R EVIEW OF L ITERATURE
2.2.3 Energy Dissipation
significant influence on the lateral stiffness of the infilled frame behavior. At the same time, it was also observed that various formulations were suggested to evaluate the initial stiffness of the infilled frames as the quantitative prediction of stiffness is quite difficult, since the stiffness of the infilled frame greatly depends on non-quantifiable parameters like workmanship. From the past literature, it was also observed that the strong and stiff infill improves the lateral load performance of the infilled frame system. But, under severe earthquake excitations, the brittle nature of the infill creates severe irregularities due to the variation in stiffness leading to the collapse of the entire building system.
variations in the behavior of the two types of infilled frames were observed under lateral loading. The areas of the hysteresis loops are smaller in case of frames where shear connectors are provided leading lower hysteretic damping. More pronounced nonlinearity of the envelope curve was reported in case of frames without shear connectors which may be attributed to the friction between infill and frame. On the other hand, Liauw and Kwan (1992) reported that the infill walls which are connected to the bounding frame (integral frames) have linear and narrow hysteretic curves at small deflections (±5 mm), whereas, at the larger deflections (±10 mm) the hysteretic loops are wider which lead to tremendous energy dissipation, and it sustained for many cycles. The energy is dissipated through both material and interface damping. For non-integral frames, the lack of fit has pronounced effect after a few cycles, when the corners of the infills crush. The hysteretic curve has a noticeable pinching and is very narrow leading to a relatively small energy dissipation capacity.
Aspect Ratio
Stylianidis (2012) investigated the influence of two aspect ratios (h/l ≈ 1.0 and 1.5), and reported that the energy dissipation capacity of the infilled frame with h/l ≈ 1.5 increased by about 30% when compared to infilled frames with h/l ≈ 1.0. Schwarz et al. (2015) from their study on frames with two different aspect ratios (h/l ≈ 2/3 and h/l ≈ 3/2) for both integral and non-integral infills reported that the narrow panel (h/l ≈ 2/3) with integral infill possesses a substantially higher plastic area (energy absorbed) than a wide panel (h/l ≈ 3/2) with non-integral infill.
Effect of Infill
Mehrabi et al. (1996) reported that the frames with strong infill panels exhibited higher energy dissipation than those with weak panels regardless of the frame design. Zovkic et al. (2013) from their study on reinforced concrete frames with various types of infills reported that the stronger infill had more energy dissipation capacity when compared to frame with weak infill. Similarly, Markulak et al. (2013) from their study on single bay steel frames with various types of masonry infills reported that the dissipation of hysteretic energy increases with increase in strength of infill. The amount of hysteretic energy dissipated in case of high strength clay block infilled frames and AAC block infilled frames was about 6.7 and 4.8 times that of the bare frames, respectively.
Kakaletsis and Karayannis (2008) reported slightly lower cumulative energy dissipation in case of frames with stronger infill when compared to frames with weak infill. At the same time, it was also reported that the effect of infill strength did not substantially influenced energy dissipation ratio especially in case of infilled frames with window openings. Misir et al. (2012) evaluated the performance of the reinforced concrete frames with two different types of masonry infills namely: standard and locked bricks. The authors reported that the cumulative energy dissipation of frames with both types of infills is higher than that of the bare frames which is mainly due to reaching higher levels of lateral forces. Another parameter called relative energy dissipation ratio was also used which is defined as the ratio of actual to ideal dissipated energy by a test specimen between the specified drift limits during the reversed cyclic response. The results showed that both types of bricks contributed to the energy dissipation capacity at drift ranges between 0.5% and 1%. Stylianidis (2012) reported that use of strong mortar leads to slight increase in the energy dissipation capacity of the infilled frames.
Zarnic and Tomazevic (1988) reported that significant influence of the infill reinforcement on the behavior of specimens was observed only in the case of infilled frames with openings. Murty and Jain (2000) reported that energy dissipation of infilled frames without horizontal reinforcement was about 22% higher than the reinforced infilled frames, which was because of the localization of sliding along the few mortar bed joints along which reinforcements were placed.
Effect of Openings
Kakaletsis and Karayannis (2008) reported that the total energy dissipation of infilled frame with openings was about 1.02 to 1.43 times the capacity of the corresponding bare frame. Further it was reported that the energy dissipation capacity of infilled frame with window openings was not substantially influenced, whereas, the energy dissipation capacity of the infilled frames found to be decreasing in case of door openings.
Effect of Vertical Load
Stylianidis (2012) reported that the presence of axial load on the columns increased the energy dissipation capacity of the infilled frames where the system dissipates energy through friction across the relatively small width infill cracks. It must be pointed out that higher values of the dissipation ratio appear in case of strong frames, i.e., when there is an
external axial compression load on the columns or an increased reinforcement ratio. This result can be attributed to the fact that when the frame is stronger the confinement of the infill is better. The ratio of energy dissipation capacity of the infilled frames to the bare frames is about 3 to 5 times at distortions of 2% and about 1.3 to 1.5 at distortions of 30%.
Summary
From the review of the past literature, it may be summarized that the energy dissipation of frame systems can be significantly enhanced by the presence of infills in addition of the formation of plastic hinges in columns by observing damage in infill by friction mechanism. Even though major importance has been given to the energy dissipation capacity as a structural parameter to understand the cyclic response of infilled frames, the evaluation of energy dissipation was only useful from a qualitative view point.