R EVIEW OF L ITERATURE
2.2.2 Lateral Stiffness
various dimensions, reported that the lateral strength for various opening configurations was found to be ranging from 0.57 to 0.83 times that of the fully infilled frame.
Summary
From the review of the past experimental investigations, it was found that the lateral strength of infilled frame depend on various parameters ranging from aspect ratio, type of infill, vertical load, reinforcement in infill, etc. The presence of infill offers strong contribution to lateral load resistance, especially in non-seismically designed frames. This the effect of infills become detrimental when the lateral force demand exceeds the lateral load resistance of the infills, as the infill walls fail suddenly, which may trigger collapse of the entire structure. This phenomenon is quite common in weak frame-strong infill configuration. From the past literature, it was also found that a clear distinction between the strong and weak frame with respect to strength of infill is not defined.
Relative Stiffness Parameter (λh)
Smith (1966) related the strength and stiffness of infilled frame with the relative stiffness parameter (Eq. 2.1). It was observed that the relative stiffness parameter increases with increase in Em and decrease with increase in Ic. It was also observed that the relative stiffness parameter is quite sensitive to masonry modulus (Em) when compared to moment of inertia of column (Ic). From the past experimental results (Moghaddam and Dowling 1987), it was reported that the lateral stiffness does not seem to be an efficient measure of lateral stiffness of the entire infilled frame as the stiffness is approximately proportional to infill stiffness and insensitive to the frame stiffness.
Aspect Ratio
Based on the past experimental results, Moghaddam and Dowling (1987) concluded that aspect ratio significantly influences the lateral stiffness of the infilled frames if aspect ratio (h/l) is greater than 0.50 and the influence is negligible for lower aspect ratios (h/l <
0.50). Similar observation was made from the study carried out by Mehrabi et al. (1996) where variation in lateral stiffness was not significant in case of infilled frames for the two aspect ratios of 0.48 and 0.67 considered. Colangelo (2005) studied thirteen infilled frames, and reported that the stiffness of infilled frames with or without seismic detailing for two different aspect ratios (h/l ≈ 0.57 and 0.75) found to be decreasing with increase in aspect ratio.
Effect of Infill
Past studies (Mehrabi et al. 1996; Al-chaar et al. 2002; Zovkic et al. 2013; Cavaleri and Di Trapani 2014) reported that the lateral stiffness of the infilled frames was found to be higher for solid brick units compared to hollow units. Mehrabi et al. (1996) from their study on infilled frames reported that the secant stiffness of both weak and strong frames infilled with solid infills under monotonic and cyclic loading has been enhanced by approximately 2 to 5 times that of their corresponding infilled frames with weak infills.
Al-chaar et al. (2002) reported that the precracked stiffness of the infilled frames with concrete masonry and brick masonry was found to be about 24 and 18 times that of the corresponding bare frame. Zovkic et al. (2013) reported the initial stiffness of the infilled frames does not depend on the infill type but the stiffness degradation was found to be lower for AAC infills when compared to clay brick infill specimens.
Murty and Jain (2000) from their study on infilled frames with full-scale and half- scale bricks reported that the average initial stiffness of the infilled frames with full-scale bricks was approximately 1.7 times that of the infilled frames with reduced-scale bricks.
This condition may be valid in case of infilled frames with high brick strength compared to that of the mortar strength. Benjamin and Williams (1958) (as referred in CEB 1996) reported that the stiffness of the infilled frames was reduced by approximately 25% when quarter scale model bricks were used in the construction of infill.
Zarnic and Tomazevic (1988) from their study on infilled frames with reinforced brick infills reported that the results did not show significant influence of the infill reinforcement on the lateral stiffness. On the other hand, Murty and Jain (2000) reported that the initial stiffness of the frames with horizontal reinforcement was reduced by about 20-40% when compared to frames infilled with full-scale bricks or reduced-scale bricks.
This was mainly due to the adverse effect of larger thickness of mortar.
Effect of Vertical Load
Considerable increase in lateral stiffness was reported by Smith (1968) with vertical uniformly distributed load imposed on the beams. Similarly, Valiasis and Stylianidis (1989) reported that the presence of a compressive axial load on the columns improved the lateral stiffness of the system investigated. On the other hand, Mehrabi et al. (1996) reported that the distribution of vertical load between column and beam does not significantly affect the lateral stiffness of the infilled frames, unless the total vertical load is increased drastically (by more than 50% of design vertical loads) which may increase the stiffness by about 30%. Cavaleri et al. (2004) evaluated the influence of vertical loads and reported that the initial stiffness of the infilled frames with partial joints reduced to 50% when compared to infilled frame with continuous joints.
Effect of Openings
Benjamin and Williams (1958) (as reported in CEB 1996) from their study on brick infilled steel frames with central openings of dimensions one-third of the length and height of the infill panel reported that upto 50% of ultimate load, the openings slightly reduced the stiffness of the infilled frame, but as the load further increased the stiffness was sharply decreased in comparison with the frame infilled by a solid panel. Mallick and Garg (1971) reported that the lateral stiffness of infilled frames reduced by approximately
85% to 90% in case of openings located close to the loading corners when compared to infilled frames with solid panels. Liauw and Lee (1977) reported no significant influence of openings on the lateral stiffness of the infilled frames tested. Mosalam et al. (1997) reported that the presence of openings reduces the stiffness by about 40% for lateral loads below the cracking load level. Kakaletsis and Karayannis (2008) reported slight variation in lateral stiffness in case of window openings, whereas, the variation was found to be approximately about 40% in case of door openings for both strong and weak infills.
Tasnimi and Mohebkhah (2011), reported that the initial stiffness of infilled frames with window openings was found to be slightly higher when compared to solid infilled frame which may be attributed to the presence of steel lintel beam, whereas, the lateral stiffness of infilled frame with door openings was found to be lesser than the corresponding solid infilled frame.
Effect of Gap between Infill and Frame
Moghaddam and Dowling (1987) from their experimental results on brick infilled steel frames reported a decrease of 40% for a 100 mm side gap. Further, the authors concluded that the provision of gap in the loaded corner lead to a considerable reduction of the stiffness of the system. Dawe and Seah (1989) studied the effect of the interface conditions between the top frame beam and the infill. Slight reduction in strength and stiffness was observed when a bond breaker (a polyethylene membrane) was adopted at the top interface. The authors also noted that a top gap of 20 mm which was about 0.8%
of the height of the infill caused detrimental effects to the cracking pattern and ultimate capacity of the infilled frame system. Approximately 50% decrease in stiffness and 60%
reduction in the strength was reported. Nazief (2014) investigated masonry infilled steel and RC frames having a full separation gap between the frame and the infill, as well as a top gap between the frame beam and the infill using finite element analysis. The sizes of the gaps considered were 5, 7, 10, and 15 mm. The author reported that the initial stiffness in case of infill steel frames experienced reduction of about 60% for 15 mm gap compared to infill walls in full contact with the surrounding frame.
Summary
From the review of past literature, it was observed that out of the various parameters, aspect ratio, strength of infill, openings and gap between the frame and infill have a
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.