Department of Civil & Structural Engineering.

Forced vibration modal testing of ‘International Bridge’

at Wayne, New Jersey, 21-23 July 2010

Prof. James BrownjohnDr Ki-Young Koo

Dr Chris Middleton

The University of SheffieldDepartment of Civil and Structural Engineering

Vibration Engineering Section

Department of Civil & Structural Engineering.

Planned test grids

Department of Civil & Structural Engineering.

Test grids for AVT and FVT using 19 sensors

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Span 2: using 6 swipes with 2×16+triax by shaker

Span 1 Span 2

Single swipe with 19 sensors

Department of Civil & Structural Engineering.

The workers ….

Chris on data acquisition, and Ki on the cherry picker

Department of Civil & Structural Engineering.

The equipment ….QA 750 servo accelerometers, APS400 shaker

Department of Civil & Structural Engineering.

Single ‘swipe’ ambient test on walkway -during afternoon and 90+F

Department of Civil & Structural Engineering.

Displacement signals from double integration of acceleration.

Are we seeing real coupling of spans?

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Department of Civil & Structural Engineering.

Mode shapes obtained using NExT/ERA:(30 minute data set)

Clear evidence of coupling here mode: 1 f=3.75Hz zeta=0.71% mode: 2 f=4.3Hz zeta=2.3% mode: 3 f=5.27Hz zeta=0.94% mode: 4 f=5.96Hz zeta=0.66%

mode: 5 f=9.5Hz zeta=0.8% mode: 6 f=10.2Hz zeta=0.61% mode: 7 f=11.7Hz zeta=2.3% mode: 8 f=12.4Hz zeta=1.5%

mode: 9 f=14.2Hz zeta=1.1% mode: 10 f=14.9Hz zeta=0.4% mode: 11 f=15.1Hz zeta=0.91%

Department of Civil & Structural Engineering.

Single vehicle free decay and

small damping and frequency

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Department of Civil & Structural Engineering.

Force vibration test/analysis methods~between 11PM and 5AM

• Excitation modes– Random (15 minutes)– Chirp (15 minutes)– Swept sine –for reference sensors only, during moves– Shaker shutdown –for reference sensors only, during moves

• Analysis methods– NExT/ERA on random excitation (MODAL) , f, – GRFP on chirp excitation (MODAL) , f, , m– OMAX on chirp excitation (MACEC) , f, , m– Circle-fit on swept sine (MODAL) f, , m– Log-dec free decay on shaker shutdown (MODAL) f,

Department of Civil & Structural Engineering.

5-swipe output only modal analysis using NExT/ERA (with 15 minute random excitation)

mode: 1 f=3.07Hz zeta=1.4% mode: 2 f=3.67Hz zeta=0.66% mode: 3 f=4.34Hz zeta=3.3%

mode: 4 f=5.14Hz zeta=0.66% mode: 5 f=5.17Hz zeta=0.59% mode: 6 f=9.42Hz zeta=0.76%

mode: 7 f=11.6Hz zeta=1.4% mode: 8 f=12Hz zeta=1.8% mode: 9 f=12.2Hz zeta=1.3%

Department of Civil & Structural Engineering.

Cross-section reveals increasing transverse mode order

mode: 1 f=3.07Hz zeta=1.4% mode: 2 f=3.67Hz zeta=0.66% mode: 3 f=4.34Hz zeta=3.3%

mode: 4 f=5.14Hz zeta=0.66% mode: 5 f=5.17Hz zeta=0.59% mode: 6 f=9.42Hz zeta=0.76%

mode: 7 f=11.6Hz zeta=1.4% mode: 8 f=12Hz zeta=1.8% mode: 9 f=12.2Hz zeta=1.3%

half-sine

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Department of Civil & Structural Engineering.

Chirp excitation & response –on last swipe (4AM?), almost zero traffic

–Shaker induced response drowned by traffic effects

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Department of Civil & Structural Engineering.

Good quality FRF; used for Global RFP curve fitting

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Department of Civil & Structural Engineering.

Global rational fraction curve

fitting for two modes …

Department of Civil & Structural Engineering.

Modes 1,2 from GRFP

mode: 1 f=5.22Hz, zeta=0.42%

mode: 1 f=3.7Hz, zeta=0.54%

mode: 1 f=3.71Hz, zeta=0.92%

mode: 2 f=5.15Hz, zeta=0.56%

Department of Civil & Structural Engineering.

…. And using MACEC/OMAX

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Department of Civil & Structural Engineering.

How did we do?

Ambient Forced mode Walkway

AVT Random AVT

Shaker shutdown

GRFP Circle fit OMAX

f /Hz /% f /Hz /% f /Hz /% f /Hz /% m1 /t f /Hz /% m1 /t f /Hz /% m /t 0? 3.07 1.4 1 3.75 0.71 3.67 0.66 3.69 0.61 3.69 0.63 81 3.70 0.51 94 3.68 0.83 71 2 4.3 2.3 4.34 3.3 3 5.27 0.94 5.17 0.59 5.22 0.63 5.22 0.57 273 5.22 0.61 300 5.20 0.96 192 4 5.96 0.66 5 9.5 0.8 9.42 0.76 9.46 0.58 9.49 0.52 354 9.48 0.53 2075 6 10.2 0.6 7 11.7 2.3 11.6 1.4 11.7 1.64 285 8 12.4 1.5 12.2 1.3 12.2 1.2 12.2 0.96 130 12.16 1.26 102 9 14.2 1.1 10 14.9 0.4 11 15.1 0.91 15 0.77 15 0.51 266 15 0.51 1237 12 15.8 0.33 843

Department of Civil & Structural Engineering.

Lessons• Expected to be able to do two spans: took much longer to move

between swipes! Could have done one span per night• We could get good quality FRFs with light ‘automobile’ traffic• OMAX proved very effective• Walkway swipe was very valuable –dynamic link between spans, no

evidence of amplitude dependence• Orthotropic nature of deck revealed by set of swipes• Movement at piers was surprising, shame we couldn’t study

horizontal movement• Step-sine testing was tedious and not good value• Shaker testing did give OK FRFs

Department of Civil & Structural Engineering.

What we would do in future• Think seriously about the logistics of moving accelerometers• Probably foregoing the luxury of full realisation of mode shapes & use

reduced cross-deck measurements• Pay more attention to the bearings, which in any case are more

accessible• Estimate modal mass, frequency and damping from shaker chirp

excitation • Use either shaker shutdown or passage of heavy vehicle to obtain

reliable frequency and damping estimates.