Investigations On Identification Of Axial Forces In Bar Members By Dynamical Vibration Tests

Structural Health Monitoring has already become an important field for the research and development of civil engineering. In practical applications, beam members subjected to axial forces are widely used in civil structures (e.g. truss girders, space truss, etc.). Therefore, the identification of actual axial forces in these elements is of great importance to verify design calculations or to estimate the spare capacity of the elements composing the structure. The work done in this dissertation is mainly focused on the following aspects:(1) By comparing the theories of Euler-Bernoulli beam, initial Timoshenko beam and traditional Timoshenko beam, the accuracy of using classical Timoshenko beam (first spectrum) can be recognized. A deformation coefficient is introduced to account for the effects of the moment of inertia in traditional Timoshenko beam theory. Thus a modified Timoshenko beam theory is developed. By comparing the accuracy of modified Timoshenko beam and traditional Timoshenko beam (first spectrum), the value of the deformation coefficient is suggested.(2) In the third chapter, a fourth order differential equation for free-free beam vibrations is established which takes into account shear deformation, the effects of inertia and the rotary inertia caused by shear deformation. Then, element mass matrix and stiffness matrix are derived based on the modified Timoshenko beam theory (considering the influence of axial force). With the above theory, modified identification of axial forces in beam members by dynamic vibration measurements is proposed when data from sensors (the fundamental frequency) along the length of the bar and the geometry parameters of the bar are available even if no information on the connection of the bar is required. In the numerical simulation, three different boundary conditions:clamped beam, hinged-hinged beam, clamped-free are used. Only the fundamental frequency of the bar can be used to identify axial force, this method is simple and feasible, then can be used in practical engineering.(3) Based on frequency method mentioned in chapter three, vibration-based modified estimation of axial force for bar members is proposed, making modal information into consideration. This method not only takes the rotational inertia, shear deformation, the rotational inertia caused by shear deformation into consideration, but also takes the influence of sensor mass into account to establish an improved dynamic beam equation. Then, the characteristic equation can be solved to estimate axial force. The first five natural frequencies and modal parameters of a bar are computed through numerical simulation with MATLAB. The improved accuracy of estimating axial force by modified Timoshenko beam theory compared to that of traditional Timoshenko beam theory are discussed from four aspects.(4) As mentioned in the fifth chapter, laboratory experiments of different state bar (state space, slenderness ratio, thickness ratio, etc.) are performed to verify the feasibility of the theory in the third and fourth chapters. A MTS810 material servo instrument from Mechanics Laboratory of Dalian university of technology is used to provide axial force. Acceleration response of measuring bar can be tested under impulse excitation. The first five natural frequencies and modal parameters of a bar are computed to identify axial force under different case, which verify the effectiveness of the proposed method.One significant advantage of the proposed method is that when data from five or more sensors along the length of the bar are available, no information on the connection of the remainder structure and the boundary condition of the bar is required, therefore the method can be widely applied to any beam or truss elements.