Download - BRIDGE ENGINEERING ASSOCIATION Informing …
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7th New York City Bridge Conference
August 26-27, 2013
BRIDGE ENGINEERING ASSOCIATION
Informing Infrastructure Decisions through Large-Amplitude Forced Vibration Testing
Nenad Gucunski, Franklin Moon, John DeVitis and Sharef Farrag
Civil and Environmental Engineering
Center for Advanced Infrastructure and Transportation (CAIT)
Brady Cox and Farnyuh Menq – NHERI, The University of Texas at Austin
NHERI@UTexas Structural Testing WorkshopNew Brunswick, NJ, August 3, 2017
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Outline
• NSF EAGER Project – Objectives and general scope of activities
• Foundation dynamics and dynamic soil-foundation-structure interaction (DSFSI)
• Numerical simulation of the dynamic response of a bridge-soil system
• Field implementation using UTA NHERI mobile shakers
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Informing Infrastructure Decisions through Large-Amplitude Forced Vibration Testing
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Motivation - Civil Infrastructure Evaluation
Aging Infrastructures
– Need for the development of reliable safety assessment approaches
– Structural-Identification (St-Id) has evolved over the past few decades as a result of:
• Adoption of sensing technologies (global and local)
• Development of highly refined simulation models
• Development of model calibration techniques (both deterministic and probabilistic)
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Motivation - Civil Infrastructure Evaluation
Current St-Id for Structure-Foundation Systems
– Based largely on the response data using various inputs (static loading, wind, temperature changes, pull-release, impact, shakers, etc.)
– Those low-level demand inputs are leading to responses similar to operational limit states =>the use of such responses to inform the safety assessment of systems under extreme events requires significant extrapolation
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Motivation - Civil Infrastructure Evaluation
=> Large-amplitude mobile shakers offer significant potential to improve the reliability of St-Id by overcoming low-level mechanisms in a controlled manner
NH
ERI
Mobile
Shakers
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NSF EAGER (Early-concept Grants for Exploratory Research) Project
Overarching Aim
– To explore and establish the ability of large-amplitude,
forced vibration testing to reveal the current performance
and forecast the future system performance of structures,
with the consideration of dynamic soil-foundation-structure
effects.
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NSF EAGER (Early-concept Grants for Exploratory Research) Project
More Focused Objectives
– Develop, evaluate, and refine a series of:
1. Forced vibration testing and control strategies to capture response measurements indicative of key performance attributes of substructure/foundation and superstructure systems
2. Data interpretation frameworks for structural system identification and assessment
– Perform a validation of the testing/control strategies and data interpretation frameworks on an operating structure with known substructure, foundation, and soil characteristics.
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Research Plan
• Development of Forced-Vibration Testing Strategies
– Parametric study to examine the correlation between certain measurable responses (both foundation and superstructure) and the foundation/substructure type/condition
• Development of Data Interpretation Frameworks
– Model-Free Frameworks - Methods based primarily on data processing, data visualization, and data fitting techniques
– Model-based Frameworks - Methods that update simulation models of the system being identified, and then employ these to examine behaviors that cannot be directly observed (St-Id)
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Research Plan
• Field Implementation and Validation
– Field implementation of the most promising testing strategies and data interpretation frameworks
– Since the deployment of large shakers is the easiest to accomplish on bridges (A bridge in Hamilton, NJ was selected as the implementation structure)
– To be carried out with a UTA NHERI shaker (T-Rex) and dense instrumentation arrays
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Envisioned Field Implementation
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Foundation Dynamics and Dynamic Soil-Foundation-Structure Interaction (DSFSI)
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Objective of Dynamic Soil-Foundation-Structure (DSFS) Interaction Analysis
The fundamental
objective of soil-
foundation-structure
interaction analysis is to
evaluate the dynamic
response by
encompassing the
radiation of energy of
the waves propagating
into the soil.
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Effects of Soil and DSFSI on the Response
• Site amplification of ground motion (earthquake loading).
• Soil flexibility will effect the flexibility of the overall SFS (Soil-Foundation-Structure) system and, thus, reduce the fundamental frequency. (In comparison to a structure founded on rock.)
• Radiation of energy through soil will lead to a significantly higher damping. (Exceptions are shallow soil layers.)
• DSFSI (Dynamic SFS Interaction) increases as soil becomes softer, and the structure becomes more rigid. And vice versa.
• Generally, DSFSI gives smaller response amplitudes under earthquake and other dynamic loads than modeling on a rigid base. Displacements at the top of a structure may be larger due to rocking of the structure (foundation).
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Modeling of DSFSI Problems - Direct and Substructure Methods
Direct Approach
Substructure Approach
Free Field Interaction
(after Wolf, 1985)
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Impedance
Foundation Dynamics Problem – Impedance Functions
F
C Ku
𝒖 =𝑭
𝑲+ 𝒊𝝎𝑪
Rigid and massless foundation
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Kvs - static stiffness
k - stiffness impedance coefficient
c - damping impedance coefficient
a0 - dimensionless frequency (wR/Vs)
Impedances are functions of frequency!
K K k ia cv vs ( )0 - vertical impedance
Response of Foundations to Vertical Loading
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Factors Affecting Dynamic Response of Foundations
• Soil properties (primarily shear modulus, damping and Poisson’s ratio)
• Geometry (shape) of the foundation
• Depth of embedment of the foundation
• Presence of a rigid base
• Mode of vibrations (translation or rotation)
• Dynamics and frequency of loading
• Foundation flexibility (stiffness)
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Modes of Vibrations
Sliding/SwayingSliding/
Swaying
RockingRocking
Twisting
Vertical
(from Fang, 1991)
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Static Stiffness for a Rigid Circular Footing on an Elastic Half-Space
(from Gazetas, 1983)
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Impedance Coefficients for a Circular Footing on an Elastic Half-Space - Vertical
a0a0
2 4 6 2 4 6
(Luco and Westman 1971, from Gazetas 1983)
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Damping Ratio for Surface Foundations
(from Richart et al., 1970)
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Some Elements Affecting Foundation Impedance Functions
• Embedment significantly increases the dynamic stiffness and equivalent damping ratio. => An effective way to reduce the anticipated high amplitudes of vibrations.
• For a foundation on a stratum, static stiffness in all modes increases (more for translational than rotational modes) with the relative radius to depth to bedrock ratio R/H.
• Impedance coefficients have undulations (instead of smooth functions for H-S) associated with natural frequencies of the stratum.
• Below the first resonant frequency of each mode of vibration, c is zero or negligible. (No surface waves to radiate energy, while bedrock prevents “vertical” radiation.) Vertical and sliding modes affected more.
• Foundation flexibility affects soil reaction and displacement distributions, and impedance coefficients.
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Impedance Coefficients of a Rigid Circular Foundation on a Stratum - Vertical
2 4 6 a0 2 4 6 a0
(after Kausel and Ushijima, 1979)
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Effect of Foundation Flexibility Expressed Through Stiffness Ratio on Soil Reaction Distribution
0 0.2 0.4 0.6 0.8 1 1.2-2.5
-2
-1.5
-1
-0.5
0N
orm
alli
ze
d s
oil
rea
ctio
n
REAL PART =0.0%
=0.35
a =2.69
0
=0.0
Normalized radius
s =0.008 s =0.125 s =1 s =8 s =125r r r r r
sE h
V Rr
p
s
3
1 1
2
0
3a
R
Vs
0
0w
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0 0.2 0.4 0.6 0.8 1 1.2-0.2
-0.1
0
0.1
0.2N
orm
aliz
ed d
ispla
ce
me
nt
REAL PART
s =0.008 s =0.125 s =1 s =8 s =125r r r r r
=0.0%
=0.35
a =2.69
0
=0.0
Normalized radius
Effect of Stiffness Ratio on Displacement Distribution
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0 1 2 3 4 5 6 7-2
-1.5
-1
-0.5
0
0.5
1
1.5
Dimensionless frequency, a 0
V
V
s1
s2
V =2
=0.35 =0.0 =0.0
R /d =10 1
s2
_
s =0.008 s =0.125 s =1 s =8 s =125r r r r r
Impedance c
oeff
icie
nt k
Effect of Stiffness Ratio on Impedance Functions
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Numerical Model and Parametric Study of Dynamic Response of Bridge-Soil Systems
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Parametric Study - 2D Model of Hypothetical Bridge
Variables
Vs: Soil S-Wave Velocity; 200-400 m/s
R: Footing Radius; 2-4 m
Hc: Column Height; 3-12 m
Constants
Wf: Footing Width; 1.3m
Tf: Footing Thickness; 0.5 m
Wc: Column Width; 0.5 m
Ts: Slab Thickness; 0.2 m
Fs: Half Column Spacing, 3 m
ρs: Soil Density; 1900 kg/m3
ν : Poisson’s ratio; 0.333
Foundation3 m 6 m 3 m
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Bridge Swaying Response Under Horizontal Harmonic Loading
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Sample Displacement Time Histories - Deck
-1.50E-04
-1.00E-04
-5.00E-05
0.00E+00
5.00E-05
1.00E-04
1.50E-04
2.00E-04
2.50E-04
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Ux
(m)
Time (s)
-3.00E-05
-2.00E-05
-1.00E-05
0.00E+00
1.00E-05
2.00E-05
3.00E-05
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Uy
(m)
Time (s)
Horizontal
Vertical
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Sample Displacement Time Histories - Foundation
-1.50E-05
-1.00E-05
-5.00E-06
0.00E+00
5.00E-06
1.00E-05
1.50E-05
2.00E-05
2.50E-05
3.00E-05
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Ux
(m)
Time (s)
-0.005
-0.004
-0.003
-0.002
-0.001
0
0.001
0.002
0.003
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Ro
tati
on
(ra
ds)
Time (s)
Horizontal
Rotation
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0
2
4
6
8
10
12
0 0.5 1 1.5 2 2.5 3 3.5 4
Load
(kN
)
Frequency (Hz)
Sample Loading Spectrum
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Sample Horizontal Displacement, Velocity and Acceleration Response Spectra
-0.01
-0.005
0
0.005
0.01
0 1 2 3 4 5
Dis
pla
cem
ent
(m)
Frequency (Hz)-0.00006
-0.00005
-0.00004
-0.00003
-0.00002
-0.00001
0
0.00001
0 1 2 3 4 5
Dis
pla
cem
ent
(m)
Frequency (Hz)
-0.0002
0
0.0002
0.0004
0.0006
0.0008
0.001
0.0012
0 1 2 3 4 5
Vel
oci
ty (
m/s
)
Frequency (Hz)-0.005
0
0.005
0.01
0.015
0.02
0 1 2 3 4 5
Vel
oci
ty (
m/s
)
Frequency (Hz)
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
0 1 2 3 4 5
Acc
ele
rati
on
(m
/s^2
)
Frequency (Hz)-0.005
0
0.005
0.01
0.015
0.02
0 1 2 3 4 5Acc
ele
rati
on
(m
/s^2
)
Frequency (Hz)
Real Part Imaginary Part
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Sample Foundation Vertical Displacement and Rotation Response Spectra
Vertical Displacement
0.00E+00
2.00E-02
4.00E-02
6.00E-02
8.00E-02
1.00E-01
1.20E-01
1.40E-01
1.60E-01
0 0.5 1 1.5 2 2.5 3 3.5 4
Uy
(m)
Frequency (Hz)
0.00E+00
2.00E-04
4.00E-04
6.00E-04
8.00E-04
1.00E-03
1.20E-03
1.40E-03
1.60E-03
1.80E-03
2.00E-03
0 0.5 1 1.5 2 2.5 3 3.5 4
Ro
tati
on
(ra
ds)
Frequency (Hz)
Rotation
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Fundamental Swaying Frequency Vs. Pier HeightRigid Base
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Fundamental Swaying Frequency Vs. Pier HeightFlexible Base (R=2 m, Vs=200 m/s)
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Fundamental Swaying Frequency Vs. Slenderness Ratio
0
0.5
1
1.5
2
2.5
3
0 0.5 1 1.5 2 2.5
Fre
qu
en
cy a
t p
eak
(H
z)
Slenderness Ratio
R=2, V=200
R=2, V=300
R=3, V=200
R=3, V=200
R=3, V=300
R=3, V=400
R=4, V=200
R=4, V=300
R=4, V=400
R=8, V=800
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Finite Element Model of Hobson Avenue Bridge
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Fundamental Swaying Mode for Rigid Base
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Finite Element Model of Hobson Avenue Bridge
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Fundamental Swaying Mode for Flexible Base
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Field Implementation Using UTA NHERI Mobile Shakers
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Field Implementation and Validation
• Objectives :
– To measure the response of all components: ground, foundation, substructure and superstructure in both vertical and horizontal directions, to infer the contributions of soil-foundation-substructure interaction on the overall response of the superstructure.
– To enable assessment of transmissibility (motion transfer) and force transfer between superstructure and foundation, and from the foundation to surrounding soil.
– To enable assessment of foundation impedance functions for both vertical and rocking motion.
• The results will be compared against the models within the parametric study to place the field test in context
• Secondary objective: The results obtained from the bridge excitation using a NHERI shaker (vertical mode only) and THMPER will be compared
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Hobson Avenue Bridge over I-195, Hamilton, NJ
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Hobson Avenue Bridge over I-195, Hamilton, NJ
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Hobson Avenue Bridge over I-195, Hamilton, NJ
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Hobson Avenue Bridge over I-195, Hamilton, NJ
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Geophone Array
MASW (Multichannel Analysis of Surface Waves) Testing
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MASW (Multichannel Analysis of Surface Waves) Testing
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T-Rex positionDeck Triaxial AccelerometersSubstructure and Ground Triaxial GeophonesDeck Triaxial Geophones
Swaying Test – Center Pier
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T-Rex positionDeck Triaxial AccelerometersSubstructure and Ground Triaxial GeophonesDeck Triaxial Geophones
Swaying Test – Middle of South Span
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T-Rex positionDeck Triaxial AccelerometersSubstructure and Ground Triaxial GeophonesDeck Triaxial Geophones
Swaying Test – South Abutment
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T-Rex Mobile Shaker
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T-Rex in Position for Testing
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T-Rex and THMPER
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Geophones and Accelerometers on Bridge Deck
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Central Pier
Sensors
Sensors
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Ground Triaxial Geophone Array
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What Are the Questions We Are Trying to Answer?
• Can we infer the contributions of soil-foundation-substructure interaction on the overall response of the superstructure?
• Can we develop data interpretation frameworks for structural system identification and assessment that will take into consideration DSFSI?
• Can large-amplitude, forced vibration testing using NHERI shakers (in a fully controlled manner) reveal the performance of structures beyond operational limit states? Are we entering a nonlinear range?
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Acknowledgements
• NSF EAGER– Project support through Award Id. 1650170 (Program officer: Dr. Richard J. Fragaszy)
• NSF NHERI (Natural Hazards Engineering Research Infrastructure) – Provided access to UTA NHERI mobile shakers
• NJDOT – Support in bridge selection, and providing documentation and access to Hobson Avenue Bridge (Mr. Eddy Germain)
• UTA NHERI team
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Thank You