Analysis Of Vibration Disease On A Long-Span X Arch Footbridge And Reinforcement

Over the last years, with increasing strength of new structural materials and aesthetic requirements for greater slenderness, the trend in footbridge design has been towards greater spans and increased flexibility and lightness. As a consequence, stiffness and mass have decreased, which has lead to smaller natural frequencies and more sensitivity to dynamic loads. Many footbridges have natural frequencies that coincide with the dominant frequencies of the pedestrian-induced load and therefore they have a potential to suffer excessive vibrations under dynamic loads induced by pedestrians. In the past few years the vibration serviceability problem of footbridges attracted great public attention.To analysis the vibration of footbridges by human-induced dynamic loads, this thesis introduced models for dynamic pedestrian loads, the theoretics of forced resonance vibration and the method to assess vibration serviceability. For the lacking of researches on human-induced vibration of footbridges currently, this thesis proposed a referenced method of dynamic design of footbridges by comparing and analysising current design criteria and codes for footbridges in other countries. There are two methods of dynamic design of footbridges, tuning the natural frequencies of the structure or analysising dynamic response of structures to human-induced loads. And the latter is more reasonable. when the low natural frequencies of a footbridge are in the range of 1.4~2.4Hz for the vertical direction and in the range of 0.7~1.2Hz for the horizontal direction, it is required to analysis the dynamic response to human-induced loads and assess vibration serviceability.This thesis took the Rainbow bridge in BaiShan city of JiLin province for an example to analysis the mechanical characteristics of X arch bridges. After inspecting the situation of the Rainbow bridge and assessing it, the reasons for excessive vibration under human-induced loads were found. Also, assessing vibration serviceability was done so as to retrofit the bridge. At last, several reinforcement strategies for the bridge were provided. This thesis is to propose referenced methodology for inspecting and assessing and reinforcing similar bridges. And some important conclusions were included in this thesis. The deck of half-through X arch bridges composed of rigid arch and flexible deck is subject to damage. The flexible deck has such bad dynamic performance that excessive vibration response to pedestrian loads will occur. Furthermore, resonant vibration may be induced by synchronized human walking. A conclusion is drawn from the dynamic characteristics analysis of a real bridge that X arch bridges'first several natural frequencies are determined by the characteristics of arch and others by the characteristics of deck. Increasing the stiffness of the deck can't move the natural frequencies of the structure out of pedestrian footfall frequency range, but it can effectively decrease the dynamic response to the pedestrian loads and improve the serviceability and safety of the bridge. Dynamic response analysis method can effectively diagnose the disease due to human-induced vibration and compare the effect of reinforcement with primary dynamic characteristics. Increasing stiffness is often difficult without considerable structural additions that almost always affect the aesthetics of the bridge and cost much. Another measure against vibration problems of footbridges is to increase the overall damping of the structure. Usually installing dampers is a wonderful measure.