Improving Understanding and Prediction of Camber of Pretensioned Concrete Beams -...
Transcript of 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
Civil, Construction &
Environmental Engineering
• 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
Civil, Construction &
Environmental Engineering
1-3
days
1-3
Months
1-2
Months1 Month
Changes in Support Conditions
and Environmental Variations
Different Stages
Civil, Construction &
Environmental Engineering
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
Civil, Construction &
Environmental Engineering
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
Civil, Construction &
Environmental Engineering
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.
8
Civil, Construction &
Environmental Engineering
Recommendations for HPC
Average creep coefficient: φ(t) = 1.9t0.48
8+ t0.54
Average shrinkage strain: ɛ(t) = 480t0.60
12+ t0.62
Civil, Construction &
Environmental Engineering
Measurement Technique
• A tape measure reading is taken at the midspanof the beam immediately after release.
• Recorded to the nearest 1/16 in.
Civil, Construction &
Environmental Engineering
Recorded Historical Data
Civil, Construction &
Environmental Engineering
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
Civil, Construction &
Environmental Engineering
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
Civil, Construction &
Environmental Engineering
Camber Measurement Continued
Civil, Construction &
Environmental Engineering
Instantaneous Camber – BTB 100
-1
0
1
2
3
4
0 2,000 4,000 6,000 8,000 10,000 12,000
Vert
ical D
ispla
cem
ent, in.
Time, sec
Bed at Midspan Right End of Bed Top Flange at Midspan Events
Har
ped
str
ands
rele
ase
beg
an
Har
ped
str
ands
rele
ase
com
ple
ted
Increase in camber due
to PPCB ends
overcoming friction
Increase in camber
due to lift/set of
PPCB
Total increase in
camber due to
friction
Bott
om
str
ands
rele
ase
beg
an
Bott
om
str
ands
rele
ase
com
ple
ted
Bea
m l
ift
Top s
tran
ds
rele
ase
beg
an
Civil, Construction &
Environmental Engineering
Bed Deflection
Civil, Construction &
Environmental Engineering
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
Civil, Construction &
Environmental Engineering
Effect of Bed FrictionMax. = 5/8 in.
or 25%
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Environmental Engineering
Uneven surface along the top
flange (max. = ¾”)Inconsistency in the depth of the
troweled surface (max. = ¼ to ¾”)
Civil, Construction &
Environmental Engineering
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
in.)
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.)
Civil, Construction &
Environmental Engineering
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
5.00C
am
ber
(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
Civil, Construction &
Environmental Engineering
Modulus of
elasticity
Sacrificial prestressing
strands
Designed prestress
force
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
Civil, Construction &
Environmental Engineering
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
Civil, Construction &
Environmental Engineering
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
Civil, Construction &
Environmental Engineering
MIDAS Model – Instantaneous camber
Camber
Civil, Construction &
Environmental Engineering
MIDAS – Instantaneous camber
0.00
1.00
2.00
3.00
4.00
5.00
0.00 1.00 2.00 3.00 4.00 5.00
Pre
dic
ted
Ca
mb
er (
in.)
Measured Camber (in.)
Average
Predicted Camber = Measured Camber
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Environmental Engineering
MIDAS ModelConcrete Creep and Shrinkage
Steel/Concrete Relaxation
Use appropriate support location
Civil, Construction &
Environmental Engineering
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
Civil, Construction &
Environmental Engineering
Measured and MIDAS Long-term Camber
0
2
4
6
8
10
0
50
100
150
200
250
0 100 200 300 400 500 600
Mea
sure
d C
amb
er (
in.)
Mea
sure
d C
amb
er (
mm
)
Time (day)
FEM- C80 FEM- D105 FEM- BTE110 FEM- BTC120
FEM- BTD135 FEM- BTE145 C80 D105
BTE 110 BTC120 BTD135 BTE145
Civil, Construction &
Environmental Engineering
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
Civil, Construction &
Environmental Engineering
Parameters Affecting Long-term Camber Measurements
Temporary
Support
Overhang
Length
Civil, Construction &
Environmental Engineering
Thermal Camber
Civil, Construction &
Environmental Engineering
Quantifying Thermal Camber
Instrumented PPCBs in Summer
Instrumented PPCBs in Winter
Civil, Construction &
Environmental Engineering
Civil, Construction &
Environmental Engineering
Temperature Effects
Civil, Construction &
Environmental Engineering
Average Temperature Gradient
Civil, Construction &
Environmental Engineering
Sample Results
Civil, Construction &
Environmental Engineering
Comparison
Zero temperature difference 15 F temperature difference
Civil, Construction &
Environmental Engineering
View of the bridge spans: 3 BTD135
Midas Model – Long-term camber
Civil, Construction &
Environmental Engineering
MIDAS Model
Civil, Construction &
Environmental Engineering
Civil, Construction &
Environmental Engineering
Determining Multipliers
42
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.80
2.00
0 100 200 300 400 500 600
Mult
ipli
er,
M
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
Civil, Construction &
Environmental Engineering
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.
Civil, Construction &
Environmental Engineering
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.
Civil, Construction &
Environmental Engineering
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
Civil, Construction &
Environmental Engineering
Publication
http://www.researchgate.net/