鋼筋混凝土結構之耐震能力評估 –非線性推覆分析之研究

Abstract

九二一震害顯示，中小學校舍為震害之高危險群，其破壞模式幾乎都是沿走廊長向之剪力破壞，此乃RC結構之系統性問題，值得詳予探究。對於既有老舊RC結構進行耐震能力評估，依據評估結果決定適當之補強方案，兼顧安全與經濟效益，乃當前營建署積極推動之政策。惟現行耐震詳評方法仍未能提供可靠之分析與評估結果，有鑑於此，本研究乃以ATENA 2D軟體為分析工具，以中小學校舍RC結構為對象進行耐震行為研究，並與ETABS推覆分析法之結果進行比較。ATENA 2D軟體乃基於鋼筋混凝土破壞力學理論所建立之材料非線性應力–應變關係﹙組合律﹚，較諸ETABS僅著眼於桿件局部位置之塑角行為更為全面而嚴謹，因此其分析結果應具更佳之可信度。根據本文研究結果，窗台之束制導致RC柱之短柱效應而引發結構之剪力破壞與韌性降低，與校舍在九二一地震之震害模式完全相符。本文進行之參數研究亦顯示，RC柱之配筋量及主筋直徑均會影響其極限剪力強度及韌性；此外，柱軸力會影響其非線性行為，進行耐震詳評時不容忽略。惟本研究亦發現，儘管短柱效應導致結構之韌性降低，但其同時亦能提升結構之極限側向強度(ultimate lateral strength)，因此未必不利於整體結構抗震強度之提升，此與過去吾人對於短柱效應減損結構抗震能力的認知大不相同，值得深思。相較於ETABS之分析結果(容量曲線)，由ATENA推估之極限強度及韌性均較趨保守，建議工程界在應用「鋼筋混凝土建築物耐震能力詳細評估分析方法」時須更為謹慎，以免因高估結構之耐震能力而錯失補強先機。

Statistics from 1999 Ji-Ji earthquake indicated that elementary and high school building structures were among the most seismically vulnerable of all, and the failure mechanism was found to be of a shear-type failure along the corridor without exception. This is a systematic problem of reinforced concrete structures worthwhile exploring. Conducting seismic capacity assessment on the existing RC structures and accordingly, selecting the cost-effective retrofit solutions is the policy currently adopted by the Construction and Planning Agency, Ministry of the Interior. Nevertheless, the current seismic capacity assessment methods are not providing reliable assessment. Therefore, the objective of this study is to explore the seismic behavior of typical RC structures for elementary schools using ATENA 2D. The results will be compared with those obtained by ETABS. ATENA 2D is based on the nonlinear stress-strain relationship (constitutive law) established from the fracture mechanics of reinforced concrete, it is more sound and reliable than ETABS, in which the inelastic behavior of the reinforced concrete is considered only by pre-defined plastic hinges at a certain localized positions. Simulation results indicate that the short-column effects caused by window stages lead to a shear-type failure and reduction of ductility of the reinforced concrete structures, as exactly revealed from many damaged elementary or high schools in Ji-Ji earthquake. The parametric study also indicates that the longitudinal reinforcement, either area-wise or size-wise, of a RC column affects both the ultimate shear capacity and ductility. In addition, the axial force in the RC column affects its nonlinear behavior and should not be ignored in the assessment. It is found that the short-column effect reduces ductility of the reinforced concrete structures while increasing its ultimate lateral strength. The existence of short columns is not necessarily detrimental to the overall seismic strength of the structures. This observation contradicts to what we used to recognize about short columns before. The results by ATENA in terms of the seismic capacity curves imply more conservative estimates of the ultimate strength and ductility of the RC structures than those by ETABS. The industry should be more cautious when using the ETABS-based pushover version of the “Seismic Capacity Assessment Method of RC Structures” for analysis to avoid overestimation on the seismic capacity of the structures and miss the chance for seismic retrofit.