Slab-on-Ground & Foundations Convention/Session... · Slab-on-Ground & Foundations ... • Intermediate anchors allowed cables ... Edition that are a part of the 3rd Edition The 3rd

TECHNICAL SESSION 5

Slab-on-Ground & Foundations

Moderator

Ken Bondy Consulting Structural Engineer

West Hills, CA

Use of Post Tensioning in a North Carolina Warehouse Floor Slab

Ian Blair Chief Executive Officer

Blair Concrete Services, Inc.

This presentation will discuss Blair Concrete Services’ involvement in the construction of the new Universal Leaf Tobacco plant in Tarboro North Carolina. The 1.3 million square foot structure contained approximately 750,000 square feet of post-tensioned slab on grade. The project also contained approximately 550,000 square feet of conventional steel reinforced slab on grade. Mr. Blair will highlight the logic of the design compared with the conventionally steel reinforced design, and will review the details of the project and the implementation and installation process.

2004 PTI Technical Conference Session 5 Ian Blair - Use of PT in a Warehouse Floor Slab

Copyright - Post-Tensioning Institute 65 All rights reserved

Design Logic

• Green Storage and Shipping are sensitive to floor scarring and damage. Required a joint-free, crack-free surface.

• Process area would accommodate tremendous amount of machinery and would be subject to slab retrofits and equipment attachment and therefore a conventional non-PT design would be used.

Post Tensioning Calculations:How Much Cable?

• The design to meet a specified mid-length PSI is very theoretical.

• Variables to be considered: Friction coefficient, sub-base modulus, minimal tendon force requirements, and slab length.

• Nobody has ever tested mid-length PSI to know what forces are actually attained.

• Objective is to have a slab in compression that does not crack. Is the 250 PSI important?

Design:

•An eventual design was arrived at in terms of cable amount and spacing

•2 way post tension design with east/west cables spaced approximately 16 inches apart and north/south cables spaced 13 inches apart.•5 inch slab with a sub-grade friction factor of 0.5.•Cables were tensioned to approximately 30 kips.

PSI Test:

• McKinney and Company set out to measure PSI at mid-length.

• Results were too high.

Dock Pits: Pits formed larger than needed to accommodate floor movement.



Pour back strips: Pour back strips were covered with an abuse curb which sat on top of slab allowing slab to move under it.

Door Openings: Door openings were tied into PT slab and a PNA armor joint allowed movement.

Details:

• Columns, penetrations wrapped: All columns and penetrations were wrapped with 2-inch foam to allow for movement. It was well utilized.

• Slip-sheets: 2 layers of 15-mil poly under all slabs and at dock levelers.

Installation:• Onsite batch plant was utilized to accommodate

schedule and production requirements.

Installation:• Proof rolled every inch to insure excellent sub-

grade. ½ inch ruts were replaced.• Laser graded the slabs for uniform surface.

Installation

• Cables had to be laid for entire room in order to accommodate continuous cable requirements. This also required slabs to be checker-boarded, and thus added logistical challenge in terms of multiple work areas simultaneously working. 60 men worked 24 hrs. around the clock. Poured 50,000 sf. per day.



Installation:• Intermediate anchors allowed cables to be tensioned

on adjacent pours.• Laser-screeded floors with two screeds. Poured 150

yards per hour.

Installation:

• Compressive tests were done around the clock. Key was to get early tensioning!

Installation:• Cured with Wet Cure method. “Transguard”



Finished product was FF 60 FL 45, crack-free, joint-free floors!!



Simpson Gumpertz & Heger Consulting Engineers

PostPost--Tensioned ConcreteTensioned ConcreteSlabsSlabs--onon--GroundGround

The 3The 3rdrd Edition of the PTI ManualEdition of the PTI Manual

2004 PTI Technical Conference, 18 May 20042004 PTI Technical Conference, 18 May 2004

By Rene W. Luft, P. E.By Rene W. Luft, P. E.San Francisco, CASan Francisco, CA

ConsultingEngineersSlabsSlabs--onon--GradeGrade

•• ObjectiveObjectiveThe PTI slab-on-ground committee voted the 3rd Edition of the PTI Manual on 16 May 2004This presentation gives some of the changes to the 2nd

Edition that are a part of the 3rd EditionThe 3rd Edition contains design recommendations in code language; parallel columns give the design recommendations and the commentary

ConsultingEngineersMaterialsMaterials

•• SOGs are Structural Plain Concrete MembersSOGs are Structural Plain Concrete MembersPlain concrete is structural concrete with no reinforcement or with less reinforcement than the minimum amount specified for reinforced concreteThe minimum specified 28-day concrete compressive strength shall be 2,500 psi (17.3 MPa)The minimum specified 28-day concrete compressive strength shall be 3,000 psi for concrete exposed to freezing and thawing and deicing chemicalsFor residential SOGs on soils with Severe sulfate exposure, Type V cement shall be used and the minimum specified 28-day concrete compressive strength shall be 3,000 psi (20.8 MPa) (per PTI)

ConsultingEngineersMaterials (continued)Materials (continued)

•• PostPost--tensioned SOGs typically use ½ inch diameter, tensioned SOGs typically use ½ inch diameter, unbonded, single strand tendons of 270 ksi specified tensile unbonded, single strand tendons of 270 ksi specified tensile strength (A = 0.153 sq in.) conforming to ASTM A 416strength (A = 0.153 sq in.) conforming to ASTM A 416

SOGs on soils with high chloride levels (> 0.08 % acid soluble, ASTM C 1152) shall utilize encapsulated unbonded tendons

•• PostPost--tensioned SOGs tensioned SOGs maymay use nonuse non--prestressed prestressed reinforcement (ASTM A 615) or welded wire fabric (ASTM reinforcement (ASTM A 615) or welded wire fabric (ASTM A 185)A 185)

ConsultingEngineersGeometries of Type III SOGGeometries of Type III SOG

Post-tensioned Type III SOGs are either Ribbed foundations or Uniform thickness foundations

ConsultingEngineersExpansive ClaysExpansive Clays

•• Expansion Index EI > 20 (ASTM D 4829)Expansion Index EI > 20 (ASTM D 4829)•• OROR•• Plasticity Index PI Plasticity Index PI 15 (ASTM D 4318)(ASTM D 4318)• Soil particles that pass No. 200 sieve 10 % (ASTM D 422)• Soil particles smaller than 5 micrometers 10 % (ASTM D 422)

2004 PTI Technical Conference Session 5 Rene Luft - Provisions of the New PTI Design Procedure


ConsultingEngineers

Behavior of Expansive Clays Behavior of Expansive Clays ––Climate ControlledClimate Controlled

ConsultingEngineersClimate Controlled Soil Climate Controlled Soil –– Stage 1Stage 1

Surface of constant suction, pF ~ 3.2 Surface of constant suction, pF ~ 3.2 –– 3.63.6

Evaporation andEvapotranspiration

ShrinkageOccurs

Rain

Depth to constant suction

Soil surface, summer, pF ~ 4.5 Soil surface, summer, pF ~ 4.5 –– 6.06.0

Soil surface, winter, pF ~ 2.5 Soil surface, winter, pF ~ 2.5 -- 3.03.0




Rain



Soil surface, winter, pF ~ 2.5 Soil surface, winter, pF ~ 2.5 -- 3.03.0

Impermeable cylinder and lid

Frictionless




Rain



Impermeable lid

Equilibrium suction profile Summer suction profile





Impermeable lid

Moisture flow to equilibrate suctionEquilibrium suction profile

Transition suction profile

ConsultingEngineers

Determination of Soil Parameter,Determination of Soil Parameter,eemm, Edge Moisture Variation Distance, Edge Moisture Variation Distance

•• Soil parameters neededSoil parameters neededLiquid limit, LLPlasticity Index, PI% passing #200 sieve, %-#200% passing 2 microns, %-2μFabric factor, Ff

•• % Fine clay is (%fc) = {(%% Fine clay is (%fc) = {(%--22μμ)/(%)/(%--200#)}*100200#)}*100•• Find Mineral Classification (see chart) with LL and PIFind Mineral Classification (see chart) with LL and PI•• Find Find 00 from charts, and calculate from charts, and calculate h = h = 0 * 0 * (%fc)(%fc)•• (( h swellh swell)) = = h h * exp(* exp( h h ))•• (( h shrinkh shrink)) = = h h * exp(* exp(-- h h ))



ConsultingEngineers


•• Method I: Use an unsaturated soil mechanics Method I: Use an unsaturated soil mechanics computer programcomputer program

Estimate the distance em over which shrinkage or swelling occurs. Use the suction profiles at the dry and wet locations

•• Method II: Method II: Use the PTI approach given in the 3rd Edition of the PTI Manual (following slides)

ConsultingEngineersAssumptions for Method IIAssumptions for Method II

•• (( hh)) is not highly variable in is not highly variable in the soil layerthe soil layer

•• Suction profile in the soil Suction profile in the soil layer has typical trumpet layer has typical trumpet shapeshape

•• The initial suction is in The initial suction is in equilibrium from the depth equilibrium from the depth z at constant suction to the z at constant suction to the surfacesurface

Then becoming wet, orThen becoming dry

ConsultingEngineers


•• Calculate S = Calculate S = --20.29 + 0.1555 * (LL) 20.29 + 0.1555 * (LL) -- 0.117 * (PI) + 0.0684 * 0.117 * (PI) + 0.0684 * (%(%--200#)200#)

•• Calculate Calculate , the unsaturated diffusion coefficient, for , the unsaturated diffusion coefficient, for (( h swellh swell) ) and and h shrinkh shrink), for each soil layer, where), for each soil layer, where

= 0.0029 – 0.000162 * S – 0.0122 * ( h )’ = * Ff

•• Calculate the weighted average of Calculate the weighted average of for the top nine for the top nine ft of soil (see slide on yft of soil (see slide on ymm for weighting approach)for weighting approach)

•• Determine Thornthwaite Moisture Index, IDetermine Thornthwaite Moisture Index, Imm

•• Determine eDetermine em m for center lift and edge lift, based on Ifor center lift and edge lift, based on Imm, , ’’h swellh swell, and, and ’’h shrinkh shrink

Use larger em based on weather (Im) and unsaturated diffusion coefficient ( )

ConsultingEngineers

Determination of Soil Parameter,Determination of Soil Parameter,yymm, Differential Soil Movement, Differential Soil Movement

•• Method I: Use an unsaturated soil mechanics Method I: Use an unsaturated soil mechanics computer programcomputer program

Estimate change in soil surface elevation for two locations separated by a distance em with the suction profiles at the two locations

•• Method II: Method II: Estimate the soil suction at the surface

Wettest, 2.5 pF ; Driest, 4.5 pF to 6.0 pF; Estimate the soil suction at depth of constant suction (usually in range 3.2 pF to 3.6 pF)

ConsultingEngineers

Determination of Soil Parameter,Determination of Soil Parameter,yymm, Differential Soil Movement, Differential Soil Movement

•• Method II (continued)Method II (continued)Calculate stress change factor SCF for

Wettest surface and constant suction (edge lift)Driest surface and constant suction (center lift)

Calculate ym = ( h)mod * SCFWhere ( h)mod is the weighted average of ( h) in each soil layer up a depth of nine feet (similar to Fig 18-III-9 of UBC 1997)

– Use a weight factor of 3 for the upper one-third of the soil layer, a factor of 2 for the middle one-third, and a factor of 1 for the bottom one-third

– Use the product of the thickness (depth) and weight factor of each soil layer for the weighted average of ( h)

– Use ( h swell) for edge lift and ( h shrink) for center lift

ConsultingEngineersShape of Soil over Distance eShape of Soil over Distance emm



ConsultingEngineersYYmm = = hh * SCF* SCF Consulting

EngineersExample of Calculation of eExample of Calculation of emm

Determination of Mineral ClassificationDetermination of Mineral Classification

Zone II

ConsultingEngineers

Example of Calculation of eExample of Calculation of emmDetermination of Determination of 00

o = 0.11

ConsultingEngineersExample of Calculation of eExample of Calculation of emm

em = 2.7

em = 5.0

em = 3.2

em = 6.1

ConsultingEngineers

Soil Soil –– Structure Interaction in Structure Interaction in PostPost--Tensioning Institute MethodTensioning Institute Method

•• Based on curve fitting the results of multiple finite Based on curve fitting the results of multiple finite element analyses of rectangular ribbed foundations.element analyses of rectangular ribbed foundations.

•• Assumptions of modelAssumptions of modelThe perimeter load P is uniform and in the range of 600 to 1,500 plf. A uniform dead load is 65 psf and a uniform live load is 40 psf are included in curve fitting.em and ym are uniform along the perimeter; em is less than or equal to 9 feet

ConsultingEngineers

S

Limitations for Ribbed Foundation ModelLimitations for Ribbed Foundation Model

LL

LS

A

A

t 4in.

P 2500 lbs/ft

h1h2

h1(t + 7) in.11 in. h2

h11.2

SF = (LL + LS)2

4LLLS 1.5

Design: 15 ft S 6 ftActual: S < 6 ft OK- Sdesign = Avg.if 1.5 Slargest/Ssmallest- Sdesign = 0.85 * Avg.if Slargest/Ssmallest 1.5

Design:14 in b 8 inActual & bearing:b 14 in. OK

b



ConsultingEngineersPerimeter Load PPerimeter Load P

•• P includes Dead Load and Live Load in both swell P includes Dead Load and Live Load in both swell modesmodes

Dead Load alone in edge lift is unnecessarily conservative

•• P is assumed uniform along perimeterP is assumed uniform along perimeterWhen P varies and Plargest/Psmallest > 1.25, use

Largest value for center liftSmallest value for edge lift

ConsultingEngineersLimitations for Rib ContinuityLimitations for Rib Continuity

•• Ribs must be continuous. Options Ribs must be continuous. Options to meet continuity are as follows:to meet continuity are as follows:

(a) Continuous(b) Rib overlaps parallel rib with adequate length and proximity to be effectively continuous(c) Rib is connected to parallel rib by a perpendicular rib which transfers by tension the bending moment in the rib

(a) (b) (c)

a

b

ConsultingEngineersUniform Thickness ConversionUniform Thickness Conversion

•• Equivalence of ribbed slab and Equivalence of ribbed slab and uniform thickness slab is equal uniform thickness slab is equal moment of inertiamoment of inertia

••

•• Minimum prestress of 0.05A Minimum prestress of 0.05A (kips) applies to both ribbed and (kips) applies to both ribbed and uniform thickness slabuniform thickness slab•• If H < 7.5 in, add perimeter rib, If H < 7.5 in, add perimeter rib, but neglect rib in calculating “H”but neglect rib in calculating “H”•• Tendon eccentricity of PTendon eccentricity of Pee e is e is the same in both ribbed and the same in both ribbed and uniform thickness slabuniform thickness slab

3H12)(W121I

3WIH

h (in)

W (ft)

I

H (in)

W (ft)

I

11in.

b

H < 7.5 in.

H 7.5 in.

ConsultingEngineersAnalysis of Nonrectangular SlabsAnalysis of Nonrectangular Slabs

•• Model slab with overlapping rectangles (e.g., LModel slab with overlapping rectangles (e.g., L-- or Tor T--shaped slabs)shaped slabs)

Overlapping rectangles shall be as large as possible within the actual foundation footprintEach rectangle shall meet the limitations for a rectangular slab, in particular, the Shape Factor.The properties of the controlling rectangle apply to the entire foundation

ConsultingEngineersInternal Moments and DeflectionsInternal Moments and Deflections

•• The PTI Method gives expressions for maximum The PTI Method gives expressions for maximum moments and shears in the long and short directions moments and shears in the long and short directions for both center lift and edge lift conditionsfor both center lift and edge lift conditions

Expressions in 3rd Edition are the same as in 2nd

Edition•• The limits on “The limits on “deflectionsdeflections” are given as a minimum ” are given as a minimum

stiffnessstiffnessMeasured deflections of the built slab are not a performance criterion

ConsultingEngineersMinimum Foundation StiffnessMinimum Foundation Stiffness

•• The minimum foundation stiffness isThe minimum foundation stiffness isEcI 18,000 M L C z , where

M, I, L, and z are taken in the long and in the short direction of the slabM is taken in edge lift and center lift swell modesz is the smaller of L or 6 in the corresponding direction

EcI in the short direction must meet the above inequality for both swell modesEcI in the long direction must meet the above inequality for both swell modes



ConsultingEngineersAllowable StressesAllowable Stresses

•• Concrete flexural tensile stressConcrete flexural tensile stressft = 6 f’c

•• Concrete flexural compressive stressConcrete flexural compressive stressfc = 0.45 f’c

•• ShearShearv = 1.7 f’c + 0.2 * fp

ConsultingEngineersCracked SectionsCracked Sections

•• Cracked section flexural strength, in both swell modes, Cracked section flexural strength, in both swell modes, shall be larger than or equal to:shall be larger than or equal to:

0.9 ML or 0.9 MS for the appropriate direction withfactor equal to 1.0

Any combination of reinforcing bars or post-tensioned tendons may be used to achieve the flexural strength

ConsultingEngineersThank you Thank you



4/19/20044/19/2004 Robert Anderson Consulting EngineersRobert Anderson Consulting Engineers 11

Welcome Welcome To PTI’sTo PTI’s

Technical ConferenceTechnical Conferenceand Exhibitionand Exhibition


The Use of The Use of Post Tensioned Post Tensioned

SlabsSlabs--onon--Ground Ground --MultiMulti--Family HousingFamily Housing

In FloridaIn Florida


What are Stable Soils?What are Stable Soils?

GranularGranularWell CompactedWell CompactedNegligible Volume ChangeNegligible Volume ChangeStrength Strength > > 1500 psf1500 psfK K > 100 > 100 pcipci


Specify Fill and CompactionSpecify Fill and Compaction

4/19/20044/19/2004 Robert Anderson Consulting EngineersRobert Anderson Consulting Engineers 55 4/19/20044/19/2004 Robert Anderson Consulting EngineersRobert Anderson Consulting Engineers 66

Recommended Recommended PrecompressionPrecompression in Slabin Slab

> 75 psi> 75 psi

2004 PTI Technical Conference Session 5 Robert Anderson - PT Slabs in Multi-Family Housing in Florida



Force Imparted to Slab:Force Imparted to Slab:

Net p/a + Subgrade DragNet p/a + Subgrade Drag


Subgrade Drag =Subgrade Drag =

0.5 0.5 x W x x W x

Where : W = Where : W = wgtwgt. Per sq. ft. of slab. Per sq. ft. of slab= Length of Slab= Length of Slab= Coefficient of Friction= Coefficient of Friction


Ranges from 0.15 to 0.5 Ranges from 0.15 to 0.5 with properly prepared basewith properly prepared base



Net Net PrecompressionPrecompression ==Tendon Force Tendon Force –– Subgrade Drag Subgrade Drag >> 75 psi75 psi











Post Tensioning forPost Tensioning forCurl ControlCurl Control


a

b

c


Control of Curl at Control of Curl at Post Tensioned JointsPost Tensioned Joints

N.A.

1” – 1 ”21


Shrinkage and Creep:Shrinkage and Creep:

Slabs Shorten Approx 1/8” per 20’ of LengthSlabs Shorten Approx 1/8” per 20’ of Length




Thickness DeterminationThickness Determination

PCA MethodPCA MethodWRI MethodWRI MethodCOE MethodCOE MethodMATSMATSAirportAirportOtherOther



WS = Working StressWS = Working Stress

==[ 9[ 9 ff’’cc + p/a ] + p/a ] S.F.S.F.


S. F. (Safety Factor) = 1.5 to 1.7S. F. (Safety Factor) = 1.5 to 1.7


Advantages of Advantages of Post TensioningPost Tensioning

Improved Rupture ModulusImproved Rupture ModulusControl of CracksControl of CracksFewer Joints in SlabFewer Joints in SlabDeflection ControlDeflection ControlCurl ControlCurl Control


Economics ofEconomics ofPost TensioningPost Tensioning

Less ConcreteLess ConcreteReinforcing SavingsReinforcing SavingsExcavation SavingsExcavation SavingsFaster CleanFaster Clean--upupTime SavingsTime Savings






Documents

Slab-on-Ground & Foundations Convention/Session... · Slab-on-Ground & Foundations ... • Intermediate anchors allowed cables ... Edition that are a part of the 3rd Edition The 3rd