Abstract: Experimental investigation was planned and conducted to study the influence of brick masonry infill in a reinforced cement concrete frame. The analytical methods available needs validation by comparison with experimental results and more accurate methods of analysis like finite element analysis has to be used for the above purpose. In this study, RC frame with middle bay brick infilled representing a five-stories, three bay building in quarter-scale has been taken for experimental investigation and the available methods of theoretical analysis and finite element analysis using ANSYS software for the frames have been carried out. Until the cracks developed in infills, the contribution of the infill to both lateral stiffness and strength was very significant. The change in lateral stiffness, strength, ductility and natural period of the framed structure due to the presence of infills change the behaviour of the building under seismic action. The object of this study was to investigate the behaviour of such infilled frames under seismic loads. For this purpose, five stories, three bay frames with central portion infilled with brick were tested under static cyclic loading simulating seismic action. Analytical works was done to understand the stiffness, strength and behaviour of these types of frames.
INTRODUCTION
The behaviour of the infilled frame under seismic loading is very complex and complicated. Since the behaviour is nonlinear and closely related to the link between the frame and the infill, it is very difficult to predict it by analytical methods unless the analytical models are supported and revised by using the experimental data.
Due to the complex behaviour of such composite structures, experimental research is of great importance to determine the strength, stiffness and dynamic characteristics at each stage of loading.
In this study M30 concrete was adopted for beams and M50 concrete was adopted for column. High strength mix design according to IS method was done to achieve the required strength.
A pre-cast foundation block available in the structural engineering laboratory was used for fixing the specimen for testing. It was cast earlier in such a way that the same unit could be used for testing any number of frames. The foundation block had twelve numbers of bolt holes of 50 mm diameter to enable the fixing of frame specimens to be tested. The fixity of foundation block was obtained by fastening the foundation block with the structural test floor by twelve number of 50 mm diameter rods. IS 1893 (part 1) (2002) code was used to base shear calculation. Madan et al. (1997) carried out modeling of masonry infill panels for structural analysis. Satyanarayanan and Govindan (1989) have studied the effect of opening on the load carrying capacity of the infilled frame. Kodur et al. (1998) presented a numerical example that illustrates the steps associated with seismic design of masonry infilled frames. The examples accounts for the effect of the infill in all the design stages.
Chiou et al. (2006) have reported the structural behaviour of a framed masonry wall subjected to in-plane monotonic loading is investigated by a full-scale test and the method of discontinuous deformation analysis. Lu (2002) has reported a comparative study of the nonlinear behaviour of reinforced concrete multistory structures is carried out on the basis of measured response of four six storey, three-bay framed structures, namely a regular bare frame, a discontinuous-column frame, a partially masonry-infilled frame.
Perumal Pillai (1994) has studied the performance of RC frames with and without brick-infill. The tests on two-quarter-scale, five storied frames brought out the lose of ductility due to infilling. Govindan (1986) has studied the behaviour of seven story infilled frames subjected to static cyclic loading. The parameters included the effect of provision of reinforcement in the plaster to improve the ductility of infilled frames.
Portland Pozzolana cement conforming to IS 269-1976 had been used for concreting. Well-graded crushed aggregate and uniformly graded sand were procured and used. TMT steel bars of 10 and 8 mm diameter had been used as flexural reinforcement for columns and beams of the frames. TMT bars of 6 mm average diameter had been used as shear reinforcement. Silica fume was used as an admixture. Good quality, burnt clay bricks were procured and used as infill in the frames. IS 13920 (1993) has been used for reinforcement detailing.
Equivalent diagonal strut approach: Paulay and Priestley (1992) suggested treating the infill walls as diagonal bracing members connected by pins to the frame members.
In this experimental study the effect of infills in the reinforced concrete frame was considered as equivalent diagonal strut. The tests that have been done by Stafford Smith (1962) on one-eighth scale models of steel frames infilled with mortar and loaded eighth diagonally or back to back. In his analysis, Stafford Smith has defined a relative stiffness factor, which is based on the analysis of a beam resting on elastic foundation as defined in equation:
| (1) |
| h | = | height of infill, |
| t | = | thickness of infill, |
| Em | = | Modulus of Elasticity of infill, |
| E | = | Young’s modulus of frame materials, |
| h | = | height of column, |
| I | = | moment of inertia of column. |
Tests on steel frame models infilled with brick masonry have been conducted by Holmes (1961, 1963). The infill is modeled by an equivalent compressive strut of width equal to one-third the diagonal length of the infill and the thickness of the strut is taken equal to that of the infill. This equivalent frame model has been used for the elastic analysis of the infilled frame.
Liauw and Kawn (1985) have proposed a plastic theory of integral infilled frames based on the results of finite element analysis.
Das and Murthy (2004a, b) have reported the design of five reinforced framed buldings with brick masonry infills for the same seismic hazard in accordance with the applicable provision given in Eurocode 8, Nepal building code 201 and Indian seismic code (with and without ductile detailing) and the equivalent braced frame method given in the literature. Sobaih and Abdin (1988), have presented a numerical method for the analysis of infilled frame subjected to earthquake excitation. The method is based on the concept of equivalent struts used to idealize the infill panels.
EXPERIMENTAL PROGRAM
General: The experimental investigation consisted of testing one-fourth full size model of five-storey, three-bay, reinforced concrete frame with central portion infill under static lateral cyclic load. The frame members are designed in such a way that plastic hinges in beams are realized before the failure of columns. The frames were cast with quality raw materials using wooden mould; sufficient care was taken to ensure quality control and they were cured as per the norms. The RC frame was analysed and designed in STAADPRO software and checked with the conventional way. The foundation portion of the frame was provided with holes to anchor the specimen to strong test floor, so as to test the frame as a vertical cantilever. The arrangement of the wooden mould was in such a way that the frame and its foundation portion can be cast monolithically.
Concreting and curing: High strength concrete was used for columns. For mixing of concrete an electrically operated concrete mixer was used and the concrete was placed immediately after mixing. Needle vibrator of 25 mm diameter was used for compaction of concrete. The casting was done at a stretch. Twelve numbers of boltholes of 50 mm diameter were provided in the footing portion of the frame at the same locations as that in foundation block. Companion specimen such as cubes, cylinders and prisms were cast for all mixes. The specimen was covered with wet gunny bags and was kept moist by periodical sprinkling of water for a period of 21 days from the day of casting. The companion specimens were also cured for the same period as that of the frames.
Brickwork construction: For infilling the central bay of the frame with brick masonry, cement mortar 1: 4 with a water cement ratio of 0.55 was used. The panel size at the bottom storey of the infilled specimen was 800x675 mm and it was 800x600 mm for other stories. The thickness of the brick masonry panel was 100 mm. The curing for brickwork was done for 7 days. The frames and the control specimens were tested after the stipulated period of curing.
Test set-up: The models were tested as vertical cantilevers under a cyclic loading program. The schematic diagram of test set-up is presented. It consists of the following arrangements,
| • | Loading arrangement |
| • | Instruments for measuring deflection (LVDT, Deflectometer etc.) |
| • | Rigid body rotation of foundation block |
| • | Strain measurements. |
Loading arrangement: Lateral cyclic loading was applied at first storey, third storey and fifth storey levels in line with the beams. The reaction frame, which is used for loading arrangements, is rigidly fixed to the test floor. Double acting hydraulic jacks (push pull jack) of capacity 500 KN were used at required levels. Loads were applied from the push pull jack at first storey, third storey and fifth storey levels of the frame. A common console controlled all the three jacks. Pressure gauges were used to measure the applied load. For the application of load through jacks two numbers of hand operated oil pumps were used for applying load in the reverse direction, if needed. The lateral movement of the test frames at the ultimate load stages was avoided by providing suitable guides using mild steel pipes.
Instruments for measuring deflection: LVDT (Linear Variable Differential Transformer) of least count 0.01 mm was used for measuring deflections at all storey levels during the initial stages of loading. When the use of LVDT needed frequent resetting the LVDT was removed and disc-type displacement meters of least count 0.1 mm were used. The LVDT/ displacement meters were connected to slotted angles that were in turn connected to the fixed type to steel reaction frame available.
TESTING PROCEDURE
Testing of frames: Figure 1 shows the complete test setup adopted for the frame model. The effectiveness of instrumentation set up and the loading were checked in the beginning by loading and unloading the frame with small loads (of the orders of 0.5 KN at all the three load points) till all the readings was repeatable.
The frame was subjected to equivalent static lateral cyclic loading. The loading sequences in the beginning for all the frames were almost same. The load increment for each cycle was 2.50 KN at the initial stages i.e., before initial cracking and 5.00 KN in the later cycles i.e., after the first cracking. The deflections at all storey levels were measured at each increment or decrement of load. The strains in steel, concrete and infill were monitored at maximum load of each cycle and at unloading conditions of frame (i.e., when the load is released fully) during all cycles of loading.
The formation and propagation of cracks, hinge formation and failure pattern have been recorded. The deflectometer readings for calculating error due to rigid body rotation of foundation block were also recorded. The load cycles were continued till loading was about two to 4 h and around 6 days of continuous testing.
| Fig. 1: | Test set up of frame |
The concrete cubes were tested for 3rd, 7th and 28th day’s strength as per IS 516 - 1964. The brick prisms were tested under compression and for modulus of elasticity.
Testing procedure: To start with, the frame was loaded with small loads and then unloaded to check the effectiveness of the instrument setup and loading. This process was repeated till the readings were consistent. The frame was subjected to equivalent static cyclic loading. A predetermined static loading was applied to the frame. The lateral loads were applied, by the use of hydraulic jacks, at first, third and fifth stories. At each increment of load level, the readings on deflectometer were measured at all floor levels. Strains in steel, concrete and bricks were measured only at maximum and zero position of loading of cyclic loading. The cracks in the beams, columns and brick works were also observed and marked in the frame simultaneously.
Investigation of brick infilled RC frame: The frame was cast, cured for 21 days lifted and erected on the test bed. Brickwork, in 1: 4 cement mortar, was constructed on the next day and was cured for a period of seven days. The various parameters like stiffness degradation, ductility factor and energy dissipation were considered for study of the behaviour of the frame. Theoretical analysis using non-linear finite element method was carried out and the results of these analyses have been compared with the experimental results.
Loading and load-deflection behaviour: The strains in steel, concrete, column-beam joint, column-infill joint and infill were measured at maximum load of each cycle and at unloading conditions of frame (i.e., when the load is released fully). The formation and propagation of cracks, hinge formation and failure pattern have been recorded.
| Table 1: | Load vs deflection |
The deflectometer readings for calculating error due to rigid body rotation of foundation block were also recorded.
The ultimate base shear was reached in the thirteenth cycle of loading. After reaching ultimate load, post ultimate cycles were performed to study the behaviour of the brick infilled RC frame after ultimate cyclic load till the final collapse of the frame. The displacement due to rigid body rotation of the footing and the foundation block were incorporated in the calculation of net deflection.
The top story deflection versus base shear (Table 1) diagram is presented in Fig. 2 for 1 to 13 cycles of loading, i.e., up to maximum load. From the hysteretic curve assuming bilinear behaviour, the yield deflection Δy was found to be 18.0 mm.
Theoretical analysis based on non-linear finite element analysis were carried out and the results of experimental studies were plotted as maximum base shear of each cycle vs top story deflection up to ultimate base shear shown in Fig. 3. From the Fig. 3 it can be seen that the top storey deflection values for some of the initial load cycles obtained from finite element analysis are higher than the experimental values, the theoretical deflection values are lower than the experimental values. At the ultimate base shear the top storey deflection was found to be 90.47 mm (experimental value) where as it was obtained as 56.175 mm from the non-linear finite element analysis.
In the FEA the brick masonry is assumed as homogeneous and isotropic material as assumed for concrete, but brick masonry is a two - phase material with brick units and mortar joints. For more precise representation of the brick masonry, individual modeling of brick, motor and the interface is required, which is laborious and practically not possible always. The type of loading the frame has been subjected to in experimental investigation (Cycle loading) and the considered in analytical study (monotonic loading) would have been one of the reasons for the difference in the values other than those stated above.
| Fig. 2: | Base shear vs top storey deflection |
| Fig. 3: | Load cycle vs deflection |
| Fig. 4: | Base shear vs deflection |
When the maximum base shear of each cycle is plotted against top storey deflection (Fig. 4) there exist a good relation between the base shear and the top storey deflection whose R2 value works out to be greater than 0.95 (very good relation). Also when the maximum base shear of each cycle is plotted against residual top storey deflection (Fig. 4), there exist a good relation between the base shear and the residual top storey deflection whose R2 value works out to be to be greater than 0.35 (very good relation).
The relationships established can be utilized for estimating the lateral loads the buildings of similar nature has been subjected to and the total deflection the building might have experienced from the residual deflection of the top storey. For obtaining a generalized relationship suitable for estimation of the above values for buildings varying heights, study of similar nature has to be carried out.
Load carrying capacity: Separation cracks were first noticed in the 2nd storey left bay brick infill in the 8th cycle. In the 13th cycle, separation cracks and bed joint cracks and brick infill failure were found in the first and 2nd floor panels and also cracks developed at the junction of first floor beam and leeward column. The different types of cracks witnessed during various loading stages are shown in Fig. 10-12.
Stiffness degradation: The stiffness of the infilled frame for various load cycles were calculated and presented in Table 2 and the variation of stiffness with respect to load cycles is shown in Fig. 5. The theoretical maximum stiffness, the experimentally observed stiffness at cracking load and stiffness at service load are also found out. The stiffness of the brick infilled RC frame was found to decrease from 46.5 KN mm–1 during first cycle to 2.57 KN mm–1 during the thirteenth (final) cycle of loading. Stiffness in the 4, 5, 6, 12 and 13th cycles of loading were greater than that in the previous cycles. This may be due to the strain hardening effect of tension steel and non-uniform closing of cracks on the compression side especially in brickwork. The theoretical maximum stiffness was 23.67 KN mm–1 and it was 8.667 KN mm–1 at the cracking load. At service load (50% of ultimate load) it was 7.263 KN mm–1.
Ductility factor: The ductility factor (μ) for three bay five storey RC frame, central bay infilled with brick was calculated. The first yield deflection (Δy) for the assumed bi-linear load-deflection behaviour of the frame was obtained as 1.1 mm. The ductility factor value μ = (Δ1/Δy) for the various load cycles of the frame were worked out and presented in Table 3 and the variation of ductility factor with load cycles is shown in Fig. 6.
The cumulative ductility factor for various load cycles is also presented in Table 4 and the variation of Cumulative ductility factor with respect to the load cycles is shown in Fig. 7.
| Table 2: | Stiffness vs load cycle |
| Table 3: | Load cycle vs ductility factor |
| Fig. 5: | Stiffness degradation curve |
The cumulative ductility factor was found to increase from 0.227 to 79.91 during the thirteenth cycle of loading.
Energy dissipation capacity: The energy dissipation capacity of the frame during various load cycles was calculated similar to RC frame as the sum of the area under the hysterisis loops from the base shear versus top storey deflection diagram obtained. The energy dissipation capacity during first cycle of loading was 0.0026 KN m and that during 13th cycles was 2.15 KN m.
| Table 4: | Load cycle vs cumulative ductility factor |
| Fig. 6: | Ductility factor for the frame vs load cycle number |
| Fig. 7: | Cumulative ductility factor for the frame |
| Table 5: | Energy dissipation capacity and cumulative energy dissipation capacity |
| Fig. 8: | Energy dissipation capacity of the frame |
| Fig. 9: | Cumulative energy dissipation capacity of the frame |
The energy dissipation capacity values calculated for all cycles are given in Table 5 and the variation of energy dissipated by the frame during each cycle is shown in Fig. 8. The variation of cumulative energy dissipated by frame is shown in Fig. 9.
Behaviour and failure modes: The first crack was witnessed in the second storey left bay brick infill when the base shear was 43.2 KN. The cracking occurred during loading reflect the fact that the infilled frame behaved as an integral unit. At failure, the infilled frame exhibited spalling of brick fragments. The formation of plastic hinges in the floor beams observed after severe cracking of brickwork in the ground storey. The leeward shear also in addition to compression because of the diagonal strut effect of the infill. At the junction of the diagonal strut, leeward column and foundation, the leeward column suffered shear and local buckling. This initiated the final collapse. Second and third storey brick works were slightly damaged, but the fourth and fifth storey bricks were intact. However separation cracks between frame and infill occurred in all storey panels. The infilled frame at failure stage is shown in Fig. 10-12.
Finite element analysis: Asteris (2003) has reported a new finite element technique for the analysis of brickwork infilled plane frames under lateral loads. Ghosh and Amde (2002) have reported the design of infilled frames to resist lateral loads on buildings in terms of their failure modes, failure loads and initial stiffness using procedures proposed by previous authors and verified. This verification is made by comparing the results of the analytical procedures of the previous authors with those of a new finite element model for infilled frames, which are verified by using experimental results. The influence of the masonry infill panel opening in the reduction of the infill frames stiffness has been investigated by means of this technique. Non - Linear finite element analysis has been carried out using ANSYS software. The RC members of the frame have been modeled with SOLID 65 element (Reinforced concrete element) elements, the infill was modeled with SOLID 45 element (Brick element) and the connection between the RC element and infill was made with LINK 10 element (link element) available in the elements library of the ANSYS software. The deflected shape of the finite element model of RC frame at ultimate load is shown in Fig. 13. The top story deflection was 56.173 mm from the finite element analysis where as it was 90.47 mm from the experimental study.
| Fig. 10: | Failure pattern of frame |
| Fig. 11: | Diagonal crack formation of brick infill in the bottom story |
| Fig. 12: | Joint failure |
| Fig. 13: | Deflected shape of the frame |
Separation of infill is visible in the bottom three-storey panels of the frame in the tension side, which was identical to that of the experimental behaviour of the frame.
DISCUSSION
In the literature review, the works were carried out in the performance of RC frames with and without brick-infill. The tests on two quarter-scale, five storied frames brought out the loss of ductility due to infilling. Study of the nonlinear behaviour of reinforced concrete multistory structures on the basis of measured response of four six story, three-bay framed structures, namely a regular bare frame, a discontinuous-column frame, a partially masonry-infilled frame. Behaviour of seven story infilled frames subjected to static cyclic loading.
In this study, three bays, five stories from with central bay brick infill under static cyclic loading were carried out. The combined effect of infill and bare frame action at one stretch were obtained in this study. The effects of ductility, energy dissipation capacity, stiffness, load carrying capacity, stress-strain behaviours differ significantly. The behaviours in the infill bay were not same in the plain bay. Load carrying capacity and stiffness of the infill bay were much more than the plain frame. These different studies were obtained in this frame. The differences in behaviours of the column with infill in one side and without infill on the other side were studied. The behaviour of frame with central bay with diagonal effect and the other two bay without diagonal effect have been carried out.
CONCLUSIONS
The frame developed beam and column hinging near beam column interfaces before they reached their maximum story shear force and they eventually failed due to joint shear, exhibiting successive strength drops. The frame exhibited diagonal crack failure at brick infill in the bottom story at a relatively slow rate of increase before they started to break down. After collapse of brick infill in the bottom story, it was act as a soft story. The contribution of brick infill was significant up to breaking. It is recommended that brick infill should be considered in the analysis of moment frames, especially in the lateral load resisting systems. The strains measured in infilled beams and columns are 20% lesser than bare frame beams upto failure of brick infill. The windward column of the infilled frame in the bottom story was affected to shear and local buckling severely. It is suggested that windward column should be strong enough in the ground floor. It was found that the bottom story columns were more severe than the other stories. Very minute cracks were found at the infill in the top story. The leeward shear also in addition to compression because of the diagonal strut effect of the infill initiated the final collapse of the frame. The diagonal cracks were found in the infills. After this diagonal crack, the infill in the bottom story was inactive. Also the infill in the bottom story was separated from the beam after the formation of diagonal crack. The slope of the failed leeward column in the bottom story was high than the other leeward columns in the top stories.