Direct Lightning Strikes To The Lightning Protection System Of Truss Bridge: Modeling, Analysis And Experimental Investigation

With the fast growth of national economy, as the essential part of the transportation construction the bridge technology has been developing rapidly. The large span bridges are almost built on the land-water interface, where the surroundings area is open terrain. In thunderstorm days, the bridges are extremely vulnerable to the lightning strikes. The accurate study on the transient phenomenon and the impulse effect of bridges struck by lightning is essential part in lightning protection project of bridges.According to the requirements of the Nation Key Basic Research Program of China(973 Program), the calculation and experimental investigation of truss bridges’ transient electromagnetic phenomena due to direct lightning strikes are carried out in this thesis. The electromagnetic effects, electrodynamic force and the heating effect of truss bridges caused by direct lightning strikes are evaluated. The main research contents are summarized as follows:1. The modeling method of the lightning protection system(LPS) of truss bridge based on the approach of a coupled transmission line network is proposed. The solution to the whole LPS is formulated by using an iterative approach in the frequency domain, and the transient surge current distribution in LPS is evaluated. The validation of the proposed approach is performed by comparing the results with those in other literature and experiment al results. The numerical and experimental examples show the flexibility, efficiency and accuracy of the model in the range of practical application of LPS of truss bridge, and which lays the foundation for transient lightning magnetic field calculation in side truss bridge.2. According to the transient current distribution in the brangch conductors of truss bridge LPS, the transient magnetic field distribution inside the truss bridge is calculated by means of Biot-Sarvat Law and finite difference method. And the time delay that exists between the current segment and the observat ion point is taken into account. The method proposed in this thesis is suitable for magnetic field calculation inside large size truss bridge.3. To evaluate the correct and veracity of the numerical calculation, the scaled model of truss bridge was set up in laboratory. The magnetic probe was designed and produced to measure the transient magnetic field. The sensitivity and linearity of probes were calibrated and suitable for measurement. To get better results from small signal involved in heavy interferen ces, the wavelet denoising method is more suitable in the function of interference eliminate. The influence factors of position of lightning strikes are also evaluated, when the position of lightning strike at is the middle of top edge, the degree of unifo rmity of magnetic field is better than the condition of lightning strike at the corner of top surface.4. The heating effect and electrodynamic force of direct lightning strikes to truss bridge were investigated through the experiment. The mechanical damages by electrodynamic force were studied, and the result indicated that the steel structures of the model were not damaged. The melting damage effect on steel bar due to the arc of lightning existed at contact of point of lightning strikes; temperature rise effect of lightning current would not damage the metal structure and was reversible.