problem statement concrete relaxation test-results
TRANSCRIPT
Effects of Superstructure Time-Dependent Deformations on Bridge Column DesignEbadollah Honarvar Gheitanbaf, Sri Sritharan, and Matt Rouse
Civil, Construction & Environmental Engineering, Iowa State University
0.0
200.0
400.0
600.0
800.0
1000.0
0 500 1000 1500 2000 2500 3000
Mic
rost
rain
Time (day)
Superstructure Strain Rate
B6 B7 B5
B4 B2 B1
Pi/A=6.68 Pi/A=6.81
MPa
Pi/A=6.77
MPa
Pi/A=6.18
MPa
Pi/A=5.87
MPa
Pi/A=4.73
MPa
-4000
-3000
-2000
-1000
0
1000
2000
3000
4000
0 500 1000 1500 2000 2500
Fx
(kN
) Time (days)
Base Shear
B23 B24 B25 B26
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.000 0.050 0.100 0.150
Mid
as
Dis
pla
cem
ent
(m)
Estimated Displacement (m)
0.000
0.050
0.100
0.150
0.000 0.020 0.040 0.060 0.080 0.100 0.120 0.140
V/K
eff(m
)
Estimated Displacement (m)
Cracked Coulmns
0.000
0.010
0.020
0.030
0.040
0.000 0.020 0.040 0.060 0.080
V/K
g(m
)
Estimated Displacement (m)
Uncracked Columns
Testing Testing Procedure
Force control mode
Displacement control mode
Column Test
Two specimen sizes to investigate any plausible size effect
Two various loading scenarios:
i. Instantaneous compressive axial displacement
ii. Step Loading
Beam Test
Precracking loading
Post-cracking loading:
Failed the specimen to examine the possibility of any residual strains in the
rebars
-10000
0
10000
20000
30000
40000
-1000
0
1000
2000
3000
4000
0 20 40 60 80 100 120 140 160
Load (
lb)
Mic
rost
rain
Time (min)
Load/Tensile Strains
L- L11H20L- L9H20Satec Load
Problem Statement
Analytical Investigation Results
1. Bridge superstructure shortens due to prestressing, and creep and shrinkage
2. Columns are pulled by the superstructure
3. Shear stresses are developed at the column base
Methodology/Objective
Concrete Relaxation Test
Conclusions/ObservationsConcrete relaxation resulted in significant reduction in column base shear force
Generally, the superstructure strain rate increased as the initial axial stress increased
Top of column displacement was estimated fairly accurately using superstructure strain rate when it was compared to the analytical model results
The maximum displacement and base shear force occurred for the most exterior columns, which cracked as well.
The nearest columns to PNM experienced the minimum displacement and base shear force, thus remained uncraked.The nearest columns to PNM experienced the minimum displacement and base shear force, thus remained uncraked.
Concrete Relaxation Test-ResultsFor the seven conduced test, strain remained constant, while force
reduced over time
0
0.5
1
1.5
2
2.5
3
0 24 48 72 96 120
Str
ess
(ksi
)
Time (hrs)
Comparison of Compressive Stress Relaxation
Small Column Specimen,
loading age of 48 days
Large Column Specimen,
loading age of 67 days
Small Column Specimen,
loading age of 76 days
Small Column Specimen
Step Loading, loading age of78 days
Small Column Specimen
Step Loading, loading age of84 days
Large RC Beam Precracking,
loading age of 130
Large RC Beam
Postcracking, loading age of150-550
-500
-450
-400
-350
-300
-250
-200
0 24 48 72 96 120
Mic
rost
rain
Time (hr)
Strains for Displacement-Control for 8 in.
Diameter Column
Location1Location
2Location3Location
4
-0.02
0
0.02
0.04
0.06
0.08
0.1
0
20000
40000
60000
80000
100000
0 2 4 6 8
Time (Min)
Act
uat
or
Dis
pla
cem
ent
(in
.)
Forc
e (l
b)
Loading under Force-Control
ForceDisplacement
0.06
0.07
0.08
0.09
40000
50000
60000
70000
80000
90000
100000
0 50 100 150
Act
uat
or
Dis
pla
cem
ent
(in.)
Forc
e (l
b)
Time (hr)
Loading under Displacement-Control
Force Displacement
Analyzed six bridges with
different lengths
Two Short Bridge Frames; Lf < 150 m (492 ft)
Two Medium Bridge Frames
150 m (492 ft) < Lf < 300 m (984 ft)
Two Long Bridge Frames Lf > 300 m (984 ft)
Using the calculated strain
rate, estimated the top of
column displacement with
respect to point of no
movement (PNM)
No residual strains in
longitudinal rebars after
the completion of the
relaxation test
Column relaxation led to
appreciable reduction in
column base shearEstablish a realistic strain rate for the superstructure
shortening
Identify and quantify concrete relaxation
phenomenon
Analyze different bridge frame configurations using the Finite Element Analysis
Propose a simplified design approach to calculate Propose a simplified design approach to calculate column moment/shear demands
Str
ain G
ages
Strain Gages
How to efficiently design bridge columns to accommodate stresses induced by superstructure time-dependent
deformations in posttensioned concrete box-girder bridges