Study On The Temperature Effect Of Long Span Prestressed Concrete Rigid Continuous Girder Bridge

During the service period,the concrete box girder is exposed to the environment,which involves complicated negative factors such as sunlight and clod wave exposure.In addition,as the concrete material has a relatively low thermal conductivity,a non-uniform temperature field will be generated in the bridge.This phenomenon will lead to the generation of thermal stresses and even thermal cracks.Some thermal cracks can significantly deteriorate the structural performance of bridges and eventually impact their reliability and security.In order to guarantee the performance of bridges,it is crucial to study their temperature fields and thermal effects.This thesis takes a long span prestressed continuous box bridge in Shandong as the background,while many fiber grating temperature and strain sensors and a series of acceleration sensors are installed in the structure.The real-time temperature,strain and modal of the structure are monitored online in a long term.Based on these data,the changing rules of box girder's temperature fields,thermal strain of the concrete box girder's section as well as the relation between its natural frequency and the temperature are studied.In addition,a 3D finite element model will be established,while the change of its temperature field and the corresponding thermal effects during the cold wave will be studied by ANSYS.The main contents and the conclusions are briefly summarized here:(1)According to the analysis on the measured data obtained from the temperature sensors,the bottom slabs and the web slabs of the upper-bridge is slightly affected by sunshine and the overall trend of the temperature change is mainly affected by the shift of seasons.The top slab of the box girder has the maximum temperature effect of the sunshine as its temperature is obviously the highest among all points during the monitored time.The bottom and the web slabs of the upper-bridge and the down-bridge are affected by the sunshine temperature in a similar level.When the vertical height is the same,the temperature distribution of the bridge does not change by the vertical position.Then,probability density of the temperature is fitted using the least square method.It can be obtained that the transverse temperature difference is greater than the vertical temperature difference.(2)By conducting the analysis towards the measured strain,it can be concluded that the strain in the bottom slab of the box girder is greater than in the top slab.In addition,for the measured points in the same altitude,the compressive strains in both top slab and bottom slab of the upper-bridge are larger than the corresponding position of the down-bridge.Also,the stress curve of the upper-bridge shows a more significant wave than the down-bridge.Compared with the temperature change,the strain curve appears the same tendency but with certain hysteresis.Based on the modal frequency analysis,it can be noticed that the relative frequency within day can reach 1% while the relative frequency within half a year is 4.38%.Through the analysis on the relation between different step frequencies and the temperature,the relation can be simplified as a linear correlation.(3)By simulating three cold wave conditions in ANSYS,take the temperature plummeting as the environment stimulation and study the transient thermal distribution in the bridge's box girder section.It can be found that the temperature history of the outer surface is similar to the environmental temperature history set in the model.In the meantime,the temperature history curves' change in the middle element and the inner surface are less significant with certain hysteresis.Compared with the strain values measured in the middle point of each span,the secondary side span has the maximum deflection of 1.93 cm in the condition of 20°C plummeting in 48 hours.The midpoint deflections of both main span and the secondary side spans decrease with the temperature drop,while the side spans midpoint deflections increase in the same environment.With the cold wave impact,it is the longitudinal stress that shows the most significant change in the top slab and the bottom slab of the box girder,while vertical stress is nearly constant.Transverse measure points on the top slab indicate the maximum vertical stress on both edges and the minimum stress on the middle point.However,the midpoints have the peak stresses on both longitudinal and transverse directions.In the cold wave of 20°C plummeting in 48 hours,the peak changing rate in the first ten order modal shapes impact reaches 0.56%.The one order mode frequency has a negative correlation with the temperature according to the simulation values.