Influence of Infill Masonry on a Building Frame Under Seismic Loadings and Its Hazards Vulnerability Assessment

Objectives/methodology: In this study, a four-story frame structure is modelled using finite element software in two different conditions of infill and without infill masonry effects. A pushover analysis is carried out to assess the seismic response and building performance under three different loading conditions of modal, triangular, and uniform loading. The all-possible loadings in negative and positive directions have been applied and building response is measured. A performance capacity curve in terms of base shear is developed for all possible loading scenarios. Finally, a comparison of bare and infill frame has been made and some conclusions were made. Findings/ application: Uniform loading among all three categories is found to be higher in capacity for both types of frames in positive and negative directions. The presence of non-structural masonry walls results in a better behavior of frames compared to bare frame. They initially increase the strength, stiffness, and energy dissipation of frames despite their brittle failure.


Introduction
Among all the natural disasters, Earthquake is considered as the most damaging to the ecological and building structures. Construction technologies must be advanced and modified to cope with the hazards of earthquake damages. It is observed that a linear design technique for construction buildings has been failed to the inelastic seismic responses of structures under massive earthquake actions and hence a traditional design approach is no more of importance for the long-term risk and benefits implications [1]. The basic concept of performance-based design is to construct the structure in such a way that should meet all the performance satisfaction under different ground motions and resist the seismic hazards as much as possible. This concept is not only limited to buildings but can apply for all the structures and their supported nonstructural elements as well.
In Ref. [2], Ravikumar concluded that several features like stiffness, lateral strength, ductility, and regularity define the behavior of a structure during a seismic activity. This is obvious that failure starts from the weaker points in any natural and un-natural hazard activities. These weaknesses may because of discontinuity in structural mass, stiffness, and geometry [3]. Structures with such discontinuities can be categorized as irregular structures. Irregularities are the most critical reason of failure under lateral loads of earthquakes [4]. In Ref. [5], Furtado et al. observed the impact of proving infill walls in a 15 story RC building and conclude in 20% increment in its story shear and base shear results. However, in most cases, the influence of infill walls may cause an extensive damage or collapse of structure [6].

Analysis of Frame Structure
Seismic Analysis is a basic tool of analysis in earthquake engineering used for understanding the response of a building under dynamic excitation [7]. In most of the building codes, equivalent linear static analysis is only recommended for regular and simple structures like small buildings and residential structures. For high-rise buildings, dynamic analyses of time history function and response spectra are suggested [7]. In a research [8], applied non-ductile infill walls at different story levels of 3, 6, and 9 stories in a 9 story building and compared its consequences with simple frames. He found that presence of infill wall results in a brittle failure at 9th story while in simple frame it is found at 3rd story, hence overall, the presence of infill walls increased the strength and reduced the seismic vulnerability of frame [8] a four story building frame is considered with a strong system of upper bound in fill masonry. In addition, a bare frame is analyzed to get the comparison of two models under seismic excitation. The geometrical features of studied building frame are shown in Figure 1. Both frames are considered here without soft story.
The loading pattern for this frame is shown in Figure 2. Loading is applied in a symmetrical manner but the span length is not same for both bays. The above load patters are applied in positive X (+X) and negative X (−X) directions as the frame is not symmetric. As the frame is 2D so the analysis is performed only in X Direction to calculate base shear manually, Euro code 8 and UBC 97 is utilized and results are as under, Base shear (Euro code) = 449.026 KN Base shear (UBC) = 460.697 KN SAP software is used for all type of analyses and hinges are assigned at beam column joints according to ASCE (seismic evaluation and retrofitting of existing buildings). All hinges are auto assigned using software and the infill is modelled as a diagonal strut which is the most common practice of infill modelling with the tension limit of zero [9][10][11]. The behavior for the simplicity is taken as of a diagonal compression strut. The properties of the strut are calculated and then manually defined.

Building Capacity Curve
The curves in Figure 6 show the capacity curve of the study building. The analysis is performed of +X and −X direction with Bare frame and Infill frame.
1) The figure clearly shows that the capacity of Triangular load case and Modal shows the somewhat approximately same trend but uniform load case shows somewhat high capacity.
2) The second conclusion is that the capacity of the infill frame is more as compared to the bare frame as in case of infill the infill act as a diagonal compression strut thus giving the building extra stiffness than bare frame. Tables 1 and 2 show the results of performance point analysis for bare and infill frame, respectively.  where, M+X is modal load in positive X direction M−X is modal load in negative X direction T+X is triangular load in positive X direction T−X is triangular load in negative X direction U+X is uniform load in positive X direction U−X is uniform load in negative X direction It can be seen from the tabulation (Tables 1 and 2) that uniform loading has a little larger impact on base shear while resulting in a smaller time of vibration. The sample curve for the SAP model result for performance point of U-X infill frame is shown here for understanding (see Figure 7, the intersection of orange and green curve)

Building Performance Points
The deflected mode shapes for all possible cases are generated from SAP software and are drawn for both bare and infill frames. Their results are displayed in Appendixes A and B, respectively. The performance categories are classified with different colors and labels as shown in Figure 8.

Conclusion
As from Appendixes A and B, it is clearly concluded that in bare frame almost all of the plastic hinges as well as structural elements yields under seismic actions except in the foundation level which lies in immediate occupancy category. In contrary, infill frame  1. The capacity analysis result in a conclusion that infill frame structure has more shear capacity in seismic loads compared to simple bare frames