Infrasound Measurements of a Railroad Bridge by bix18616

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									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@erdc.usace.army.mil

            Infrasound Technology Workshop 2008
                    Bermuda November 3-7
                  Overview
• Experiment
  – Seismic, Infrasound, Acoustic and
    Meteorological measurements
  – Load testing
• Modeling
• Data Analysis
• Future work
                            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)
        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.


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.                       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.
 Ft. Leonard Wood Deployment
                3 SIAM arrays
      seismic, infrasound, acoustic and
           meteorological sensors.
                       Array deployed at target bridge


2 standoff arrays
Airport Site
  2007-6-23




      Actual sensor layout with scale




  30 meter
                                  Test Area
                                      2007-June
                                                  Rolla




Wastewater
Range 19.9 km
Az 45 degrees
            Fort Leonard
               Wood

                    Airport
                    Range 26.867 km
                    Az 39.4 degrees
   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.
      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.
           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.
             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.
      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?
                Load Testing
• Bridge Description:
  – Type
     • Pratt Truss (est. 1941)
  – Material
     • Steel (built-up)
  – Span
                                  – Width
     • 7 Panels @ 23 ft.- 4 in.
                                    • 15 ft.- 9 in.
  – Height
                                  – Skew
     • 30 ft.
                                    • 65 ˚

                             13
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
                                                                      Strain (me)




                                                          -8.00E+01
                                                                                                              0.00E+00
                                                                                                                         2.00E+01




                      -1.40E+02
                                  -1.20E+02
                                              -1.00E+02
                                                                          -6.00E+01
                                                                                      -4.00E+01
                                                                                                  -2.00E+01
                  1
                211
                421
                631
                841
               1051
               1261
               1471
               1681
               1891
               2101
               2311
               2521
               2731
               2941
               3151
               3361
               3571
               3781
               3991
               4201
               4411




Measurements
               4621
               4831
               5041
               5251
               5461
               5671
               5881
               6091
               6301
               6511
               6721
                                                                                                                                    • Example: Strain Gages Results




               6931
               7141
               7351
               7561
               7771
               7981
                                                                                                                                                                      Experimental Methodology
                                                                Strain (me)




                       -2.00E+01
                                          0.00E+00
                                                                4.00E+01
                                                                           6.00E+01




                                                     2.00E+01
                                                                                      8.00E+01
                                                                                                 1.00E+02
                                                                                                            1.20E+02




                                      1
                                    228
                                    455
                                    682
                                    909
                                   1136
                                   1363
                                   1590
                                   1817
                                   2044
                                   2271
                                   2498
                                   2725
                                   2952
                                   3179




               B1208
                                   3406
                                   3633
                                   3860
                                   4087
                                   4314
                                   4541




Measurements
               B1291
                                   4768
                                   4995
                                   5222
                                   5449
                                   5676
                                   5903
                                   6130
                                   6357
                                   6584
                                   6811
                                   7038
                                   7265
                                                                                                                       • Example: Strain Gages Results




                                   7492
                                   7719
                                   7946
                                                                                                                                                         Experimental Methodology
          COMPUTER MODEL
                                         •   Mechanic of Materials
                                              •  Stress
                                                  • s=eE
• Analytical Model (Frame):                   • Axial Force
                                                  • F = sA
  – Main Steel Structural Elements            • Obtain Analytical Internal
     – Built-up Sections                         Force

                             Top Chord
                  Diagonal


        Diagonal Chord



                                               Stringer
                                    Bottom Chord

                Floor Beam
Summary of the Load Test and Analytical Model
                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)
       Ft. Leonard Wood Bridge
                 z1
            z2                          z1  z2


            z    x




• Pratt Truss Bridge
• Struts included by specifying equal z-displacements at top
  of vertical member pairs
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
          Natural Frequency: Categories
             Relatively large                            Relatively large
           deformation of many                         deformation of a few
               components                                  components




13.6 Hz                                   10.0 Hz

                                    Relatively small
                                     deformation




                                                           All results plotted w/
                                                            same deformation
                          11.9 Hz                              scaling factor
         Natural Frequencies –
           Observation 2 hz
                               x
                          z




• 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)
                    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
    Effect of CSA on Acoustics
                     Beam Model   Deck Model




     Geometry
       Effect




  Models excited
   using same
  accelerations
150 m from source,      10           4.7
                        cm            m
 normal above (y)
                   CSA Source Model
                   center to center spacing, real measurements
             76
             cm                         • Representation of
                                          stringers (plan view)
                                        • Apply source
x                                         acceleration in y
    z
                                          direction (n = 1→5)




        10        10   10      10
        cm        cm   cm      cm
     Comparison with Continuous Model




• Do small gaps between stringers affect results even at 1
  Hz (l=343 m, gap=0.76 m)?
• Small gaps (relative to l) affect acoustic response YES
Radiation Pattern
 4 Beam Model




  3                 4
               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
SIAM Data, raw infrasound
                                         IML Airport

                                         IML Airport

                                         IML Airport

                                                IML Bridge

                                                IML Bridge

                                                IML Bridge

                                                IML Bridge




      Interference infrasound generated by the train
            What frequencies does the train generate? –
                           Bridge Array
     Bridge Array – 3019 – NE of            Bridge Array – 3020
     Bridge, further from train             SW of Bridge, closest to train at
                                            this time




50
hz                                                                              40
                                                                                hz




                          From 18:34:00 to 18:34:30 UTC

                          [13:34:00 to 13:34:30 Local]
        What frequencies does the train generate? –
     Spectrogram of WTF and Airport Arrays for 18:34:00
       to 18:34:30 UTC, [13:34:00 to 13:34:30 Local]
        3015 – WTF Array                     3012 – Airport Array
                                                                            80
                                                                            hz



40
hz


                                                                                 10
                                                                                 hz




     WTF has frequencies up to and including 40hz
     Airport has low frequency (up to 6hz) and then high frequency (80hz)
     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 (>50hz)
• 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
   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)
1052 UTC            Loud times have greater
                    frequency range
                  Bridge signature is clearer at the
                  airport array despite the changing
                  dynamics at the airport
      Bridge signature from Airport Array




• Station 3010 is not usable due    FK analysis indicates
to faunal mastication (rabbit)
during data acquisition             correct azimuth and
                                    apparent velocity
• Frequencies of interest present
– with higher amplitudes than at
WTF array
• Time series shows that the
airport array is more active than
WTF array
      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.
Questions?

								
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