Live Load Resistance of a Cable-suspended Footbridge Using Photogrammetry and Finite Element Analysis

[[abstract]]人行吊橋對於臺灣山區的聯繫是不可或缺的，一旦遭受損害往往造成交通運輸不便，更會影響人民的安全，而吊橋承受載重時之變形測量對其安全評估是相當重要之課題。測量時採用如傳統位移計等，在人員與架設方法時皆有疑慮，使用精密光學測量儀器，則需考量成本上的負擔。本研究先建立一套簡易非接觸式量測方法，藉由有限元分析模型校準量測之變形，有助於應用在吊橋安全檢測與承載力之評估。 研究時採用影像測量方法，製作搭配之十字標靶，先於室內進行影像測量之精度驗證，了解誤差之控制。而現地實驗場選擇屏東縣萬新吊橋，以攝影量測吊橋之幾何外觀後，進行載重實驗，由人工放置既定活載重於吊橋中段，經由橋外端發射之雷射光點與橋上十字標靶之攝影，量測橋面垂直位移變化量。將現地影像測量各照片進行數位影像處理，取得吊橋主索幾何線型，建立承受靜載重平衡時之有限元模型，然後施加原現地實驗之活載重，進行幾何非線性分析計算，比較現地實驗之垂直位移；最後配置不同活載重分析，探討吊橋之活載重承載力。 現地活載重加載與有限元模型分析，兩者變位相差4mm為合理，驗證了應用有限元素方法建模分析之正確性，其結果與吊橋現地實驗值相當接近，故此套程序，應可適用於大位移吊橋之承載能力之評估。[[abstract]]The footbridge is vital for human and goods transport as Taiwan’s valley crossings, and its deflection measurement under live loads is essential to bridge safety. The installation of displacement gauge concerns hazard to workers whereas the sophisticated optical survey instrument involves high costs. The study employs non-contact survey by photogrammtry, and validates surveyed results by the finite element modeling. On a bridge site, the entire cable image comprises a series of photographs showing a large-size wood cross template, taken while traversing the bridge deck in sections. Next, the deck deflection experiment under a light live load is recorded as images of a small-size cross template targeted by laser dots projecting from a laser pointer fixed at the bridge entrance. Image processing produces cable geometry under self-weight that conforms to a finite element cable model, which is subsequently used in computing the cable deflection under the experimental live load. Different live load conditions are also analyzed to assess the load carrying capacity