基於推覆分析與短柱效應考量之結構耐震評估方法

Abstract

目前工程界進行耐震詳評工作時，多存在「強柱弱梁」的迷思，且未能將窗台導致的短柱效應考慮在評估分析中，不僅無法反映真實的結構行為，還可能高估結構的耐震能力。「強柱弱梁」的立意甚佳，惟梁因樓板的加勁作用使得勁度大幅提昇，以致即使在強震下其主筋無法降伏而未能形成塑鉸，導致整體結構之韌性降低。此由九二一地震之災後調查中，從未有人表示看到過梁端產生塑鉸即為明證。此外，許多中小學校舍的破壞模式幾乎都是沿走廊長向之柱剪力破壞，其主因乃窗台下的填充牆對柱形成局部加勁作用而導致短柱效應所造成。有鑑於此，本研究乃發展一套結構耐震詳細評估方法，以剪力屋為分析架構排除「強柱弱梁」的破壞機制，並以等值斜撐模擬填充牆，將其引致之短柱效應反映在分析模型中。本評估方法主要利用SAP2000進行推覆分析，根據實際之混凝土強度、斷面尺寸、配筋及填充牆計算結構之極限耐震容量(Capacity)。另一方面，結構之耐震需求(Demand)則配合我國現行之建築耐震設計規範求得，但其中韌性容量係根據推覆分析結果而非規範建議值。凡耐震容量與耐震需求之比值大於一者即表示結構物之耐震能力無虞。本文並針對兩棟校舍進行耐震詳評，分析結果顯示，教室隔間牆有助於結構耐震能力之提升；沿走廊之矮牆則會造成柱之剪力破壞，降低結構韌性，證明本文所提結構耐震能力評估方法之合理性。

The current practice of seismic capacity assessment is commonly obsessed with the illusive idea of “strong-column weak-beam” while failing to take into account the effects of short-column due to window stages. As a result, the analyses are not reflecting the actual behavior of the structures, and may in turn overestimate the seismic capacity. Theoretically, the “strong-column weak-beam” design concept is not a bad idea. However, the beams are much stiffer than expected as reinforced by concrete slabs, and no plastic hinges will be formed in the beams since the reinforcing steel bars are far from yielding even during severe earthquakes. As a consequence, ductility of the overall structure is reduced. This argument is supported by the fact that no one has ever claimed observing any plastic hinge in beams in the after-shock reconnaissance of 1999 Ji-Ji earthquake. In addition, the collapse mechanisms of most buildings in the elementary and high schools are found to be of a shear-type failure in the columns along the corridor. This is primarily due to the short-column effects with the partial restraint of columns by the infill walls for window stages. In view of the aforementioned problems, this study develops a structural seismic capacity assessment scheme based on shear building structures by which the failure mechanism of beam-sidesway is excluded. In addition, the concept of equivalent diagonal struts is adopted to structurally represent the infill wall so as to sufficiently reflect the short-column effects in the analysis. The proposed method is based on the pushover analysis by SAP2000 utilizing the actual material strength, dimension and reinforcement of the RC members as well as the infill walls to estimate the ultimate seismic capacity of a structure. While the seismic demand is to be estimated in accordance with the current seismic design standard, however, discarding the code-recommended allowable ductility capacity with that obtained by the pushover analysis. For those structures with a capacity-to-demand ratio of greater than one are considered seismically safe. In this study, seismic capacity assessments of a couple of RC school buildings have been conducted. Results indicate that the partition walls contribute positively to the seismic capacity of the structures, while the partial infill walls under the windows along the corridor may introduce shear-type failure to the columns and decrease the ductility of the structures. These further confirm adequacy of the proposed seismic capacity assessment method.