Improving Understanding and Prediction of Camber of Pretensioned Concrete Beams -...

Department of Civil, Construction and Environmental Engineering

Sri Sritharan

Wilson Engineering Professor

October 21, 2016

Improving Understanding and

Prediction of Camber of

Pretensioned Concrete Beams

Department of Civil, Construction and Environmental Engineering

Sponsor:

Project webpage: http://sri.cce.iastate.edu/Camber/

Partners: Andrews

Prestressed

Concrete

Team: M. Rouse, E. Honarvar, J. Nervig, W. He

Civil, Construction &

Environmental Engineering

Background

Primary Tasks

Instantaneous Camber

Long-term Camber

MIDAS Analyses

Key Findings

Outline

• Net upward deflection resulting from the applied prestress force

after subtracting the downward self-weight deflection.

• Exists from the time the prestress is transferred until the dead

and live load deflection exceeds that due to prestress.

• It is affected by variations in several parameters at different

stages of a PPCB.

• Creates construction challenges.

Camber of PPCBs

Months

Months1 Month

Changes in Support Conditions

and Environmental Variations

Different Stages

Underestimating Camber:

Require addition of haunches

Additional nonprestressed reinforcement

(haunches exceeding four inches)

Causes delays and increase costs

Disputes in the field

Challenges in the Field

1- Material characterization

2-Instantaneous camber

measurements

3- Long-term camber

measurements

4- Instantaneous camber

predictions

5-Long-term camber

predictions

MIDAS simulations

& Simplified methods

Primary Tasks

Material Characterization 4 HPC and 3 NC concrete mixes were

evaluated for compressive strength, creep and

shrinkage.

Use AASHTO LRFD 2010 equation for

concrete modulus of elasticity with appropriate

modifications to concrete strength.

Recommendations for HPC

Average creep coefficient: φ(t) = 1.9t0.48

8+ t0.54

Average shrinkage strain: ɛ(t) = 480t0.60

12+ t0.62

Measurement Technique

• A tape measure reading is taken at the midspanof the beam immediately after release.

• Recorded to the nearest 1/16 in.

Recorded Historical Data

Rotary Laser Level

• A rotary laser level is used to measure the beam from the top flange, bottom flange, and the bed.

• Kept stationary

• Accurate up to 1/16 in. at 100 ft

String Potentiometers • Continuous monitoring of the beam and precasting bed during release

• Accurate up to 0.015 in.

String potentiometer from top flange at Plant A String potentiometer on the

precasting bed at Plant B

Camber Measurement Continued

Instantaneous Camber – BTB 100

0 2,000 4,000 6,000 8,000 10,000 12,000

ical D

ent, in.

Time, sec

Bed at Midspan Right End of Bed Top Flange at Midspan Events

Increase in camber due

to PPCB ends

overcoming friction

Increase in camber

due to lift/set of

Total increase in

camber due to

friction

Bed Deflection

Example – BTB 100

• Predicted camber = 3.19”

• Plant Tape Measure Reading = 2-1/2”

• ISU Laser Level without bed

deflections = 2.52”

• ISU Laser Level accounting for bed

deflections and friction = 2.88”

• ISU String Pot = 2.937

Effect of Bed FrictionMax. = 5/8 in.

or 25%

Uneven surface along the top

flange (max. = ¾”)Inconsistency in the depth of the

troweled surface (max. = ¼ to ¾”)

New measurement technique has been recommended to

minimize these measurement errors

Factors affecting the instantaneous

camber measurements

Bed deflections

(Error: 0.030 in. ± 0.062

Friction

(Error: 0.392 in. ± 0.294)

Inconsistent top flange

surfaces along the beam

length

(Error: 0.099 in. ± 0.142 in.)

Inconsistencies in the top

flange surfaces resulting from

local effects

(Error: 0.113 in. ± 0.119 in.)

Variety of PPCBs arranged in Increasing Length

Tape Measure Reading from PrecasterCamber (accounting for bed deflections, friction, and inconsistent top flange surfaces)String Potentiometers

Improved camber data

Modulus of

elasticity

Sacrificial prestressing

strands

Designed prestress

Transfer length

Prestresslosses

Section properties

Use AASHTO LRFD 2010 Equation

Consider elastic shortening, seating

losses, and relaxation

Use AASHTO LRFD 2010 Equation

Use transformed

section

Factors affecting instantaneous camber predictions

Influence of Release Strength• 40% higher

measured release

strength for 4500-

5500 psi designed

release strength

• 12% higher

release strength

for 6000-8500 psi

designed release

strength

MIDAS SimulationsModeling Features Results may be affected by

• Accurate section properties

• Accurate tendon profiles

• Accurate transfer of prestress

• Change in support location

• Creep and shrinkage effects

• Stage construction

• Change in boundary conditions

• Thermal effects

• Measurement errors

• Variation in material properties

• Complex thermal gradient

MIDAS Model – Instantaneous camber

Camber

MIDAS – Instantaneous camber

0.00 1.00 2.00 3.00 4.00 5.00

Measured Camber (in.)

Average

Predicted Camber = Measured Camber

MIDAS ModelConcrete Creep and Shrinkage

Steel/Concrete Relaxation

Use appropriate support location

𝜀𝑡 𝑡 =𝜎𝑐 𝑡0

𝐸𝑐 𝑡01 + 𝜑 𝑡, 𝑡0 + 0

∆𝜎0 𝑡 1+𝜑 𝑡,𝜏

𝐸𝑐 𝜏𝑑𝜎𝑐 𝜏 + 𝜀𝑠ℎ 𝑡, 𝑡0 + 𝜀𝑡ℎ

𝜀𝑐 𝑡 = 0𝑡𝐶(𝑡0, 𝑡 − 𝑡0)

𝜕𝜎(𝑡0)

𝜎(𝑡0)𝑑𝑡0

Elastic and Creep Strains Shrinkage Strain Thermal Strain

∆𝜀𝑐,𝑛= 𝜀𝑐,𝑛 − 𝜀𝑐,𝑛−1 =

𝑗=1

𝑛−1

∆𝜎𝑗𝐶 𝑡𝑗 , 𝑡𝑛−𝑗 −

𝑗=1

𝑛−2

∆𝜎𝑗𝐶(𝑡𝑗 , 𝑡𝑛−𝑗

Use a combination of:Finite Element Analysis (FEA)

Time-Step Method

Total Strain

Creep Strain

Measured and MIDAS Long-term Camber

0 100 200 300 400 500 600

Time (day)

FEM- C80 FEM- D105 FEM- BTE110 FEM- BTC120

FEM- BTD135 FEM- BTE145 C80 D105

BTE 110 BTC120 BTD135 BTE145

Support Location Varies

4”x4” supports;

overhang the depth

of the beam.

4-ft long plywood;

overhang 0 to 5 ft.

4”x8” supports;

overhang 5% of

the beam length

Parameters Affecting Long-term Camber Measurements

Temporary

Support

Overhang

Length

Thermal Camber

Quantifying Thermal Camber

Instrumented PPCBs in Summer

Instrumented PPCBs in Winter

Temperature Effects

Average Temperature Gradient

Sample Results

Comparison

Zero temperature difference 15 F temperature difference

View of the bridge spans: 3 BTD135

Midas Model – Long-term camber

MIDAS Model

Determining Multipliers

0 100 200 300 400 500 600

Age (Day), t

Average BTE 110

Average BTC 120

Average BTD 135

A set of multipliers without overhang (0-60 days; 60-180 days; and over 180 days)

Temperature gradient multiplier, λT ( Use ΔT= 15°F)

A set of multipliers with an average overhang length of L/30 (0-60 days; 60-180 days; and over 180 days)

A Single multiplier (Average at-erection age: 120 days)

Multipliers as a

function of time

Multipliers were produced by comparing the instantaneous camber to

long-term MIDAS camber

Long-term Multipliers

Method 1, (M1): Multiplier Function with adjusted data for overhang

Method 2, (M2): Set of multipliers- zero overhang

Method 3, (M3): Set of multipliers- average overhang

Method 4, (M4): Single Multiplier-zero overhang

Method 5, (M5): Single Multiplier-average overhang

Method 6, (M6): Current Iowa DOT approach

Acceptable difference between the measured and design camber is within ±1.0 in.

Key FindingsCamber estimate is significantly affected by basic material

properties (i.e., Ec, sh, Ccr).

Instantaneous camber is often inaccurately captured due to

the construction practices and measurement techniques

used at precast plants.

When compared to accurate instantaneous camber

measurements, both simplified methods and MIDAS FEMs

produced good predictions when realistic Ec is used.

Key FindingsLong-term camber measurements are significantly affected

by support location and solar radiation.

Accuracy of multipliers is often compromised due to errors in

the instantaneous and long-term camber measurements.

MIDAS FEMs provided insight into the effects of support

location, solar radiation, and the change in support condition

as a function of time.

MIDAS FEMs led to more realistic design multipliers

Publication

http://www.researchgate.net/

Improving Understanding and Prediction of Camber of Pretensioned Concrete Beams -...

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