Research On Natural Frequency Of Simply Supported Beam Basis On Multiple Temperature Fields

With the great development of China’s infrastructure construction, the bridge project hasalso leap across. But large cracks began to appear in many built bridge structures. And it causeswidespread concern of scientific researchers and engineers. Studies have shown that thetemperature constraints stress caused by natural environment temperature, in which the bridgesare placed, is the main reason for surface cracks in the bridge structures. While the temperaturefield also led to the changes of bridge natural frequency. It has important practical significanceto analyze the temperature effect on the bridges, and the influence degree of temperature on thenatural frequency.Departure from the distribution form of the temperature field, three main types of naturalenvironment temperature field are discussed one by one. They are day-change temperature field,sudden-drop temperature field and annual-change temperature field. The theory calculationmethod and simplified practical application of temperature field are summarized. By ANASYSfinite element analysis software, a simply supported beam model is established to simulate theeffect of actual temperature field. The results of model analysis are consistent with thedistribution form provided in standard, and get well simulation results.Then, the temperature stress, equivalent temperature axial force and equivalenttemperature moment are discussed and calculated qualitatively. Temperature stress is essentiallyrestraint stress in micro or macro. When the temperature changes, if various deformation of thebridge structure can be free to expand to release, the cross section of the structure will not causetemperature stress. Due to the mutual constraint of the cross section of each fiber, the stressgenerated by the nonlinear temperature field in the cross-section of simply supported beams isthe self-restraint temperature stress. The practical effect of the temperature field is the same asequivalent external loads imposed at both ends of the bridge, including equivalent temperatureaxial force and equivalent temperature moment. In this chapter, the temperature stress underdifferent distribution form of temperature fields are studies by cases, and the formula of theequivalent temperature load is given. Equivalent temperature field load is converted to the axial eccentric force acted at bothends of the beams in this paper, and the expression of the natural frequency based on the Eulerbeam vibration principle is obtained. The two influence factors of temperature on naturalvibration frequency are analyzed. The two factors are changes of elastic modulus andtemperature stress. the influence degree of the two factors o n natural frequency are discussedcombined with examples, respectively.Besides, based on a real simply supported box girder bridge with the length of30m in the#22-#23pier of Yuanda Bridge, By using theoretical analysis method formed in this paper,aswell as ANSYS modeling analysis, the change degree of natural vibration frequency underdifferent temperature difference are obtained. Finally, the natural frequency under differenttemperature difference obtained from a real bridge dynamic load test is used to verify thenumerical results. Conclusions are as follows:1. The influence of temperature changes on thenatural frequency of simply supported beams are mainly due to the reduction of the elasticmodulus of the concrete material under the sunshine temperature field. The changes ofboundary conditions caused by temperature stress has weak relationship with frequency;2.Theinfluence of temperature on first-order natural frequency of simply supported beam bridge isbiggest. Under sunshine temperature field, when the temperature difference between bridge roofand floor is20°C, the frequency is reduced by4.85%from theoretical value of less consideringtemperature effect;3.Temperature rises, and the vibration frequency of the bridge will decrease.