOR

DRY AIR SPACEDRY AIR SPACE

SHRINKAGESHRINKAGE

AIR SPACE BENEATH THE FLOORAIR SPACE BENEATH THE FLOOR

emem emem

ORIGINALGROUND LEVELORIGINALGROUND LEVEL

TOTAL MOVEMENT PATTERNSWITH RAISED WOOD FLOOR FOUNDATIONS


NOTE

AM OUNT OF HEAVE DEPENDS ON THE AM OUNT OF WATER THAT IS M ADEAVAILABLE TO THE SOIL AT THE EDGE OF THE BUILDING AND THE TYPEOF SOIL. e m IS THE EDGE M OISTURE VARIATION DISTANCE WHICH VARIESTYPICALLY BETWEEN 3 AND 9 FEET DEPENDING ON THE TYPE OF SOIL.

NOTE


VERTICAL AND HORIZONTALMOISTURE BARRIERS


FLOORFLOORP OORDRAINAGEP OORDRAINAGE

WATE RWATE R

WATE RFLOW

WATE RFLOW

SLOPESLOPE

WATE RFLOW

WATE RFLOW

WATE RINGWATE RING

FLOWE R BE DFLOWE R BE D

emem e me m

HE AV EHE AV E HE AV EHE AV E

NOTE

AM OUNT OF HEAVE DEPENDS ON FREQUENCY OF LAWN IRRIGATION AND TYPEOF SOIL. e m IS THE EDGE M OISTURE VARIATION DISTANCE WHICH VARIESTYPICALLY BETWEEN 3 AND 9 FEET DEPENDING ON THE TYPE OF SOIL.

NOTE

AM OUNT OF HEAVE DEPENDS ON FREQUENCY OF LAWN IRRIGATION AND TYPEOF SOIL. e m IS THE EDGE M OISTURE VARIATION DISTANCE WHICH VARIESTYPICALLY BETWEEN 3 AND 9 FEET DEPENDING ON THE TYPE OF SOIL.

LAWN IRRIGATIONLAWN IRRIGATION

FLOORFLOOR

LAWNIRRIGATION

LAWNIRRIGATION

LAWNIRRIGATION

LAWNIRRIGATION

emem e me m

HE AV EHE AV E HE AV EORIGINALGROUND LE V E LORIGINALGROUND LE V E LHE AV E

NOTE

AM OUNT OF SHRINKAGE DEPENDS ON T HE DEPTH OF THE TREEROOT Z ONE, HOW FAR T HE ROOT S PENET RATE BENEATH THEBUILDING, AND T HE T YPE OF SOIL .

NOTE


TREE ROOT ZONETREE ROOT ZONE

WATER IS DRAWN INTOTHE ROOT ZONE DURINGDRY WEATHER


TREESTREES

FLOORFLOOR

ORIGINALGROUND LEVELORIGINALGROUND LEVELSHRINKAGESHRINKAGE

NOTE

DOWNHILL CREEP OCCURS ON SLOPES THAT ARE STEEPER THAN 1 FOOT RISE IN 10 HORIZONTAL FEET. CREEP RATE IS GREATER THESTEEPER THE SLOPE AND THE WETTER THE SOIL IS .

NOTE


DOWNHILL CREEPDOWNHILL CREEP

FLOORORIGINALGROUND LEVELORIGINALGROUND LEVEL

FLOOR

NO MOVEMENT PLANE

NO MOVEMENT PLANE

DOWNHILL CREEPM OVEM ENT




BUILDINGDISTORTION

BUILDINGDISTORTION


• em: edge moisture variation distance Not as important as with slabs on ground except around

the perimeter

24

NOTE


NOTE


FLOORFLOOR


SHRINKAGESHRINKAGE


emem emem





• zm: depth of the moisture active zone Same factors as with slab on ground Interest is in the total movement

26


• zA: depth of movement active zone Important to determine movement of supports

• Pads• Spread footings• Posts• Piers

Which to use? Determined by costs

28

Objective:design for expected movement

Objective: small support movement

Slab-on-Ground: Site HazardsSlab-on-Ground: Site Hazards

• Trees Shrinkage Affects zm, zA, em, ym

• Slopes Like differential shrinkage Rate depends on moisture Affects zm, ym

• Drainage Swelling Affects em, ym

29

Raised Floor Foundations:Raised Floor Foundations:Site HazardsSite Hazards

• Trees: shrinkage reduces total differentialmovement

• Slopes Like differential shrinkage Rate depends on moisture Affects zm, ym

• Drainage Swelling Increases total differential movement Affects ym

30

Slab-on-Ground: Building GeometrySlab-on-Ground: Building Geometry

• Shape factor affects structural design quantities Moment Shear Differential deflection

• Shape factor

Circle = 12.56 Square = 16 Rectangle = 3.7 ~ 1 SF = 24

31

Raised Floor Foundations:Raised Floor Foundations:Building GeometryBuilding Geometry

With adjustable floor elevation, does not matter

32

Slab-on-Ground:Slab-on-Ground:Structural RequirementsStructural Requirements

This is a soil-structure interaction problem.

• Soil is not a uniform pressure• em is not a cantilever distance• Structural properties of slab and soil are creep

(long-term) properties

35


• Ground pressures never come close to bearingcapacity

• zm is dictated by site conditions and not byregional conditions

• ym for both edge shrinking and edge swelling (> 1inch) must be estimated accurately

36


• em: for both edge shrinking and edge swelling isthe most sensitive predictor of design moments,shears, and required stiffness, EI.

• zA: is smaller than zm and is dictated by siteconditions and not by regional conditions

37


• β – length = relative stiffness length

• Dictates what is a long- and short-slab (> 6β islong)

• Controls design quantities for long slabs

48

Ec = creep stiffness of concreteEs = creep stiffness of soil

Design Criteria:Design Criteria:Deflections / ToleranceDeflections / Tolerance

• Sensitivity of foundation and super structure (Δ/L)• Differential (Δc+ Δs)

As built (Δc) Soil movement (Δs)

• Total• Twisting

49

Design Criteria:Design Criteria:StiffnessStiffness

• Substitute for deflection tolerance

• Enough concrete section to handle soil movement

50

Raised Floor Foundations:Raised Floor Foundations:Structural RequirementsStructural Requirements

This is NOT a soil-structure interaction problem.

• Moment• Shear• Deflection

are the same as for column-supported structures.

51

criteria

Raised Floor Foundations:Raised Floor Foundations:Structural RequirementsStructural Requirements

• Allowable deflections before adjusting supportelevations dictate the design quantities of the floorsupports

• Design of supporting footings, posts or piers is tomake rough estimate of total movement Vertical Rotation

52

Movement PatternsMovement Patterns

53

Movement PatternsMovement Patterns

54

UPWARDMOVEMENT

ANCHORZONE

REINFORCING

DRILLED PIER

0

5

10

15

20

pF3.0 3.5

pF2.53.5

DRILLED PIER

0

5

10

15

20

Shear Stress Shear Stress

Horizontal StressDistribution

DRILLED PIER

0

5

10

15

20

Axial Tension Deflection Moment

DRILLED PIER

DRILLED PIER REINFORCEMENT

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 2000 4000 6000

Perc

ent o

f Ste

el d

ue to

Sid

e Fr

ictio

n St

ress

Side Friction Stress, αc, (psf)

d = 1.0 ’

d = 1.0 ‘

d = 1.5 ‘

d = 2.0 ‘

d = 1.5 ‘

d = 2.5 ‘

d = 1.0 ‘d = 2.0 ‘

d = 2.5 ‘

d = 1.5 ‘

d = 2.0 ‘

d = 2.5 ‘

L = 15 ’ L = 10 ’

L = 10 ’

L = 15 ’

L = 10 ’

L = 15 ’

L = 5 ’L = 10 ’

L = 10 ’

L = 5 ’

L = 5 ’

L = 5 ’

Temperature and Shrinkage Reinforcement

Concrete Tensile

Strength Governs

Steel Yield

Stress Governs

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

0.000 1.000 2.000 3.000 4.000 5.000

Soil Suction, pF

Copyright John T. Bryant (2008)

Empirically Measured SuctionsEmpirically Measured SuctionsBCI 2002 to 2008 = 26,000+ Data PointsBCI 2002 to 2008 = 26,000+ Data Points

Reasonable Suction Change RangeReasonable Suction Change Range

2003 Total Soil Suction Data (4776 Observations)

0

200

400

600

800

1000

1200

Unsaturated Soil MechanicsTypical Suction Envelope

Initial Profile

EquilibriumProfile

Final Profile

Typical “Trumpet”shape based onMitchell and Lytton.

Exponential Suction Profile for Extreme WettingExponential Suction Profile for Extreme Wettingand Drying Conditionand Drying Condition

0

2

4

6

8

10

12

14

2 3 4 5

Suction (pF)

Dep

th (f

t)

Wet

Equil.

Dry

0

2

4

6

8

10

12

14

0 0.2 0.4 0.6

Volumetric Water Content

Wet

Dry

0

2

4

6

8

10

12

14

0 1 2 3

Vertical Movement (in)

Wet

Dry

Fort Worth Interstate 820

Mitchell (1979)

MoistureActivezone

( , ) exp - cos 2 -e on n

U Z t U U Z nt Z! !

!" "

# $ # $= + % & % &% & % &

' ( ' (

( ) exp -e on

U Z U U Z!"

# $= + % &% &

' (

NOTE


NOTE


FLOORFLOOR


SHRINKAGESHRINKAGE


emem emem




Raised Floor FoundationRaised Floor FoundationBoundary ConditionsBoundary Conditions

OUTSIDE pFWET

FLOWER BED 2.9LAWN WATER 3.0TREE 4.5

pF DRY R.H.BENEATH,%

6.50 10 6.35 20 6.22 30 6.10 40 5.98 50

NOTE


NOTE


TREE ROOT ZONETREE ROOT ZONE



TREESTREES

FLOORFLOOR

ORIGINALGROUND LEVELORIGINALGROUND LEVELSHRINKAGESHRINKAGE

Laboratory Tests

Atterberg limits Hydrometer Water content Dry density Sieve analysis

Typical Clay Properties

Property Range ExampleLiquid Limit 55-90 86Plasticity 30-60 59Plastic Limit 25-30 27Percent Passing #200 80-99 97Percent Passing 0.002mm - 65

Effects of Flower Beds

NOTE

VERTICAL AND HORIZONTAL M OISTURE BARRIERS DECREASE THE AM OUNTOF M OISTURE THAT GETS BENEATH THE WALLS OF THE BUILDING. EFFECTIVEDEPTHS OF VERTICAL M OISTURE BARRIERS ARE BETWEEN 2.5 AND 4 FEET.EFFECTIVE WIDTHS OF HORIZONTAL M OISTURE BARRIERS ARE BETWEEN 4 FEETAND e m, THE EDGE M OISTURE VARIATION DISTANCE.

NOTE

VERTICAL AND HORIZONTAL M OISTURE BARRIERS DECREASE THE AM OUNTOF M OISTURE THAT GETS BENEATH THE WALLS OF THE BUILDING. EFFECTIVEDEPTHS OF VERTICAL M OISTURE BARRIERS ARE BETWEEN 2.5 AND 4 FEET.EFFECTIVE WIDTHS OF HORIZONTAL M OISTURE BARRIERS ARE BETWEEN 4 FEETAND e m, THE EDGE M OISTURE VARIATION DISTANCE.



FLOOR

ORIGINALGROUND LE V E LORIGINALGROUND LE V E L

FLOOR

WATE RFLOW

WATE RFLOW

WATE RFLOW

WATE RFLOW

V E RTICALM OIS TURE BARRIE RV E RTICALM OIS TURE BARRIE R

HORIZONTALM OIS TURE BARRIE RHORIZONTALM OIS TURE BARRIE R

Effects of Trees

NOTE


NOTE




FLOORFLOORP OORDRAINAGEP OORDRAINAGE

WATE RWATE R

WATE RFLOW

WATE RFLOW

SLOPESLOPE

WATE RFLOW

WATE RFLOW

WATE RINGWATE RING

FLOWE R BE DFLOWE R BE D

emem e me m

HE AV EHE AV E HE AV EHE AV E

NOTE


NOTE


DOWNHILL CREEPDOWNHILL CREEP

FLOORORIGINALGROUND LEVELORIGINALGROUND LEVEL

FLOOR

NO MOVEMENT PLANE

NO MOVEMENT PLANE





BUILDINGDISTORTION

BUILDINGDISTORTION

Shallow Slope Failure

GRASS

ROOTS

During dry periods roots extract waterFrom the soil and cause shrinkage cracks

101

ROOT ZONE

Crack Spacing Gets Larger with Depth

102

RUNOFF WATER

4.0

2.0

4.0

2.5

pF

SUCTION RANGEBETWEEN CRACKS

WATERSOAKSINTOSOIL 2.0

Foundation soil propertiesFoundation soil properties

Moisture Conditions Beside Raised Floors Beneath Raised Floors

Differential MovementsSoil PropertiesSite Investigation

Raised Floor FoundationRaised Floor FoundationBoundary ConditionsBoundary Conditions

OUTSIDE pFWET

FLOWER BED 2.9LAWN WATER 3.0TREE 4.5

pF DRY R.H.BENEATH,%

6.50 10 6.35 20 6.22 30 6.10 40 5.98 50

OutlineOutline

• Elements of design• Site conditions• Slabs-on-ground• Raised floor foundations• costs

109

Elements of DesignElements of Design

• Structural requirements Moments Shear Deflection

• Costs

110

As determined by site conditions

Site ConditionsSite Conditions

• Soils

• Site hazards Trees Slopes Drainage

• Building geometry

111

Contrasting Design Approaches forContrasting Design Approaches forSlab-on-Ground and Raised FloorSlab-on-Ground and Raised Floor

Foundations on Expansive SoilsFoundations on Expansive Soils

Robert L. Lytton

Houston Foundation Performance AssociationHouston, Texas

December 9, 2009

contrasting design approaches for slab-on ... -...

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