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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

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

Informing Infrastructure Decisions through Large-Amplitude Forced Vibration Testing

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)


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


=> 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

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.

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.

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)

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

Envisioned Field Implementation

Foundation Dynamics and Dynamic Soil-Foundation-Structure Interaction (DSFSI)

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.

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).

Modeling of DSFSI Problems - Direct and Substructure Methods

Direct Approach

Substructure Approach

Free Field Interaction

(after Wolf, 1985)

Impedance

Foundation Dynamics Problem – Impedance Functions

F

C Ku

𝒖 =𝑭

𝑲+ 𝒊𝝎𝑪

Rigid and massless foundation

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

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)

Modes of Vibrations

Sliding/SwayingSliding/

Swaying

RockingRocking

Twisting

Vertical

(from Fang, 1991)

Static Stiffness for a Rigid Circular Footing on an Elastic Half-Space

(from Gazetas, 1983)

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)

Damping Ratio for Surface Foundations

(from Richart et al., 1970)

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.

Impedance Coefficients of a Rigid Circular Foundation on a Stratum - Vertical

2 4 6 a0 2 4 6 a0

(after Kausel and Ushijima, 1979)

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

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

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

Numerical Model and Parametric Study of Dynamic Response of Bridge-Soil Systems

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

Bridge Swaying Response Under Horizontal Harmonic Loading

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

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

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

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

)


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

)


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

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

Fundamental Swaying Frequency Vs. Pier HeightRigid Base

Fundamental Swaying Frequency Vs. Pier HeightFlexible Base (R=2 m, Vs=200 m/s)

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

Finite Element Model of Hobson Avenue Bridge

Fundamental Swaying Mode for Rigid Base

Finite Element Model of Hobson Avenue Bridge

Fundamental Swaying Mode for Flexible Base

Field Implementation Using UTA NHERI Mobile Shakers

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

Hobson Avenue Bridge over I-195, Hamilton, NJ




Geophone Array

MASW (Multichannel Analysis of Surface Waves) Testing

MASW (Multichannel Analysis of Surface Waves) Testing

T-Rex positionDeck Triaxial AccelerometersSubstructure and Ground Triaxial GeophonesDeck Triaxial Geophones

Swaying Test – Center Pier


Swaying Test – Middle of South Span


Swaying Test – South Abutment

T-Rex Mobile Shaker

T-Rex in Position for Testing

T-Rex and THMPER

Geophones and Accelerometers on Bridge Deck

Central Pier

Sensors

Sensors

Ground Triaxial Geophone Array

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?

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

Thank You