摘要
梁柱节点是框架结构中的一个薄弱构件,且在地震作用下节点破坏的模式对结构整体破坏模式有着重要的影响。而遵照“强柱弱梁”等原则按规范进行严格抗震设计的梁柱节点,其钢筋纵横交错,施工极为困难,而且混凝土浇筑后不易振捣,工程质量难以得到保证。
有学者将钢纤维混凝土应用于节点,取得了一定的成果。而超高韧性水泥基复合材料(UHTCC)是一种纤维体积掺量在2%以下的纤维增强复合材料,具有准应变硬化特性,变形能力较好,且在单轴拉伸作用下具有多缝开裂特性。为简化梁柱节点的抗震设计提供了一个新的选择方案。
为了研究UHTCC材料代替部分箍筋应用于钢筋混凝土框架节点处的可行性,本文针对9个不同的全尺寸钢筋混凝土框架梁柱节点试件进行了低周往复加载试验。试验设计的研究参数有轴压比,UHTCC的使用区域,节点核心区体积配箍率,梁端箍筋加密区的箍筋间距等。
在9个试件中,有3个是完全由传统混凝土浇制而成,并按照我国相关规范经过严格的计算合理配置纵筋和箍筋。另外6个试件则是在梁柱节点核心区及附近用UHTCC代替传统混凝土,减少了梁端箍筋的配置,且在梁柱节点核心区的箍筋也有所减少甚至未在节点核心区配置箍筋,以考察UHTCC是否能够代替部分箍筋的作用承担剪力,甚至对构件抗震性能有所改善。
在实验结束后,对各个试件的滞回曲线、骨架曲线、刚度退化、能量耗散等相关数据和指标进行处理分析和比较,结果发现UHTCC试件即使在配箍减少的情况下,依然能够稳定承受低周往复荷载作用,且其极限承载力与普通混凝土试件相比有所提高。UHTCC试件的刚度退化过程也保持基本稳定,而且其节点核心区的变形能力较普通混凝土试件更好,其在单个荷载循环内的总耗能也普遍高于混凝土试件。
The beam-column connection is a vulnerable element in the reinforced concrete frames. And the failure mode of the joints does have significant influences on the sidesway mechanism of the entire building under earthquake. However, if the beam-column connections were fully designed and detailed according to the "strong column-weak beam" principle to withstand seismic actions, the longitudinal and transverse reinforcements would cross with each other. This would make the construction work extremely hard. What's more, due to the congestion of the reinforcement in the beam-column joint region, it is more difficult to place and consolidate the concrete, therefore the quality of the whole construction cannot be guaranteed.
It is reported that steel fiber reinforced concrete was used in the joint region and certain results were achieved by some researchers. In recent years, a kind of innovative fiber reinforced cement-based composite draws intensive attention, which is called ultrahigh toughness cementitious composite (UHTCC). UHTCC is a type of composite containing about2%volume fraction of short fibers, with properties such as pseudo strain hardening, superior deformation capacity and multiple cracking behaviors under uniaxial tensile loading, which promises an alternative for seismic design of structures.
In this paper,9specimens of full-scale beam-column joints with different designs were tested under the earthquake-type reversed cyclic loading, in order to investigate the feasibility of reducing the transverse reinforcement in the beam-column connections by using UHTCC. The parameters to be researched in the whole experiment program include axial load ratio, the area where UHTCC was used, the stirrup ratio in the joint core and the stirrup spacing in the stirrup densification area of the beams.
Of the total9specimens,3were made of common concrete, which are strictly designed according to the current codes in China. In the other6beam-column connections, UHTCC was used in the joint core region, as well as parts of nearby columns and beams. And in order to examine whether the UHTCC material could bear the shearing force instead of partial transverse reinforcement, or even improve the seismic-resistance performance of the structure element, less or even no stirrups were used in the joint core and nearby region.
After the experiment, the hysteresis property, spine curve, stiffness degradation and the energy dissipation property of the9specimens was presented, analyzed and compared. It is noted that, the specimens with UHTCC, even though with less stirrups, still could undertake the reversed cyclic loading steadily. Moreover the ultimate bearing capacity of the UHTCC specimens was apparently higher than the normal concrete ones. The stiffness degradation process of the UHTCC specimens is basically stable, and the deformation performance of the UHTCC joints is better than that of the normal concrete ones. Besides, higher energy dissipation is obtained in the UHTCC specimens, too.
引文
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