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

Recently, ambient vibration test (AVT) is widely used to estimate dynamic characteristics of large civil struc-tures. Dynamic characteristics can be affected by various environmental factors such as humidity, intensity of wind, andtemperature. Besides these environmental conditions, the mass of vehicles may change the measured values when traffic-in-duced vibration is used as a source of AVT for bridges. The effect of vehicle mass on dynamic characteristics is investigated through 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 according 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%.