Modal Damping Ratio Identification Of Long-span Bridges Based On Continuous Vehicle Bumping Method

Damping represents the inherent capacity of energy dissipation in a structure,it plays a significant role on dynamic response of a structure to wind,earthquake and traveling vechiles.However,due to lacking general mathematical model,the structural damping is the most difficult parameter to determine among other modal parameters,and the field test is the only way to estimate it accurately.Numerous data had been gained from full-scale dynamic testing of large bridges by ambient vibration testing,but it provides poor estimation due to nonstationarity and nonlinearity of excitation,short period of meausrement data.While steady-state sinusoidal excitation can be used to determine accurately structural damping,but it is difficult to execute on large-scale bridges and is expensive.Based on above limitations,the present work attempts to provide a convinient and reliable technique to excite large bridge structures with good operability and reliable test results of damping ratio.To achieve this,a new method called continuous vehicle bumping excitation method is proposed.Theoretical analysis and numerical simulation work have been perfomed.The main work and conclusions include as follows:(1)A literature review is made of dynamic field testing methods as well as existing mathematical models for structural dampings,the achievements and limitations associated with the referenced work is highlighted.The damping ratio data,collected from some long-span suspension bridges field tests and compared with codes of several countries,is expected to provide reference for researchers.(2)A technique called continuous vehicle bumping excitation is proposed to excite large bridge structures,which is basaed on placement of a series of obsatles on bridge decks.The response of the structure under the excitation of the continuous vehicle bumping is derived,and reasonable arrangement of obstacles is given from the viewpoint of energy.By using the simplified single degree-of-freedom model of a vehicle,differences in modal properties of a bridge with and without presense of vehicles are obtained from complex eigenvalue analysis.The results show that the presense of vehicles on bridge only affect the modes of a structure whose frequencies are close to that of the vehicle.Further,in order to evalute wind effects on damping of long-span bridges,the variation of aerodynamic damping with wind velocity is evulated from quansi-steady mdoel and unsteady model of aerodynamic forces.The dynamic response analysis of a simple supported beam and a long-span suspension bridge were simulated under the continuous vehicle bumping excitation to idnetify the damping ratio of the first antisymmetric vertical mode.The effect of vehicle parameters and aerodynamic damping on the structure of the damping factor are quantitatively analyzed.It is confirmed that the vehicle parameters should be choosed such that natural frequency of vehicle is sufficiently larger than that of the structural mdoe in study.(3)Due to numerous members in a stiffening truss section,it is usually complicated and imprecise in the mass moment of inertia calculation process.Therefore,a new method is formulated,which is based on the change in the first torsional frequencie of a cantilever truss girder caused by attaching the additional mass moment of inertia on the truss nodes.The theoretical background is given and the applicable conditions are investigated.The influence of the slenderness ratio of steel truss beam and the position of additional mass moment on the calculation results of torsional moment of inertia mass are discussed in detail.