Design of Reinforced Concrete Infilled Frames
Most reinforced concrete (RC) buildings constructed before modern safety standards are liable to suffer significant damage or even collapse in the event of a major earthquake, as they have not been designed according to capacity design concepts and ductility principles. Therefore, they most probably have inadequate ultimate strength, stiffness, and energy dissipation capacity. The presence of unreinforced brick masonry partition walls in RC frame buildings in Greece and other earthquake-prone areas around the world is probably the reason why many older buildings have withstood major earthquakes in these regions over the past years. Masonry infills, considered as nonstructural elements, are not taken into account in the analysis. Nonetheless, their contribution to the reserve capacity of RC structures is well acknowledged and has been verified both experimentally (Stavridis et al. 2012) and analytically (Moretti et al.2013). Rehabilitation is required for existing RC buildings that are not designed according to modern codes, to upgrade their seismic performance. Retrofit of the masonry infills results in mod- erate strength enhancement (Koutromanos et al. 2013), whereas strengthening of RC frames with RC infill walls leads to considerable increase in lateral stiffness, strength, and to a certain extent, ductility. Hence, this method is often opted for instead of strength- ening the individual bearing elements (Fardis and Panagiotakos 1997; Chrysostomou et al. 2012).
At low levels of in-plane horizontal force, the frame and the in-fill panel will act in a fully composite manner as a structural wall with boundary elements. When load increases, usually the frame attempts to deform in a flexural mode, whereas the infill deforms in a shear mode. This results in the separation of the panel from the frame (at the opposite corners along the diagonal under tension) and the development of a diagonal strut along the diagonal under compression, as shown in Fig. 1 (Paulay and Priestley 1992).
The behavior of the frame/infill interface is significant for the overall performance of the infilled frames (Koutromanos et al. 2011; Shiohara et al. 1985). The connection of the infill to the frame along the interface has been dealt with either by placing concrete shear keys (Hayashi et al. 1980), or by embedding dowels(mechanical or adhesive) along all the frame/infill interfaces (Sugano and Fujimura 1980) or only along the beams (Anil and Altin 2007; Altin et al. 2008; Kahn and Hanson 1979), with or without roughening the old concrete at the interface (Bass et al. 1989). Apparently, the best and easiest method is the embedment of dowels because they lead to a more stable behavior (Altin et al. 1992; Hayashi et al. 1980). Furthermore, adhesive anchors were proven superior in comparison with mechanical anchors (Aoyama et al. 1984).
The detrimental effect of relative slippage along the construction joints has also been extensively investigated in the case of RC structural walls (Synge 1980). Tests have shown that slip at the base construction joint remains relatively constant as a percentage of shear deflection, and that the use of stiff boundary elements in- creases the hysteretic performance of structural walls (Oesterle et al. 1976).
Models for the simulation of in-plane behavior of infilled frames when subjected to racking loads are either micromodels describing the local behavior, or simplified macromodels depicting the global structural behavior. The most commonly used model of the latter category is a single strut along the compression diagonal of the infill connected to the opposite corner joints of the frame through pins.
Because of the lack of design provisions regarding specifically the design of RC infilled frames, as compared with masonry infilled frames for which extensive research has been conducted, a research program was carried out in the RC Structures Laboratory at the University of Thessaly with the objective to verify the adequacy of the provisions for the design of RC infilled frames included in the new Greek code for rehabilitation of existing structures (KAN.EPE. 2012). Significant observations and conclusions in modeling the response of the specimens tested through the appli- cation of existing design provisions (ASCE 2006; CSA 2004),which have been developed primarily for the design of masonry infilled frames, are presented in this work.
The specimens tested are 1=3-scale physical models of a prototype RC frame in the ground floor of a three-story building (with layout of 15 m × 15 m) constructed in Greece before the 1990s. The prototype RC frame sections assumed were designed according to the older Greek codes (1954, 1959) with a seismicity level valid at the time of construction, with nonductile reinforcement provisions. The beam was designed to be stronger than the columns, and the design of the columns was such that bending failure precedes shear failure. The 1=3 scale was chosen because of the limitations of the testing equipment in the laboratory (loading setup and hydraulic jack capacity) combined with the relatively large number of specimens (10). Furthermore, 1=3 scale has been used widely in similar research programs (Altin et al. 2008; Anil andAltin 2007; Aoyama et al 1984; Canbay et al. 2003; Erdem et al.2006; Kara and Altin 2006; Higashi et al. 1980; Sonuvar et al. 2004; Sugano and Fujimura 1980). Two wall aspect ratio values, hinf=Linf , equal to 0.58 (Series A) and 0.83 (Series B) were studied, as shown in Fig. 2. The cross-section of the model frame’s columns and beam is shown in Fig. 3. The longitudinal steel reinforcement in the columns was fully anchored in the foundation block. The columns of specimens A6 and A7 were strengthened by a RC jacket cast simultaneously with the infill, as suggested by the Greek code when an existing RC frame is reinforced with a RC infill wall(KAN.E
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