| 摘要: | 根據美國2019年版ACI 318規範(ACI，2019)將擴頭鋼筋耐震伸展長度公式修訂為l_(dtE,19)=(f_y ψ_e ψ_p ψ_o ψ_c)/(75√(f_c^′ )) d_b^1.5(in, psi)，其中梁縱向鋼筋降伏強度以1.25f_(y )取代1.0〖 f〗_y，並將鋼筋直徑乘上1.5次方進行計算，相較於ACI 318-14擴頭鋼筋耐震伸展長度l_(dtE,14)=(f_y ψ_e)/(62.5√(f_c^′ )) d_b(in, psi)，當鋼筋尺寸大於#8(D25)時，l_(dtE,19)之擴頭鋼筋耐震伸展長度將大於l_(dtE,14)。本研究共進行13組外柱梁柱接頭試驗，探討採用l_(dtE,14)=(f_y ψ_e)/(62.5√(f_c^′ )) d_b(in, psi)耐震伸展長度錨定於梁柱接頭之耐震性能，除探討鋼筋伸展長度外，也考量包含梁縱向鋼筋淨間距及接頭剪力需求-容量比與柱構件軸力比對梁柱接頭耐震性能的影響。
試驗結果顯示，採用l_(dtE,14)=(f_y ψ_e)/(62.5√(f_c^′ )) d_b(in, psi)之耐震伸展長度，儘管梁縱向鋼筋淨間距為1.5d_b，所有試體也皆可發展至層間位移角4%弧度的耐震性能，且擴頭鋼筋未發生錨定失敗的情形。由試體破壞模式發現，當試體接頭剪力比約為0.8時，最終破壞模式為梁塑鉸破壞，而接頭剪力比介於1.0至1.25時，最終破壞模式為梁降伏後接頭剪力破壞，顯示出儘管設計之接頭剪力比高達1.25，採用l_(dtE,14)=(f_y ψ_e)/(62.5√(f_c^′ )) d_b(in, psi)耐震伸展長度埋置於梁柱接頭，可提供試體發展至層間位移角4%弧度的耐震性能。比較高軸力與低軸力試體之強度發展與破壞模式發現，當梁柱接頭本身已能發展良好梁端塑鉸時，施加高柱軸力無法明顯提升梁柱接頭的試體強度，但根據破壞模式顯示，施加高柱軸力對於接頭剪力比達1.0之試體，能有效提升接頭勁度，並改善試體之最終破壞模式。
;This study investigates the seismic performance of beam-column joints by examining the use of different development lengths of headed reinforcing bars, in accordance with the American Concrete Institute (ACI) 2019 code. The revised formula for the development length of headed reinforcing bars is given as l_(dtE,19)=(f_y ψ_e ψ_p ψ_o ψ_c)/(75√(f_c^′ )) d_b^1.5(in, psi), where 〖 f〗_y represents the yield strength of longitudinal beam reinforcement, which is taken as 1.25 times the original yield strength 〖 f〗_y and the bar diameter is multiplied by 1.5 for the calculation. A comparison is made with the ACI 318-14 code, which uses an unmodified yield strength and a different formula for the development length, l_(dtE,14)=(f_y ψ_e)/(62.5√(f_c^′ )) d_b(in, psi). It is observed that when the size of the steel reinforcement is larger than #8 (D25), the development lengthl_(dtE,19) is longer than l_(dtE,14). Thirteen sets of exterior beam-column joint tests were conducted to evaluate the seismic performance when using the development length from ACI 318-14, with the reinforcing bars anchored in the beam-column joint. The study not only examines the influence of the development length but also considers the effects of clear spacing of longitudinal beam reinforcement, joint shear demand-to-capacity ratio, and axial load ratio of column elements on the seismic performance of the joints.
The test results demonstrate that using the development length from ACI 318-14 allows all specimens to achieve seismic performance up to an inter-story drift angle of 4%, even with a clear spacing of longitudinal beam reinforcement set at 1.5 times the bar diameter. Moreover, no anchorage failure of the headed reinforcing bars was observed in any of the specimens.
The analysis of the failure modes indicates that when the joint shear ratio is approximately 0.8, the ultimate failure mode is beam plastic hinge failure. On the other hand, when the joint shear ratio ranges from 1.0 to 1.25, the ultimate failure mode is joint shear failure after beam yielding. Remarkably, even with a high design joint shear ratio of up to 1.25, using the development length from ACI 318-14 can still provide seismic performance up to an inter-story drift angle of 4%. |
