繫筋配置對於鋼筋混凝土梁耐震性能影響之實驗研究

Experimental Study on the Effect of Crosstie Configuration on the Seismic Performance of Reinforced Concrete Beams

李台光(Tai-Kuang Lee) ； 陳正誠(Cheng-Cheng Chen)

《結構工程》 38卷2期 (2023/06) Pp. 39-64

https://doi.org/10.6849/SE.202306_38(2).0002

摘要

現行「混凝土結構設計規範」規定，鋼筋混凝土梁於塑性鉸區域，梁縱向鋼筋在各角隅處之梁縱向鋼筋及每隔一根梁縱向鋼筋，均須以閉合箍筋或閉合肋筋之轉角或繫筋之彎鉤作側向支撐，惟國內鋼筋混凝土工程施工實務，RC梁縱向鋼筋配置較為密集，致使難以施工。本研究針對繫筋配置對於鋼筋混凝土梁耐震性能影響，規劃4座大型鋼筋混凝土梁試體進行實驗驗證，分別為S6D試體橫向鋼筋間距為6倍最小梁縱向鋼筋直徑（D25），且梁中間縱向鋼筋不配置繫筋；S4D試體橫向鋼筋間距為4倍最小梁縱向鋼筋直徑（D25），且梁中間縱向鋼筋不配置繫筋；S6D-SHB試體橫向鋼筋間距為6倍最小梁縱向鋼筋直徑（D25），梁中間縱向鋼筋配置繫筋其耐震彎鉤勾住梁底部縱向鋼筋；S6D-SHT試體橫向鋼筋間距為6倍最小梁縱向鋼筋直徑（D25），梁中間縱向鋼筋配置繫筋其耐震彎鉤勾住梁頂部縱向鋼筋。S6D-SHB及S6D-SHT試體橫向鋼筋（3根SD 280W之D10鋼筋）總圍束力與S6D試體（2根SD 420W之D10鋼筋）相同。研究結果發現：（1）負方向（梁頂部縱向鋼筋受拉及底部縱向鋼筋受壓）韌性表現，以S6D-SHB試體為最優，S6D-SHT及S4D試體次之，S6D試體為最差。S6D-SHB試體為最優，其原因在於配置中間垂直繫筋且其耐震彎鉤勾住梁底部縱向鋼筋，延緩底部縱向鋼筋受壓挫屈之效應；（2）在相同橫向鋼筋總圍束力作用下，配置外閉合箍筋及中間垂直繫筋且其耐震彎鉤勾住梁底部縱向鋼筋試體之極限層間位移角及塑性轉角較採用外閉合箍筋的試體高出5%及12%，建議施工時，接近RC梁柱接頭2倍梁深度的範圍內，先不組立底模及側模，待繫筋完成組裝後，再進行底模及側模之組立，應可解決施工困難的問題；（3）橫向鋼筋間距為4倍最小梁縱向鋼筋直徑的試體之極限層間位移角及塑性轉角較橫向鋼筋間距為6倍最小梁縱向鋼筋直徑的試體相等及高出8%，RC梁橫向鋼筋間距對於RC梁耐震性能之影響並不顯著；（4）在相同橫向鋼筋總圍束力作用下，配置外閉合箍筋及中間垂直繫筋且其耐震彎鉤勾住梁底部縱向鋼筋試體之極限層間位移角及塑性轉角較採用外閉合箍筋的試體高出5%及12%，配置外閉合箍筋及中間垂直繫筋且其耐震彎鉤勾住梁頂部縱向鋼筋試體之極限層間位移角及塑性轉角較採用外閉合箍筋的試體高出2%及4%。本研究發現「混凝土結構設計規範」第15.4.3.3節RC梁於塑性鉸區域，在各角隅處之梁縱向鋼筋及每隔一根梁縱向鋼筋，均須以閉合箍筋或閉合肋筋之轉角或繫筋之彎鉤作側向支撐之規定對於RC梁耐震性能之影響並不顯著；（5）所有4座試體正向包絡線強度無明顯衰減的情形，負方向側向強度則有明顯衰減的情形，此外Pinching（收縮）效應明顯，且極限側向位移角均可達到4.0%以上，惟參考現行「鋼結構極限設計法規範及解說」第13.6.1節韌性抗彎矩構架梁柱接頭所需塑性轉角為0.03弧度，因此所有試體的韌性（塑性轉角）表現基本上皆未達耐震結構3.0% rad之所需，推測原因應為撓剪破壞所致；（6）無論是矩形或T型斷面梁柱接頭，梁頂部縱向鋼筋量一般會略大於梁底部縱向鋼筋量，因此T型梁主要破壞模式應為梁底層縱向鋼筋受壓破壞，撐開閉合箍筋及繫筋所致，而配置垂直繫筋且其耐震彎鉤勾住梁底部縱向鋼筋，也應該有延緩底部縱向鋼筋受壓挫屈之效果。

關鍵字

鋼筋混凝土梁 ； 繫筋 ； 耐震性能

並列摘要

The current Building Code Requirements for Structural Concrete stipulates that in the plastic hinge zone of reinforced concrete beams, the longitudinal reinforcement at each corner and every other longitudinal reinforcement must be enclosed by the corner of the closed hoop or the hooks of crossties used as lateral support. However, in Taiwan reinforced concrete construction practice, the longitudinal reinforcement of RC beams is densely arranged, making construction difficult. In this study, four large-scale reinforced concrete beam specimens were planned and fabricated to conduct experiments to verify the effect of crosstie configuration on the seismic performance of reinforced concrete beams. The transverse reinforcement spacing of S6D and S4D specimens is 6 and 4 times the minimum beam longitudinal reinforcement diameter (D25) respectively, and the longitudinal reinforcements in the middle of the beam are not enclosed with crossties. The transverse reinforcement spacing of S6D-SHB S6D-SHT specimens is 6 times the minimum beam longitudinal reinforcement diameter (D25), the longitudinal reinforcement in the middle is enclosed with crossties, and the seismic hook engages the longitudinal reinforcement at the bottom and the top of the beam, respectively. The total confining force of the transverse reinforcement of S6D-SHB and S6D-SHT specimens (3-D10 reinforcements of SD 280W) is the same as that of S6D specimen (2-D10 reinforcements of SD 420W). It is found that: (1) With respect to the seismic performance in the negative direction (the longitudinal reinforcement at the top of the beam is under tension and the longitudinal reinforcement at the bottom is under compression), the S6D-SHB specimen is the best, the S6D-SHT specimen is second, and the S4D specimen is the third. S6D specimen is the worst. It is because that the middle vertical crosstie is configured in the S6D-SHB specimen and its seismic hook engages the longitudinal reinforcement at the bottom of the beam, delaying the effect of buckling of the longitudinal reinforcement at the bottom. (2) In the same transverse reinforcement total confining force, the ultimate drift angle and plastic rotation angle of the specimen with outer closed hoop and middle vertical crosstie and its seismic hook engaging the longitudinal reinforcement at the bottom of the beam are 5% and 12% higher than those of the specimen with outer closed hoop. It is recommended that during construction, within a range close to 2 times the depth of the beam at the RC beam-column connection, the bottom and side forms are not assembled in advance, and then after the crossties are assembled, the bottom and side forms are installed to solve the construction difficulties. (3) The ultimate drift angle and plastic rotation angle of the specimen with the transverse reinforcement spacing of 4 times the minimum beam longitudinal reinforcement diameter are equal to and 8% higher than those of the transverse reinforcement spacing of 6 times the minimum beam longitudinal reinforcement diameter. The transverse reinforcement spacing of RC beams has no significant effect on the seismic performance of RC beams. (4) Under the same total confining force of transverse reinforcements, when the outer closed hoops and the middle vertical crossties are configured and the seismic hooks engage the longitudinal reinforcement at the bottom of the beam, the ultimate drift angle and plastic rotation angle of the specimen are 5% and 12% higher than those of the specimen with outer closed hoop. When the outer closed hoops and middle vertical crossties are configured and the seismic hooks engage the longitudinal reinforcements at the top of the beam, the ultimate drift angle and plastic rotation angle of the specimen are 2% and 4% higher than those of the specimen with outer closed hoops. This study found that in Section 15.4.3.3 of Building Code Requirements for Structural Concrete, for the plastic hinge zone of RC beams, the rule that the longitudinal reinforcement at each corner and every other longitudinal reinforcement must be enclosed by the corner of the closed hoop or the hooks of crossties used has no significant effect on the seismic performance of RC beams. (5) All four specimens have no significant attenuation in the positive lateral strength, and the negative lateral strength has significant attenuation. In addition, the pinching effect is obvious, and the ultimate drift angle can reach more than 4.0% rad. The required plastic rotation angle is 0.03 rad, so the seismic performance (plastic rotation angle) of all specimens basically does not meet the requirement of 3.0% rad for earthquake-resistant structures. It is speculated that the reason should be caused by flexural-shear failure. (6) For RC beam-column connections, the amount of longitudinal reinforcement at the top is generally slightly greater than the amount of longitudinal reinforcement at the bottom. Therefore, the main failure mode of the T-shaped beam should be the compression failure of the longitudinal reinforcement at the bottom, and the expansion of the closed hoops and crossties. The seismic hook of vertical crosstie engaging the longitudinal reinforcement at the bottom of the beam should also have the effect of delaying the compression and buckling of the longitudinal reinforcement at the bottom.

並列關鍵字

reinforced concrete beams ； crossties ； seismic performance

參考文獻

內政部營建署 (2021). “109 年營建統計年報 ” ，內政部營建署網站http://www.cpami.gov.tw/。

內政部 (2021). “混凝土結構設計規範”，臺灣臺北市。

NIST (2016). “Seismic design of reinforced concrete special moment frames: A guide for practicing engineers, Second Edition, GCR 16-917-40, NEHRP Seismic Design Technical Brief No. 1”, the Applied Technology Council and the Consortium of Universities for Research in Earthquake Engineering for the National Institute of Standards and Technology, Gaithersburg, MD, USA.

中國土木水利工程學會 (2021).“混凝土工程設計規範與解說(土木 401-110)”，臺灣臺北市。

ACI Committee 318 (2019). “Building code requirements for structural concrete (ACI 318-19)(SI) and commentary”, Michigan, USA.