Infrasound Measurements of a Railroad Bridge

1
Infrasound Measurements of a Railroad Bridge

• Dr. Mihan McKenna
• Ms. Sarah McComas, Ms. Alanna Lester, and Dr.
  Paul Mlakar
• U.S. Army Engineer Research and Development
  Center
• Geotechnical and Structures Laboratory
• Mihan.H.McKenna_at_erdc.usace.army.mil
• Infrasound Technology Workshop 2008
• Bermuda November 3-7

2
Overview

• Experiment
• Seismic, Infrasound, Acoustic and Meteorological
  measurements
• Load testing
• Modeling
• Data Analysis
• Future work

3
Motivations

• Two prior papers indicated that infrasound was
  generated by bridges at sufficient energy to be
  detected over background noise at certain times
  of the day/year.
• Traffic was not thought to be the source driver,
  perhaps natural sources such as wind excites the
  structure.
• Acoustic ducting was required for propagation of
  infrasound energy.
• Bridge may act as a dipole very close to source.
• Donn, W., N. Balachandran, and G. Kaschak.
  Atmospheric Infrasound Radiated by Bridges. J.
  Acoust. Soc. Am., Vol. 56, No. 5, Nov. 1974.
  1367-1370.
• Kobayashi, Y. (1999) Infrasound Generated by a
  Highway Bridge. Butsuri-Tansa Vol. 52, No. 1.
  54-60. (in Japanese)

4
Research Goals and Motivations

• Purpose
• To determine the feasibility of remote assessment
  of bridges using infrasound acoustics in
  combination with seismic, meteorological and
  audible acoustic methods.
• Desired Payoff
• Field personnel can deploy small-aperture
  infrasound arrays to listen to a target
  structure and reliably analyze the situation
  without having to come into direct contact with
  the structure.

Potential Results Understand the physics of
structure/atmosphere interactions resulting
infrasound propagation. Foundation to create a
catalog of bridge signatures to formulate
algorithms for rapid remote assessment of
infrastructure from bridges and other man-made
structures.
5
Ft. Leonard Wood Deployment

• 3 SIAM arrays
• seismic, infrasound, acoustic and meteorological
  sensors.

Array deployed at target bridge
2 standoff arrays
6
Airport Site 2007-6-23
Actual sensor layout with scale
7
Fort Leonard Wood
Test Area 2007-June
Rolla
Wastewater Range 19.9 km Az 45 degrees
Airport Range 26.867 km Az 39.4 degrees
8
The Infrasound Source Driver

• Two 75 ton engines with eight flat cars of known
  weight
• Series of passes eight, four, two, no cars with
  two engines, one engine, stopped and moving
• Controlled source with limited access during the
  experiment

• Generates the vibrational modes of the bridge
  used to discriminate against other background
  noise including several other bridges in the
  area, both military rail and civilian interstate.

9
Meteorological Measurements

• Three met stations deployed with one per array
  consisting of temperature, pressure, wind speed,
  wind direction, dew point, humidity, soil
  moisture at two levels 0.5 m and 2 m to estimate
  surface roughness.
• Five local environmental monitoring stations on
  post, recording temperature, wind speed, wind
  direction every 15 minutes at two heights 3 m
  and 10 m.
• Collaboration with Hanscomb AFB for balloon
  radiosonde measurements. Total of five launches
  over the day of the test to 30 km by
  state-of-the-art technology.

10
Meteorological Analysis

• Only one inversion existed at the time of the
  train loading, at 0600 local time, before the
  test. There are no other ducting possibilities
  that day.

11
Propagation Modeling

• InfraMAP modeling of the radiosonde data yielded
  only one successful run, at 6AM local time.
• Data analysis searching for the bridge signature
  will focus on the time frame from 4 AM to 8 AM
  local time.

12
Integration of Source and Propagation Modeling

• Identified the optimal time for observing a
  possible signal from the target bridge between 4
  AM and 8 AM local time.
• What would the train signature look like?
• Frequencies?
• Continuous wave vs. discrete?
• How does the source driver affect the signal?

13
Load Testing

• Bridge Description
• Type
• Pratt Truss (est. 1941)
• Material
• Steel (built-up)
• Span
• 7 Panels _at_ 23 ft.- 4 in.
• Height
• 30 ft.

• Width
• 15 ft.- 9 in.
• Skew
• 65

13
14
Experimental Methodology

• Experimental load rating tests
• Strain Gage (44 Used)
• Main Structural Elements
• One Train Engine

• Strain Gage

• BDI

• Base Station

• Auto Clicker

• PC

14
15
Experimental Methodology

• Example Strain Gages Results

Strain (me)
Measurements
16
Experimental Methodology

• Example Strain Gages Results

Strain (me)
Measurements
17
COMPUTER MODEL

• Mechanic of Materials
• Stress
• seE
• Axial Force
• F sA
• Obtain Analytical Internal Force

• Analytical Model (Frame)
• Main Steel Structural Elements
• Built-up Sections

18
Summary of the Load Test and Analytical Model
19
Source Modeling

• COMSOL Multiphysics Structural Mechanics Module
• Key components
• Simplified source to limit computational cost in
  large model
• Accurate represents sound emitted from bridge
• Technical difficulties
• Bridge models to determine natural frequencies
  typically constructed using beam/truss elements
• Beam/truss elements appear as point sources in
  acoustic analyses
• Geometry of beam important for acoustic response
• Natural frequencies of bridge do not provide
  obvious simplification of bridge structure no
  single area dominates acoustics (e.g., bridge
  deck)

20
Ft. Leonard Wood Bridge
z1
z2
x
z

• Pratt Truss Bridge
• Struts included by specifying equal
  z-displacements at top of vertical member pairs

21
Natural Frequencies - Overview

• Bridge shows 230 modes between 2 and 20 Hz
• Three General Categories of Modes
• Category 1 Relatively large deformation of many
  components (10)
• Category 2 Relatively large deformation of few
  components (33)
• Category 3 Relatively small deformation of
  components (57)
• First two categories should dominate acoustical
  energy

22
Natural Frequency Categories
Relatively large deformation of a few components
10.0 Hz
13.6 Hz
Relatively small deformation
All results plotted w/ same deformation scaling
factor
11.9 Hz
23
Natural Frequencies Observation 2 hz

• Modes show deformation in z direction, stringer
  stays in plane
• No modes show significant deformation in y
  direction
• Bridge design requires large stiffness to resist
  deformation in y direction (designed to prevent
  cantilever bending)

24
Methodology

• Rank bridge components based on source strength
• Cross sectional area (CSA) perpendicular to
  direction of motion
• Relative acceleration identified from natural
  frequency analysis
• Develop acoustic model of critical components of
  bridge using shell elements of CSA
• Develop detailed model using simple shape of beam
  CSA
• Apply deformation mode from natural frequency
  analysis to each component
• Use solution on outer boundary of acoustic model
  to drive infrasound solution over large domain

25
Effect of CSA on Acoustics
Deck Model
Beam Model
Geometry Effect
Models excited using same accelerations 150 m
from source, normal above (y)
26
CSA Source Modelcenter to center spacing, real
measurements

• Representation of stringers (plan view)
• Apply source acceleration in y direction (n 1?5)

27
Comparison with Continuous Model

• Do small gaps between stringers affect results
  even at 1 Hz (l343 m, gap0.76 m)?
• Small gaps (relative to l) affect acoustic
  response YES

28
Radiation Pattern4 Beam Model
4
3
29
Summary

• Ft. Leonard Wood bridge shows complex frequency
  response
• Cross sectional area of beams has strong effect
  on acoustics
• Small gaps relative to wavelength have effect on
  acoustics
• Simple representation of bridge deck shows strong
  directionality

30
SIAM Data, raw infrasound
IML Airport
IML Airport
IML Airport
IML Bridge
IML Bridge
IML Bridge
IML Bridge
Interference infrasound generated by the train
31
What frequencies does the train generate?
Bridge Array
Bridge Array 3019 NE of Bridge, further from
train
Bridge Array 3020 SW of Bridge, closest to
train at this time
50hz
40hz
From 183400 to 183430 UTC 133400 to
133430 Local
32
What frequencies does the train generate?
Spectrogram of WTF and Airport Arrays for
183400 to 183430 UTC, 133400 to 133430
Local
3015 WTF Array
80hz
40hz
10hz
WTF has frequencies up to and including
40hz Airport has low frequency (up to 6hz) and
then high frequency (80hz)
33
Bridge signal from WTF array

• Includes frequencies of interest (2, 10 and
  13hz) - extremely low amplitude
• Time series does not appear to have high
  activity
• Additional higher frequencies (42 and 56hz)
  present and other additional transients (gt50hz)
• Higher frequencies (40 50hz) are persistent
  through the two hour time period

Difficult to find arrivals in signal 2hz signal
in time series FK analysis results correspond
with bridge azimuth
34
Airport array frequencies
1005 UTC (0505 AM Local)
Greater dynamic of frequencies present at
airport Quiet times are clearer than WTF array
(30hz signal visible) Loud times have greater
frequency range
1052 UTC
Bridge signature is clearer at the airport array
despite the changing dynamics at the airport
35
Bridge signature from Airport Array

• Station 3010 is not usable due to faunal
  mastication (rabbit) during data acquisition
• Frequencies of interest present with higher
  amplitudes than at WTF array
• Time series shows that the airport array is more
  active than WTF array

FK analysis indicates correct azimuth and
apparent velocity
36
Future Modeling and Data Synthesis

• The finite element model of the bridge created
  during the load testing will be uploaded to a
  multi-physics finite element package.
• The bridge will be coupled into the atmosphere
  and vibrated at the frequencies observed during
  the test.
• A representative source package will be
  developed for use in infrasound propagation
  modeling software.

37
Questions?