鋼結構梁柱彎矩接頭於梁端腹板開孔之力學行為

Abstract

於特殊彎矩構架中當梁柱接頭經過適當之設計，其梁構件便可在地震力作用下發展出足夠的塑性變形，使其具有可靠的韌性行為來消散地震引致之能量。設計上梁柱接頭常藉由減弱梁端撓曲強度或補強梁柱交界面等方式來迫使塑鉸產生於梁端，以防止接頭破壞改善其耐震性能。 本研究旨在針對腹板開孔減弱式梁柱接頭之可行性及其耐震性能進行研究，並深入瞭解其力學行為以建立一套合理之設計流程。有鑑於以上構想，本研究遂以非線性有限元素分析軟體，建立梁腹板開孔梁柱接頭之數值分析模型，並將分析之結果與試驗結果進行驗證。其後，本研究再以開孔形狀、開孔位置、開孔長度、開孔深度及腹板厚度等參數，探討其對梁柱接頭耐震性能之影響。最後本研究依據腹板開孔之力學行為，進行開孔斷面彎矩-剪力互制公式之解析，並將解析與有限元素分析之結果進行驗證，再依據研究成果建立腹板開孔梁柱接頭之設計流程。 有限元素分析結果顯示，有限元素分析確可準確的模擬梁柱接頭之整體及局部之行為。同時，分析結果亦顯示，於各種不同之開孔形狀中，採用本研究建議之中間矩形兩端半圓形的「圓-矩形開孔」型式梁柱接頭擁有最佳之韌性表現。此外，增加開孔長度及開孔深度等參數則可提升接頭之韌性，但同時亦有開孔區提早發生剪力破壞之風險，故本研究建議開孔長度之中間矩形段長度應為0.5至0.75倍梁深，而開孔深度則應以0.5倍至0.6倍梁深為宜，如此所設計出之開孔尺寸將可同時滿足梁柱接頭之韌性需求，且不致過早於開孔區段發生剪力破壞。再者，梁腹板厚度的增加亦可增加開孔斷面剪力強度，延緩開孔區段發生剪力破壞，但須注意不可過度增加，以免過度補強降低接頭之韌性。 另一方面，根據理論解析之結果顯示，解析所得之設計公式可準確地預測開孔斷面彎矩-剪力互制強度，故本研究可依據此設計公式建立相關之設計流程。而當所設計出之開孔斷面符合「開孔斷面之標稱剪力強度Vn小於平衡標稱剪力強度Vn,b」及「開孔後梁柱面撓曲強度Mn,f小於梁未開孔斷面之撓曲強度Mp」等二條件時，其設計結果即為可用之梁腹板開孔梁柱接頭。 本研究所建議之梁腹板採圓-矩形開孔經有限元素分析後證實，確實可成功的應用於減弱式梁柱接頭，使梁柱接頭具有良好之耐震性能。再者，依據理論解析所得之設計公式亦可充分地反應腹板開孔彎矩-剪力之互制效應，應可做為梁腹板開孔梁柱接頭相關設計流程之用。

Well-designed special moment frames are expected to be capable of dissipating numerous energy during earthquake through the development of plastic flexural deformation on the beams. In order to prevent the brittle failure and improve the seismic behavior, beam-to-column connections are generally designed to force the plastic hinge formed on the beam at a distance away from the column by reducing beam section close to the column or reinforcing the connection at beam-column interface. This study was conducted to explore the feasibility and seismic performance of a reducing beam section connection by introducing an opening on the beam web, and to establish an efficient design process by investigating the mechanical behavior of the connection. Therefore, nonlinear finite element analysis was employed to simulate the beam-to-column connections with beam web opening. Analysis results were then verified by experimental results to validate the analysis model. Subsequently, the effects of relevant parameters, such as opening shape, plastic hinge location, opening length, opening depth, and web thickness, on the seismic performance of the connections were further investigated. Finally, based on the mechanical behavior, an analytical flexural-shear strength interaction equation for the beam web opening was derived, and the analytical results were verified by the finite element analysis results. Accordingly, a design procedure for the connections with beam web opening was established. The analysis results indicated that the finite element analysis can accurately predict the global and local behavior of the connections with beam web opening. Further, the results of parametric study showed that, among the various web opening shapes, the circular-rectangular opening which the shape is rectangular in the middle and semi-circular at the two ends attained the best ductility performance. In addition, increasing the length and depth of the web opening can enhance the ductility of the connections. However, excessive increase would cause shear failure at the opening corners due to inadequate shear strength. Therefore, to satisfy the ductility requirement for the connection and prevent the premature shear failure on the beam opening section, it is suggested that the opening length should be 0.5 to 0.75 times the beam depth for the rectangular region, and the opening depth should be 0.5 to 0.6 times the beam depth. Moreover, increasing the beam web thickness can enhance the shear strength on the opening section and delay shear failure. However, excessive increase could reduce the ductility of the connection. The analytical results showed that the equation derived can accurately predict the flexural-shear interaction strength of the web opening section. Thus, this equation is suitable to establish relevant design processes. To design a usable connection with beam web opening, two criteria must be satisfied: (1) Nominal shear strength at the web opening cross-section, Vn, is less than the balance nominal shear strength, Vn,b; and (2) The beam flexural strength at column face of the beam with opening, Mn,f, is less than the plastic flexural strength of the beam without opening, Mp. Based on finite element analysis results, the recommended circular-rectangular web opening can be utilized as a weakened beam-to-column connection and to achieve a satisfactory seismic performance. Furthermore, the analytically derived equation can accurately calculate the flexural-shear interaction strengths on the web opening section, and, further, can be applied to design beam-to-column connections with beam web opening.