Seismic performance of steel X-knee-braced frames equipped with shape memory alloy bars

Highlights

• Some structural properties like stiffness, ductility, strength and recentering with OpenSees.

• An approximate method of analysis is presented to control the strain of SMAs.

• 12 different diameters investigate the effect of various size for SMA bars.

• Using nonlinear pushover analysis, the state of plastic hinges in SMA bars, and other structural components studied.

• Excellent reversibility due to the use of superelastic elements in the frames.

Abstract

The utilization of smart materials in construction, to improve the seismic response of structures is one of the most interesting topics in earthquake engineering. Accordingly, this study aims to investigate the use of shape memory alloy (SMA) bars within X-knee-braced frames (X-KBFs). To fulfill the objectives of this research, three 3-, 5-, and 7-story frames are modeled in the OpenSees at first. Then, by placing these superelastic elements between the beam-column joints and knee members, some structural properties such as stiffness, ductility, strength, and recentering are examined against seismic loading. Moreover, by performing pushover analyses, the influence of SMA bars on the formation of plastic hinges is investigated. Considering 12 different diameters for SMA bars, a number of nonlinear time-history analyses are performed in the software framework of OpenSees; and, the results show that the employment of such members does not remarkably change the stiffness, ductility, and strength of structures. Contrary to these features, an interesting finding is that the permanent roof displacements of the structures are significantly reduced through these elements due to their excellent recentering capacity.

Introduction

Considering the fact that the earthquake loads naturally are random, according to the philosophy of performance-based seismic design of regular structures, the objective of seismic building codes and provisions is that a minor or moderate earthquake will not damage the structure and that a major earthquake will not produce collapse of the structure [1,2]. However, researchers in earthquake engineering are always looking for new ways to reduce structural damages under strong ground motions. The invention of dampers and stiffeners, for example, can be mentioned in this regard. In fact, the majority of these vibration control methods are intended to remain the constructions operational after occurring an intense earthquake [3].

Based on seismic codes and provisions, in general, a structure built in high-risk seismic zones must satisfy two main criteria. It must have sufficient stiffness capacity to control the interstory drift ratios (IDRs) to prevent any structural or non-structural damages during frequent but moderate earthquake excitations. Under intense earthquake event, the structure must have sufficient strength and ductility to prevent the collapse; although, limited non-structural and/or structural damages are permitted in this condition [4]. So, a building constructed in this approach is required to meet specific measurable or predictable performance requirements [5]. Strength, stiffness, and ductility are three principal demands in current seismic provisions to design of earthquake-resisting structures. Often, strength controls the amount of failure in the structural elements; while, stiffness is an important factor in the failure of non-structural components (e.g., walls in framed-structures), and ductility is essential to prevent the collapse [6,7]. Undoubtedly, each of the three mentioned features drastically is related to the materials properties, which are used in the structural elements. In other words, the values of yielding stress, modulus of elasticity, the ratio of ultimate strain to yield strain, and the value of residual strain in the stress-strain curve of a given material, play an important role in the seismic behavior of structures. So, the application of new materials in various structural elements always has been one of the most important issues in vibrational control methods to improve the seismic behavior of structures.

In this context, Shape Memory Alloys (SMAs), because of their unique properties (such as super-elastic behaviour), have been extensively utilized by researchers in a variety of applications in earthquake engineering such as: rebar in reinforcement of concrete structures [8], dampers for vibrational control [9,10], and base-isolation system [11], as well as, within beam-column joints in both of concrete [12] and steel [13] frames. In this regard, Song et al. [14] studied the applications of these novel materials for passive, active and semi-active controls of civil structures; also, Alam et al. comprehensively investigated the utilizing of these materials to enhance the performance of civil infrastructures [15].

Among all structural components, braces are one of the most important elements in a seismic-resistant system that have a suitable stiffness capacity compared other structural systems, which leads to a reduction in the interstory drift ratios of buildings. In this regard, to improve the remaining seismic capacities of braces (i.e., strength and ductility), many numerical and experimental research efforts have been conducted (see, for example [16,17]). Further, the application of SMAs for braced frames, due to their ability to undergo large deformations while reverting to their original undeformed shape, providing the recentering capability to these materials, has been a practical issue in earthquake engineering. This feature reduces the residual deformations, which may lead to a significant reduction in financial losses due to structural repairs after strong earthquakes. In this regard, McCormick et al. have been evaluating the seismic performance of three and six-story concentrically braced frames with super-elastic SMA braces. Their obtained results showed that the SMA braces are effective in limiting of IDRs and residual drifts during an earthquake [18]. The seismic response modification factor, R, in braced frames equipped with SMAs is also investigated by Ghaffarzadeh and Mansouri [19]. In a similar work, Ghassemieh and Kargarmoakhar studied the overstrength, ductility, and response modification factor of steel frames, by considering the effects of building height, number of spans, and different types of bracings [20]. Asgarian and Moradi also studied the seismic performance of various steel braced frames with different bracing configurations involving diagonal, split X, chevron (V and inverted V) braces that were equipped with SMAs. They reported that the implementing the SMA braces can lead to a reduction in residual roof displacement and peak inter-story drift compared to the buckling-restrained braces (BRBs) [21].

Hooshmand et al. worked on the location of SMAs in steel bracing systems. They suggested that the most suitable place for the shape memory alloy is at the ends of braces to obtain the best seismic performance level. Also, it was found that the secondary moment, which is produced in X-bracing at the connection, would be transmitted better by using the shape memory alloys within gusset plate [22]. In 2014, Omar investigated the seismic response of braced steel frames with shape memory alloy and mega bracing systems. In this comparative study, the use of SMA vs. Mega bracing was examined. They used some time-history nonlinear analyses through Seismostruct software, to evaluate the efficiency of these two seismic-resistant systems. The results indicated that both systems enhance the strength and stiffness of the original structure; although, due to the excellent behavior of SMAs in nonlinear phase and under compressive forces, the SMA system showed much better seismic performance [23]. In another study, the performance-based seismic design of self-centering steel frames with SMA braces is examined by Qiu and Zhu in 2016. They mainly focused on the variability in the hysteretic parameters of SMAs, like the phase-transformation stiffness ratio and the energy dissipation factor. Various seismic intensity levels evaluated the seismic performance of the designed frames. Their obtained results show that the considered SMA system can successfully achieve the prescribed performance objectives regarding three seismic hazard levels [24].

By general reviewing the great works has been done to improve the seismic performance of braced frames using SMAs, it can be seen that the application of these novel materials in knee-braced steel frames, which have proper seismic behavior as various references mention it (e.g. [25,26]), have received less attention in the literature. Consequently, this study aims to present a practical approach using superelastic SMA materials within knee-braced frames to enhance the seismic performance of low- and mid-rise steel buildings. To this end, some SMA bars (with different diameters) will be placed in the adjacent of beam-column connection and knee members to enhance the reversibility of the structures after extreme seismic loading. So, these superelastic elements likely would help the structures to be operational and/or reparable against strong earthquake loads [27].

Overall, the central emphasis of this research is on the increase of recentering behavior in X-KBFs by using the distinctive mechanical properties of SMA bars. Throughout this research, it is assumed that the SMA material is made from nickel and titanium (Ni-Ti or Nitinol), which is commonly utilized in practice [28]. As it is supposed in previous studies like [21,29], the SMA bars are also assumed to be buckling-restrained, and the type of SMA's compositions is well-chosen based on the ambient temperature of structure to exhibit the superelastic behavior.