Temperature Analysis Method Of Box Girder Bridge Using The Local Wind Speed Nearing The Structral Surfaces

Bridges are the important civil infrastructures.However,the long-term overload operation and harsh service environment result in damages even accidents on bridges.Temperature has been has been widely recognized as one of the most significant and negative environmental factors for the bridge,which significantly degenerate the performance,decline the carrying capacity and safety,even lead to catastrophic failure.The temperatures of bridge are susceptible to the surrounding environments.The environmental wind is one of the crucial factors dominating the convective heat-exchange between bridges and the surrounding air.In this paper,the effects of wind local properties nearing the structural surfaces on the bridge temperatures are investigated.The main contents are as followed:(1)Experimental studies are performed for determination of thermal physical parameters of building materials under natural environment.The basic principles of convective heat-exchange between solid and air are analyzed and summarized.A new set of experiment equipment is designed and fabricated for quickly measuring and analysis of the convective heat-transfer coefficient of materials.Furthermore,experiments are implemented on concrete slab specimens with different roughness(rough and smooth plaster)to determine the convective heat-transfer coefficient in the natural environment.The wind speed has a great effect on forced convective heat-transfer coefficient.The relation between the convective heat-transfer coefficient hc of rough fabricated and relative smooth surface with respect to the wind speed v can be fitted into linear function expressed as hc=6.378+3.929 v and hc=5.941+3.519 v W/(m2·oC),respectively.(2)Numerical simulations of the local properties of environmental wind nearing the bridge surfaces are carried out using computational fluid dynamics(CFD)software.The typical problem of wind flowing around a cylinder is firstly performed to confirm the validity of numerical calculation.Then,numerical simulations of wind field around bridges are performed and analysis based on two bridge girder sections corresponding to a long-span steel box-girder suspension bridge and a long-span concrete box-girder continuous rigid frame bridge,respectively.The wind speed of the nearby surfaces of box section is obviously different from the field inflowing velocity.The wind filed around the windward and leeward of box section is large disorder,but the wind distributions of surfaces paralleling the inflowing wind direction are relatively stable.Thus,the convective heat-transfer coefficient of all bridge surfaces,which calculated using wind speed of bridge field,has larger deviation to the actual situation.Considering the feasibility of wind speed measurement in engineering,it is suggested that heat-transfer coefficient can be calculated suing the wind speed which locating 0.5m above each surface along the outside normal direction.(3)Two temperature field analysis methods for bridge considering the local properties of wind nearing the surfaces of bridge are proposed.The one is that the field wind speed adjusted using the shape coefficient referring to the Chinese wind loads codes.The other method is wind speed for bridge thermal analysis using the CFD numerical results,namely,the heat-transfer coefficient are calculated using the wind speed at the height of 0.5m above each surface.Based on ANSYS finite element(FE)software APDL programming,two-dimensional fine FE thermal analysis models are developed for a large steel box-girder and concrete box-girder section,respectively.Then,time-varying temperature field of the two box section models are studied using different wind conditions(traditional method,shape coefficient correction method and CFD numerical method)under the weather conditions of sunny day.Comparing with the traditional method,the numerical results of bridge temperatures considering the local properties of wind achieve better agreements with the measurements.They also reveal that more serious vertical and horizontal temperature difference on the box-girder bridges.