Forced vibration modal testing of ‘International Bridge’ at Wayne, New Jersey, July 2010 Prof. James Brownjohn Dr Ki-Young Koo Dr Chris Middleton The University of Sheffield Department of Civil and Structural Engineering Vibration Engineering Section

Planned test grids

Test grids for AVT and FVT using 19 sensors Span 2: using 6 swipes with 2×16+triax by shaker Span 1Span 2 Single swipe with 19 sensors

The workers …. Chris on data acquisition, and Ki on the cherry picker

The equipment …. QA 750 servo accelerometers, APS400 shaker

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

Displacement signals from double integration of acceleration. Are we seeing real coupling of spans?

Mode shapes obtained using NExT/ERA: (30 minute data set) Clear evidence of coupling here

Single vehicle free decay and small damping and frequency variation

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, 

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

Cross-section reveals increasing transverse mode order half-sine full-sine half-sine

Chirp excitation & response –on last swipe (4AM?), almost zero traffic –Shaker induced response drowned by traffic effects

Good quality FRF; used for Global RFP curve fitting

Global rational fraction curve fitting for two modes …

Modes 1,2 from GRFP

…. And using MACEC/OMAX

How did we do?

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

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.