Research On Dynamic Impact Factor Of Steel Plate-concrete Composite Girder Bridge

Due to the characteristics of simple structure,light weight and large economic span,the steel plate-concrete composite girder bridge has broad application prospects in small and medium span bridges in China.At present,the research on the static performance of steel plate-concrete composite girder bridges has been relatively perfect,but the research on dynamic performance is less.However,due to the relatively poor torsional performance,it may cause a greater dynamic response under the action of eccentric load.Supported by the National Key R&D Program of China(2016YFC0701202),the dynamic impact factor of the steel plate-concrete composite girder bridge was studied.The main research contents and conclusions are as follows:(1)The dynamic balance equation of the steel plate-concrete composite girder bridge and the expression of the Newmark-βmethod were solved iteratively.The two-axis 7-degree-of-freedom car model and the four-axis 27-degree-of-freedom train model were established through ANSYS software,and the unit parameter settings were also given.The simulation of road surface and track irregularity was realized by MATLAB and the whole process of vehicle-bridge coupling vibration simulation was given based on the road surface contact method.(2)Four highway I-shaped steel-concrete composite girder bridges with a span of20～50m were designed according to the SMDI general drawing standard.The effects of fundamental frequencies,road roughness,speed and driving position on the dynamic impact factor were studied.The results showed that the impact factor increased with the increase of the fundamental frequency,and the rate of change showed a decreasing trend synchronously with the increase of fundamental frequency.The worse the pavement grade resulted in the greater the impact factor of the bridge.The change of impact factor with vehicle speed was determined jointly by the vehicle and bridge structure attributes,and there was a larger impact factor under the conditions of 20km/h and 100km/h.The torsion resistance of the 20m-span with 5.87Hz steel-concrete composite girder bridge was poor,the eccentric load conditions of the vehicle would cause the impact factor to be too large,and the driving position was not the main factor affecting the impact factor when the span was large.(3)According to the design drawing of the railway bridge in Sichuan-Tibet frozen ground area,three steel-plate composite girder bridges with a ballastless span of 32-48m were designed to study the influences of fundamental frequencies,track irregularity,vehicle speed and train composition on the dynamic impact factor.The results showed that the impact factor increased with the increase of the fundamental frequency.Track irregularity was an important source of excitation for vehicle-bridge coupled vibration and a high interference spectrum can produce a larger impact factor than a low interference spectrum.When the vehicle speed and the fundamental frequency of the bridge met a certain relationship,resonance would occur,which would greatly increase the impact factor.At the same time,due to the interval of resonance,the impact factor on the left and right sides of the resonance speed would also be significantly larger.A single-car formation crossing the bridge would not produce a resonance effect,but its impact factor was greater than that generated by a multi-car formation in the non-resonant speed range under the same conditions.When a multi-car formation crosses a bridge,a resonance effect will occur when v_i=3.6fd/i.When the total length of the multi-car formation train exceeds the span of the bridge structure,the IMs will be the same.(4)Compare the suggested formulae with the specifications of IMs.The results showed that the fitting formulas in this paper were higher than the values of IM in Chinese highway and railway bridge codes.One reason is that the fitting of the measured data of the box girder bridge in the Chinese code does not represent the IM of the steel plate-concrete composite girder bridge.Another reason is that the transverse connection stiffness of the steel plate-concrete composite girder bridge is less than that of the box girder bridge,and will further enlarge the dynamic load effect under the condition of partial load.Therefore,it is suggested that the value of the IM should be adjusted appropriately in the design of medium and small span steel plate-concrete composite girder bridge.