大型木造結構之發展與挑戰

Abstract

由於全球暖化氣候變遷議題，國際間環保意識高漲，各國推廣綠建築、綠建材的同時，木構造也備受重視。早期之木造建築多以原木材料為主，建築物尺度受材料所限，工業木材發展後始有大跨徑的木構造出現，也促成大型木造建築的新工法與市場發展契機。本研究探討木建築的發展現況與未來，包括一座大型木造拱橋的結構特性研究以及發展高層木建築的可能挑戰。本文將隨機子空間系統識別技術應用至大跨徑木拱橋的參數識別，以點移動荷載模擬行人載重引發人行木橋之動態反應進行數值模擬分析。在穩態行人載重擾動下，以識別出之模態數量、阻尼比與MAC值評比，顯示取樣頻率較高者擁有較好的系統識別結果。由於該座木拱橋的邊界條件以鉸支承為主，沿橋走向的模態較不易被激發；當同一時間橋上的行人數減少時，則無法激發出高頻模態。因此，人行橋系統識別之現地試驗，應分別選擇於尖峰及離峰時段進行監測，俾能取得較完整之橋梁系統參數。另一方面，交錯層積材 (Cross-Laminated Timber, CLT)之發展與應用為高層木建築的實現提供了可能性，惟其在抗震、防火、抗風特性上的研究仍不夠周全，例如CLT牆版試驗侷限於面內行為、側向剛性不足引致變形過大與二次應力效應之安全與舒適性問題、接合構件的變形控制與韌性問題、火害試驗侷限於木材本身而忽略與連接件的整體行為等，在相關問題未釐清與克服之前，高層木建築之安全仍存在高度不確定性。當前歐美若干代表性的高層木建築都蓋在非地震帶、無颱風的城市，對於像台灣這樣有地震與颱風肆虐的國家，實不宜過度樂觀與冒然推展。

In response to global climate changes and worldwide rising awakeness for environment protection, the concepts of green buildings and materials are widely promoted among all countries, with wood structures being highly attractive as a consequence. In early days, wood structures were mostly constructed with logs so that the scale of structures was limited. Advances in wood manufacturing industry have made the erection of large-span wood structures a reality and created opportunities for the development of new construction technologies of large-scale wood structures. This study explores the current status and future on the development of wood structures, including investigation of the structural characteristics of a large arch bridge and discussion of possible challenges in the construction of high-rise wood buildings. In this study, the stochastic subspace system identification (SSI) method was adopted to identify the dynamic characteristics of a large-span arch bridge of woods subjected to pedestrians excitation simulated by moving loads. Under the disturbances of pedestrians in a steady state, the cases with higher sampling frequency prove better in terms of the number of modes identified, accuracy in the estimated damping ratio as well as the MAC values. As the boundary conditions of this wood bridge are mostly hinges, the vibration modes related to the longitudinal direction of the bridge are found difficult to excite, while the high frequencies modes are not excited with less pedestrians on the bridge simultaneously. Therefore, it is suggested to conduct field tests of pedestrian bridges in both rush hour and off-peak time so as to get more insights of the structural characteristics via system identification. On the other hand, the advances and application of Cross-Laminated Timber (CLT) provide the possibility towards the construction of high-rise wood buildings. Nevertheless, comprehensive studies are not yet available in exploring the characteristics of high-rise wood structures in earthquake-protection, fire-proof and wind-resistance. For instances, component tests of CLT wall panels were restricted to the in-plane behavior only, safety and serviceability problems introduced by large deformation and P- effects, as a result, due to the low lateral stiffness nature of wood structures, issues on ductility and deformation control of connectors, fire tests were restricted to timbers alone without the assembly with connectors as a whole, etc. The safety of high-rise wood buildings remains highly uncertain before clarification and exclusion of the aforementioned concerns. The high-rise wood structures to be named of in Europe and North America are located at cities without the threats of earthquakes and typhoons. For countries suffering from those natural hazards like Taiwan, it is not suggested to be overly optimistic on the promotion of high-rise wood buildings.