پاسخ لرزه ای فریم های RC تحت بار چرخه ای با فرکانس پایین
Abstract
1- Introduction
2- Experimental investigation
3- Seismic performance of frames
4- Analysis of test result
5- Conclusions and recommendations
References
Abstract
This paper presents an investigation on the seismic behavior of four reinforced concrete (RC) frames. The study is focused on the effect of the Aerated Lightweight Concrete (ALC) blocks infills on the seismic performance of the RC frames and the interaction between infills and surrounding frames. Four RC frames include a control specimen, frame with full-filled infills, frame with large window openings, and frame with eccentric door openings. Based on the low frequency cyclic loading experiments, hysteretic dissipation ability, stiffness degradation, characteristic displacement and load, failure pattern, flexural moments of columns, effective slab width, required ratio of column-to-beam strength, and column shear force are experimentally investigated and analyzed. Tests results indicate that the strength, the initial stiffness and the area of the hysteretic loop at the same load step were influenced significantly by infills when compared with the frame without masonry infills. This effect was reduced due to the large openings in the frame with infills. The column and beam mixed hinges failure was observed in the frame without infills while shear failure appeared at column ends in frames with infills. The testing results also show that inflection points were shifted to columns top because of the infills. Column shear forces were increased significantly due to the diagonal strut effect. Recommendations on useful effective slab width, the required ratio of column-to-beam strength and the equivalent strut width are made to take masonry infills into consideration in design.
Introduction
Masonry infills are frequently used in reinforced concrete (RC) structures. Since they are normally considered as architectural elements, the role of infills is underestimated in design code [1-4]. Seismic damage investigation indicated that RC frames were damaged showing “strong beam-weak column” failure, shear failure, and joint failure, which went against the original design intention. During strong earthquakes, the majority of RC frames collapsed or suffered a soft story failure due to few infills in the first floor (Fig. 1) [5]. Disproportionate openings led to short-column shear failure (Fig. 2) [6]. These damages illustrate that the infills have a significant effect on the whole structure and cannot be ignored. Thus, more researches should be carried out to understand the role of infills during strong earthquakes. During the last few decades, experimental studies on RC frames with masonry infills under static loads were carried out. Around 1960s, Polyakov [7] and Holmes [8] first found that brick infills could increase the strength and stiffness of frames, which brought infills effect into the sight of researchers. In order to compare seismic performance of RC frames with different infill materials and load routines, Bertero [9] carried out quasi-static cyclic load and monotonic load tests of 1/3 scaled 3-1/2 stories frames, Bertero [9] pointed out that the initial lateral stiffness, maximum lateral resistance, and effective viscous damping coefficient of frames increased significantly due to the effect of infills. Zovkic [10] also pointed out that the seismic performance was greatly influenced by the material of the infills. Armin [11] considered other influence factors such as strength of infills with respect to bounding frames, panel aspect ratio and vertical loads.