Effect of vehicle weight on natural frequencies of bridges measured from traffic-induced vibration

Recently,ambient vibration test(AVT)is widely used tu estimate dynamic characteristics of large civil struc-tures.Dynamic characteristics ean be affected by various envirnnmental factors such as humidity,intensity of wind,andtemperature.Besides these environmental conditions,tire mass of vehicles may change the measured valnes when traffic-in-duced vibration is used as a source of AVT tor bridges.The effect of vehicle mass on dynamic characteristics is investigatedthrough traffic-induced vibration tests on three bridges;(1)three-span suspension bridge(128m+404m+128m),(2)five-span continuous steel box girder bridge(59m+3@ 95m+59m),(3)simply supported plate girder bridge(46m).Acceleration histories of each measurement location under normal traffic are recorded for 30 minutes at field.These recor-ded histories are divided into individual vibrations and are combined into two groups aceording to the level of vibration;oneby heavy vehicles such as trucks and buses and the other by light vehicles such as passenger cars.Separate processing ofthe two groups of signals shows that,for the middle and long-span bridges,the difference can be hardly detected,but,forthe short span bridges whose mass is relatively small,the measured natural frequencies can change up to 5.4%.