Paste youAbstract: The current research work aims to study the behavior of the Reinforced Concrete (RC) framed structures fully, as well as, partially infilled with Unreinforced Masonry Infill (URM) panels under seismic loads. In order to achieve this goal, a numerical study of three groups of 2-D three-bays RC frames will be conducted under static Pushover analysis, as well as, dynamic time-history analysis. The three building groups are classified as nine, six and three stories representing high, medium, and low rise buildings; respectively. Different infill panels’ configurations will be studied for each group to simulate the Bare Frame (BF) representing the most used common practice which does not include the stiffness and the strength of URM panels in the analysis and design procedure, Infilled Frame (IF) taking the stiffness and the strength of the URM panels into consideration, Open Ground Story structure (OGS) which the infills in the ground story are omitted, and Partially Opened Ground Story (POGS) in which the infills are omitted from the two exterior bays in the ground story. Some selected numerical results in terms of base shear forces, lateral deflections, and inter-story drift ratios are obtained for all the considered configurations and presented in a comparative way.
Keywords: Infilled frames, Open ground story, Soft story, static pushover analysis, Dynamic time history analysis, Base shear, Story Drift.
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Introduction:
URM infill panels are commonly used in RC structures as interior and exterior partition walls for architectural purposes. Common practice has always been to ignore the infill panels during the design and the analysis of the RC framed structures due to its highly non-linear nature which is difficult to be simulated. However, they may interact with the surrounding frame when the structure is subjected to lateral loads induced by earthquake ground motions.
In several moderate earthquakes, some of these buildings have shown a better performance when subjected to earthquake even though the majority of these buildings were not designed or detailed for seismic loading conditions [1][2].On the other hand, they have been, sometimes, blamed for several undesirable effects under seismic loading such as; short column effect, soft story effect, torsion and, out of plane collapse.
Along the years, many researchers have studied the effect of URM infill walls on the behavior of the new, as well as, the existing RC building structures. Most of the researches can be classified into two groups; experimental studies and analytical studies. Several researchers have investigated experimentally the performance of the infilled frames under lateral loads. The first published experimental work on the infilled frames under racking load was by Polyakov [3] who conducted a number of large scale tests in order to determine the racking strength of infilled frames. Holmes [4] conducted a number of experimental tests to describe the behavior of single-story welded-steel frames with concrete infill under horizontal and/or vertical loading. Bertero and Brokken [5] summarized studies in which the effects of masonry and lightweight concrete infills on RC moment existing frame buildings were studied experimentally. Mehrabi et al. [6] discussed the results of experimental tests conducted on twelve separate single-bay, single-story RC frames with masonry infill. The results indicated that infill panels can improve the seismic response. Pinho and Elnashai [7] summarized the test procedure and the results of the experimental test conducted on two separate, full-scale, four-stories, reinforced concrete frames. Hashemi and Mosalam [8] conducted a shaking table experiment on a prototype selected as the middle bays of the first story of the building. Furtado et al. [9] performed experimental study of full-scale infilled masonry walls under cyclic and monotonic loading with and without in-plane damage.
While the aim is to study the behavior of the structure as a whole, most of the tested frames were single-story; single-bay plane frame. Other researchers studied the performance of the infilled frames analytically in order to study the behavior of the structure as a whole. Mallick and Severn [10] were the first to use the finite element method in order to model the infilled framed structure. Rectangular plane stress element was used to represent the infill taking into consideration the slip between the frame and the infill. Dhanasekar and Page [11] studied the influence of brick masonry infill properties on the behavior of the infilled frames using a finite element model under raking load. The model was verified using racking tests on infilled frames. Mosalam et al. [12] summarized the fragility analyses of representative buildings in the area of Memphis, Tennessee, USA. This study focused on low rise lightly reinforced concrete frame building with and without infill walls. Asteris [13] investigated the influence of the masonry infill panel opening in the reduction of the infilled frames stiffness. Changing the position and the percentage of the opening for a single-story, single-bay infilled frame was carried out. Milheiro et al. [14] presented two case studies of existing reinforced concrete buildings infilled with masonry walls located in Portugal in different seismic zones according to EuroCode 08. The buildings were studied using non-linear dynamic analysis with and without infill walls.
As the URM infill panels are used in the majority of the RC building structures, many lessons can be learned by studying their damage patterns after the occurrence of earthquakes. The common practice of removing the infill panels in the ground story for commercial purposes leads to vertical stiffness irregularities and may cause soft story mechanism as shown in Figure 1.
In Egypt, the URM infilled RC framed structures shares a considerable portion in the Egyptian real estates. The majority of these framed structures have a number of bays ranging between three to six bays in both directions with the below stories; basement, ground, and first, kept opened for commercial reasons. Furthermore, the majority of these buildings were designed and constructed according to old regulations in which no seismic design provisions were considered. Moreover, the method of construction of the infill panels in these structures does not prevent the interface with the structure behavior. This results in a different behavior rather than the bare structures. Accordingly, it is important to study the behavior of such buildings during earthquakes to investigate their performance considering the interference in the behavior with the infill panels.
In the present study, three groups of 2-dimensional fully and partially infilled framed structures have been employed. The three groups were nine stories, six stories and, three stories RC framed structures representing high, medium, and low rise buildings, respectively. The design of these frames has been carried out according to the old versions of the Egyptian regulations where no seismic design provisions were included. This is to represent the case of old existing RC building. Accordingly, only the gravity loads has been considered in the design of these frames.
Modeling of URM Infill Panels:
The URM infill panels can be modeled through two methods; macro-model method and micro-model method. Although the micro-model method is considered the most precise method to model the URM panel as it provides the possibility of real simulation of the masonry, it demands longer time of the analysis due to complexity in generating the model. Alternatively, the macro-model method studies the overall structure response by representing the URM panel as a diagonal compression strut as firstly described by Polyakov [16]. In other words, the infilled frame system is equivalent to a braced frame.
In this work, a four-node masonry infill panel element, proposed by Crisafulli [17], was used to represent the nonlinear behavior of infill panels in framed structures. Each panel is represented by six strut members; each diagonal direction features two parallel struts to carry axial loads across two opposite diagonal corners and a third one to carry the shear from the top to the bottom of the panel as illustrated in Figure 2.
Modeling Validation
In order to validate the results of the developed models, an experimental test by Pinto et al. [18] has been modeled and the accuracy of the models has been assessed through a comparison between the analytical and the experimental results. The tested frame is a reinforced concrete four-story frame consisting of two bays of 5.0 m span and one bay of 2.5 m span. The geometric configuration of the tested frame is shown in Figure 3. Further information on the tested frame can be found in [18]. The implementation of the double-strut nonlinear cyclic model for URM walls was carried out by Smyrou et al. [19].
Material properties of the reinforced concrete frame have been the same as laboratory testing. The frame has been modeled using SeismoStruct software [20]. Inelastic displacement-based frame elements divided into 200 fibers have been used for modeling beams and columns. Each structural member has been subdivided into four inelastic beam-column elements with smaller length at the member ends so as to ensure the accurate modeling of expected plastic hinge zones. The non-linear dynamic time history analysis has been carried out using the artificial record which is plotted in Figure 4.
The story displacement and the base shear have been recorded and plotted along with the experimental ones to compare them and to assess the accuracy and the reliability of the model. Figure 5 and Figure 6 provide the time histories (experimental and numerical results) of top floor displacement and base shear for the tested frame. From the figures, it can be shown that the obtained responses from the model (displacement and base shear) are very close to those obtained from the experiment showing an acceptable accuracy.
Parametric Study
In order to study the behavior of the fully, as well as, the partially infilled RC framed structures with URM panels under seismic loads, three groups of 2-dimensional framed structures have been employed. The three groups are nine stories, six stories, and three stories structures representing High Rise Buildings (HRB), Medium Rise Buildings (MRB) and, Low Rise Buildings (LRB); respectively. All of them have over all dimensions of 15.0m x 20.0m in plan with constant story height of 3.0m. Each building consists of moment resisting frames spaced at 5.0m apart and no shear walls are utilized. The typical layout plan for the three buildings is shown in Figure 7. All beams have the same dimensions (0.25m x 0.60m) in all floors. In each building, the columns have the same geometrical characteristics in all floors. Table 1 shows the cross sectional dimensions and the reinforcement of the columns for all buildings. Only the gravity loads have been considered in the design of these frames, and therefore the frames have not been expected to meet the seismic design requirements. The design has been carried out according to the Egyptian regulations [21], [22]. For the beams, distributed vertical loads have been assigned taking into consideration a flooring cover load of 1.5kN/m2 and a live load of 2kN/m2 acting on the slabs which are supported by these beams. Additional concentrated loads have been applied on the columns to simulate the reactions of the beams in the transversal direction to that of the selected frames. The unit weight of the infill walls has been considered 14kN/m3 which represents the unit weight of the widely used hollow bricks in Egypt.
A typical frame has been selected from each building to perform the parametric study. Each frame has three bays of 5.0m span. For each selected frame, four different configurations for the infill panels have been considered as follows:
The Bare Frame (BF) case in which no infill panels have been utilized at all floors to represent the most common practice of not including the stiffness and the strength of the infill walls in the analysis and design procedure.
The Infilled Frame (IF) case in which the infill panels have been located in all stories in all bays in order to take the stiffness and the strength of the walls in consideration.
The Open Ground Story (OGS) case in which the infill panels have been located in all floors except in the ground story in order to simulate the common practice of keeping the ground story open for commercial or architectural reasons.
The Partially Opened Ground Story (POGS) in which the infill panels have been located in all stories and in all bays except the two exterior bays in the ground story.
Figure 8, Figure 9, and Figure 10 show the different four configurations of the infill panels. For all frames, the reinforcement details of the beams have been kept identical in all floors. As shown in Figure 11.
Material Properties
The materials have been chosen to have properties as close as possible to those used in the construction in Egypt. The concrete used corresponds to a normal weight with cubic compressive strength of 25MPa. The used reinforcing steel is high grade steel of class 36/52 according to the Egyptian standards with nominal values of yield strength, ultimate strength and ultimate strain equal to 360MPa, 520MPa and 12%; respectively.
Nonlinear concrete model proposed by Mander et al. [23] has been employed for defining the concrete material while the Menegotto-Pinto steel model [24] has been employed for defining the steel material as presented in Table 2 and Figure 12.
The non-load bearing infill walls of hollow bricks have been assumed to be used in the modeling with dimensions 0.12×0.25×0.06m. Plaster of 15mm has been applied on both sides of the wall. Material properties adopted for masonry infill walls are presented in Table 3. The width (w) of the infill diagonal strut is computed using the expression by Paulay and Preistley [25]; given in Equation 1.
w=0.25d_inf (1)
where dinf = the diagonal length of infill.
Types of Analysis
In order to obtain a comprehensive understanding about the behavior of the infilled framed structure, the three buildings (e.g. HRB, MRB, and LRB) with the four wall configurations for each have been selected in this study. For each building, the case of the bare frame (BF) has been considered as the reference case. This is due to the fact that the design process usually assumes this case because of its simplicity for the structural engineers. Two distinct types of analysis have been employed; the nonlinear static pushover analysis and the dynamic time history analysis.
Pushover analysis
Static pushover analysis has been conducted with an inverted triangular load pattern in order to obtain the capacity curve for each model. Displacement controlled pushover procedure has been the used one in order to capture the post peak softening behavior of the structure and to capture the irregular response features. At the end, the relationship between the base shear and the lateral deflection (roof displacement), that is called capacity curve, is determined [26].
Dynamic time history analysis
The nonlinear dynamic time history analysis was proven to be the most powerful analysis type in predicting the behavior of the structures under the seismic loads. This is because it takes into account the material, as well as, the geometrical nonlinearities. In dynamic analysis, the nonlinear inelastic response of a structure subjected to earthquake loading can be predicted over time during and after the application of the load. The seismic action may be introduced by means of acceleration loading curves at the supports, which may also be different at each support so as to represent asynchronous ground excitation.
In this research work, the dynamic time history analysis has been employed to obtain more realistic results and reliable conclusions. Both material inelasticity and geometrical nonlinearity have been employed. In order to cover a wide range of the expected ground motions, three different ground motion records which they have different frequency contents (i.e. one of them has a High Frequency Content (HFC), the second has Medium Frequency Content (MFC), and the last has Low Frequency Content (LFC)) have been selected to perform the dynamic time history analysis. The basis used to classify ground motions according to their frequency content is introduced by Kwon and Elnashai [27]. The characteristics of the selected ground motions are shown in Table 4. All of them have been downloaded from the Pacific Earthquake Engineering Research Center (PEER, 2000) website [28].
By scaling each of the previous ground motions into (0.1g, 0.2g, 0.3g, 0.4g, and 0.5g), 15 artificial ground motions have been developed in order to test the structures under the different seismic hazard levels and to cover all the seismic zones. In addition, the acceleration time histories considered are presented in Figure 13.
Numerical Modeling
The frames have been modeled in SeismoStruct software [20] which is a Finite Element package capable of predicting the large displacement behavior of space frames under static or dynamic loading. Inelastic displacement-based frame elements divided into 200 fibers have been used for modeling beams and columns. A number of beam sections has been defined corresponding to beams ends and the beam mid-spans, with different reinforcement distribution in order to accurately model the reinforcement of the different regions of the concrete members. According to the Egyptian code provision [22], the effective width of slab was taken to be 0.95m for each span. Fixed supports have been used for the ground columns to represent the strong foundation. The rest of the nodes have been restrained in the out-of-plane degree of freedoms to reduce the problem to a two-dimensional structure.
Three types of performance criteria have been employed in the analyses. These three criteria are the yielding of the reinforcing steel representing slight damage state, the spalling of the concrete cover representing the moderate damage state, and the crushing of the core concrete representing the near collapse state. The used strain values for each criterion are presented in Table 5.
Numerical Results and Discussion
Pushover analysis results
The pushover analysis results were discussed in detail by Abdelaziz et al. [29]. The resulting capacity curves of the HRB, MRB, and LRB with and without the infill panels are presented in Figure 14 where the x-axis represents the roof drift ratio (The ratio of the roof displacement to the total height (”/H) (while the y-axis represents the ratio of the base shear to the total weight (P/W). The Figure indicates that, the presence of URM infill panels increases the
lateral stiffness and capacity of the RC frames despite the early failure of the masonry panels. Their presence has increased the initial stiffness of the bare frame by about 7.8, 9, and 17 times and the shear capacity by about 2.5, 2.8, and 2.5 for the cases of HRB, MRB, and LRB; respectively. The same result is also obtained in the case of POGS frames. It is notorious that the initial stiffness of the POGS is more than the BF’s stiffness by about 6.7, 7, and 7.5 times while its lateral capacity is more by about 2.5, 2.2, and 1.5 times for the cases of HRB, MRB, and LRB; respectively. For the case of the OGS, its initial stiffness is also more than the BF’s stiffness by about 5, 3.8, and 2.1 for the cases of HRB, MRB, and LRB; respectively. Its lateral capacity is more for only the cases of HRB and MRB by about 2.3 and 1.8 times, respectively. However, the shear capacity of the OGS is rather equal to the BF shear capacity but with low drift ratio.
The capacity of the partially or totally infilled frames has shown higher degradation when compared to the BF. The IF, OGS, and POGS have tended to act as a BF at later stages after the failure of the infill panels. At this stage, the infill panels are assumed to have completely collapsed.
The maximum inter-story drift profiles of the LRB under pushover analysis are presented in Figure 15. It should be noted that these drift profiles represent the envelopes of the peak drift ratios beyond the collapse state. These obtained plots illustrate the differences among the drift profiles of the building structure modeled as BF, IF, OGS, and POGS. The figure indicates that, the maximum value of inter-story drift for the BF has occurred around the middle stories. However, the maximum drift ratios for the IF, OGS, and POGS have occurred at the lower stories especially in the ground story in the cases of MRB and LRB. This can be due to the early cracking of the infill panels at the lower stories. It is worth to mention that slight difference has been observed between the drift profiles of the OGS, and POGS frames.
Dynamic time history analysis
Dynamic time history analysis predicts the real nonlinear behavior of the structures subjected to earthquake loading. Three real ground motion records have been selected in order to represent a wide range of frequency content. The records have been scaled to 0.1g, 0.2g, 0.3g, 0.4g, and 0.5g. The frames (BF, IF, OGS, and POGS) have been subjected to these fifteen records. The results are presented through the natural periods of the structures, the displacement response at the roof floor, the base shear and the maximum inter-story drift ratios.
Natural periods of the structures
Before presenting the results of the dynamic time history analysis, a modal analysis has been undertaken and the values of the periods of the first, second, and third modes have been obtained as listed in Table 6 in order to provide an initial prudence into the structures. For the four infill panel configurations of the structures, significant changes in the fundamental periods of the first three modes have been detected. The mode of vibration varied when taking the infill panels into consideration. The inclusion of the masonry panels led to reducing the fundamental period of the structures (i.e. increasing the frequency) due to the increased stiffness resulted from using the infill panels. As shown in Table 6, the period of the first mode for the BF has significantly decreased to around 65.1% to 76.1%, 25.3% to 55.7%, and 62% to 66.7% for the IF, the OGS frame, and the POGS frame; respectively. This can be due to the fact that the presence of the infill panels makes the structure have higher stiffness than the BF building model. The periods of the second and the third mode for the BF have also decreased due to the presence of the infill panels.
Maximum Inter-story drift
Plots for the maximum inter-story drift ratios are presented in Figure 16, Figure 17, and Figure 18. It is noted that the drift ratios profiles from the dynamic analysis represent the maximum inter-story drift ratios for each PGA, not actual profiles at a given instant of time or a given PGA. Differences have been observed among the drift ratios profiles of the structures. Taking the masonry infill panels into consideration has decreased the values of story drift ratio as compared to the bare frame. The infilled frames have less inter-story drift ratio at all floors than the bare ones. This may indicate the proper distribution of plasticity in the structure. The figure indicates that, the maximum value of inter-story drift ratio for the BF occurs around the middle stories. However, the maximum drift ratios for the OGS have a sudden increase at the ground story resulting in a soft story mechanism. Furthermore, the existence of a soft story at the ground story highly magnified the story drift ratio at that level with values mostly exceed those associated with the bare frame. This can be due to the fact that the OGS examines a sudden decrease in its lateral stiffness at its ground (lower) story. Due to the early cracking of the infill panels at the ground story, the maximum drift ratio for the IF and POGS mostly occurs at the ground level.
Seismic Vulnerability Assessment
Ground motions with low frequency contents were the most damaging compared to the high and the medium frequency ground motions. This can be due to the fact that low frequency earthquakes will cause resonance for structures with low natural frequencies (i.e. high natural periods) which are typically moderate height or tall buildings. However, medium frequency earthquakes cause more damage for the low rise structures due to their high frequencies (i.e. low natural periods).
The existence of soft story in the infilled frames has made the structures vulnerable to collapse especially in a moderate to severe earthquakes. On the other hand, the infilled frames have shown better performance for most of the earthquakes. These frames have been able to deform for a longer period without collapse. This can be beneficial as an alternative strengthening technique in existing old building in which no seismic design was followed. However, when the infill panels fail (in many cases at the ground story), the demands tends to largely concentrate at this story with little drift demands in the upper stories. Therefore, the demands in the RC elements in the ground story, after failure of the infill panels, may be larger. This was observed in the high rise structures during the low frequency earthquake.
Comparison between static pushover and dynamic analyses
A comparison has been implemented in order to show the differences between the outcomes of the static pushover and the dynamic analyses. The results obtained from the static pushover analysis have been compared with the drift ratios obtained from dynamic time history analysis for the HRB, MRB, and LRB in Figure 19, Figure 20, and Figure 21; respectively. For the HRB, the static pushover analysis could not predict the peak values of drift ratios. The prediction of the story at which the maximum inter-story drift ratio occurs is inadequate. Furthermore, the drift ratios of upper stories, obtained through static pushover analysis, are generally underestimated. This can be due to the fact that the conventional pushover analysis does not consider the higher modes contributions and progressive stiffness degradation which might affect the distribution of seismic story forces. As a result, maximum base shear is also underestimated, when compared to those obtained by dynamic analysis as shown in Figure 22. For the MRB, the static pushover analysis has failed to predict the peak values of inter-story drift ratios. Also, prediction of the maximum base shear is not accurate. However, the difference between the static and the dynamic analysis is less compared with the difference in the HRB. For the LRB, the static pushover analysis could predict the peak values of maximum drift ratios, for most cases, except for the case of the BF. However, the drift ratios of upper stories are under-estimated. Slight difference in the maximum base shear obtained through static pushover and dynamic analysis is observed. In other words, the static analysis could predict the maximum base shear for the LRB.
Conflict of Interest:
On behalf of all authors, the corresponding author states that there is no conflict of interest.
Conclusion
Followings are the salient conclusions obtained from all the cases studied:
The URM infill panels have a significant influence on the global performance of the RC framed structures. The performance of the bare frame under seismic loading does significantly vary with various infill panels’ configuration (i.e. partially or fully infilled).
The regular distribution of the masonry infill panels can significantly improve the seismic performance of reinforced concrete framed structures during earthquakes in
terms of lateral capacity, story drifts and displacement control despite the fact that failure of infill occurs in the early stages of the earthquake. Their presence makes the frames able to deform for a longer period without collapse. Furthermore, taking their interaction into consideration leads to reduce the story displacements and increase the lateral capacity as compared to the bare frame case.
Taking the interaction between the infill panels and structures can be beneficial as an alternative strengthening technique in existing old building in which no seismic design was followed.
The existence of soft story in the ground level due to omitting the infills leads to magnifying the drift ratios at that level with values mostly exceeding those values obtained considering the building structure modeled as a bare frame. Furthermore, the columns in this story are more vulnerable as the shear forces acting on columns are considerably higher than those associated with the bare frame.
Ground motions with low frequency contents are the most damaging compared to the high and the medium frequency ground motions for the most cases.
Static pushover analysis, in which the higher modes contributions are not considered, could not predict correctly the peak values of drift ratios, as well as, the maximum base shear, except for the case of the low rise structures.
The soft story irregularity due to omitting the infill panels in the ground story should be considered by the national Egyptian building codes. This can be by magnifying the shear by a factor of 2 to 3 in the story where the infill panels are removed.
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