Comparative seismic fragility assessment of mid-rise steel buildings with non-buckling (BRB and SMA) braced frames and self-centering energy-absorbing dual rocking core system

Highlights

• SMABF and SEDRC system can achieve comparable capacity on controlling the maximum inter-story drifts compared to BRBF.

• SEDRC system shows better collapse-prevention capacity than SMABF and BRBF under far-field and near-fault ground motions.

• SMABF and EDRC system show much smaller residual inter-story drift than BRBF under far-field and near-fault ground motions.

• SEDRC system shows comparable maximum floor acceleration responses compared to BRBF, while SMABF shows much higher ones.

Abstract

During the past two decades, many emerging smart lateral force-resisting systems have been proposed for achieving better post-earthquake recoverability than the conventional ones in steel buildings. The Self-centering Energy-absorbing Dual Rocking Core (SEDRC) system has been developed recently by the authors for achieving excellent self-centering behavior with large deformability in steel buildings. This paper intends to investigate the benefit of steel buildings using the SEDRC system compared to that using other emerging smart lateral force-resisting system (e.g., shape memory alloy braced frame (SMABF)) and conventional one (e.g., buckling restrained braced frame (BRBF)). For this purpose, first, three six-story steel buildings with BRBFs, SMABFs, and SEDRC systems, were designed to obtain similar inter-story drifts subjected to the design basis earthquakes. Second, nonlinear static and dynamic analyses were conducted to preliminarily study the seismic performance of the designed buildings. Then, incremental dynamic analyses (IDAs) with far-field and near-fault ground motion inputs were conducted to study the seismic responses of the designed buildings under different seismic hazard levels. Finally, the results from the IDAs were analyzed in a probability framework. The inter-story drifts, residual inter-story drifts, and floor acceleration responses of the considered systems were analyzed to investigate the seismic fragility of the structural and nonstructural components in the designed buildings. The analysis results indicate that the SEDRC system can achieve the best structural collapse-resistant capacity under both far-field and near-fault ground motions because of the excellent deformability. The SMABF and SEDRC system show better seismic resilience performance by controlling the residual inter-story drift responses than BRBF. Moreover, the SEDRC system can obtain a smaller exceedance probability of damage states associated with nonstructural components depending on floor acceleration responses than SMABF because of uniform inter-story drift responses.

Introduction

Steel concentrically braced frames (CBFs) are widely adopted as seismic-resistant systems because of sufficient stiffness for limiting the structural inter-story drift. However, the past investigations [1,2] show that CBFs may collapse with limited ductility under earthquakes because of the brace buckling failure subjected to compression and brace fracture caused by low-cycle fatigue. Thus, the buckling-restrained braces (BRBs) were introduced in CBFs to develop buckling-restrained braced frames (BRBFs) to achieve desired ductility and stable energy-dissipation. The BRBFs designed with the proper method can successfully achieve life safety or collapse prevention performance objective but lead to considerable residual inter-story drifts (RID) under rare seismic events [3]. The investigation conducted by McCormick et al. [4] shows that it is uneconomic and difficult to repair the buildings with a maximum RID larger than 0.5% after earthquakes. During the past 2011 Christchurch earthquake [5], hundreds of buildings had been demolished because of unacceptable repair costs caused by large RID.

Self-centering technology has been considered as an effective solution to reduce the RID of building structures under rare seismic events. The self-centering structural systems can show a flag-shaped hysteretic behavior with small or nearly zero RID, which can be achieved via various methods. Shape memory alloy (SMA) and post-tensioned tendons (PT) are the two most popular self-centering technologies, which have been investigated by researchers in recent years. The shape memory effect and the superelastic effect are the two main characters of SMA [6]. Based on the shape memory effect, the SMA can recover the deformation through heating. Based on the superelastic effect, the SMA can recover the deformation via unloading, which is similar to the PT technology. Many self-centering braces (SCB) using the PT technology [7] or the superelastic effect of SMA [[8], [9], [10], [11], [12]] have been proposed to enhance the seismic resilience of CBFs. Compared to the SCB with PT technology, the SCB with SMA can achieve larger deformability because the superelastic strain of SMA is up to about 8% - 10% [6]. The past shaking table test [13] and numerical studies [14] show that the CBFs with SMA-based SCB (denoted as SMABFs) can achieve stable self-centering hysteretic behavior and desired seismic performance objectives under earthquakes. The performance-based design procedure has also been proposed for SMABFs [15,16]. Moreover, Qiu and Zhu [17] showed that the SMABFs with the flag-shaped hysteretic behavior might suffer from larger higher-mode effects than BRBFs under seismic events, which will lead to the inter-story drift concentration at the upper stories of SMABFs. The severe inter-story drift concentration at a single story may cause the soft-story failure of braced frames under strong earthquakes while the other stories remain essential elastic [18].

The Self-centering Energy-absorbing Dual Rocking Core system (denoted as SEDRC system) is another emerging smart structural system proposed by the authors [19] for seismic resilience based on the recent rocking structure technology [2,[20], [21], [22], [23], [24], [25], [26], [27], [28]] and self-centering technology [[29], [30], [31], [32], [33]]. Fig .1(a) shows the sketch of the SEDRC systems. The rocking core (RC) and the shear friction spring damper (SFSD) are two major parts included in the SEDRC systems. The SFSDs provide the self-centering mechanism and the main energy-dissipation capacity for the SEDRC systems. The two RCs with pin bases can be steel braced frames, moment-resisting frames, concrete walls, and steel trusses, which have sufficient strength and stiffness to avoid inter-story drift concentration of buildings. The two RCs can also act as the backup energy-dissipation capacity after the bearing action development of SFSDs. Here, the two RCs are considered as two BRBFs with pin bases. The idealized monotonous nonlinear behavior of the SEDRC system is shown in Fig. 1(c). The past experimental investigation [19] indicates that the SEDRC system can achieve stable self-centering hysteretic behavior before the bearing action development of SFSDs.

This paper intends to investigate the benefit of steel buildings using the SEDRC system compared to that using BRBF and SMABF. For this purpose, this paper focuses on the comparative fragility assessment of BRBFs, SMABFs, and SEDRC systems in a probabilistic framework under far-field and near-fault ground motions. First, a six-story BRBF designed in the past investigations [3,34] is revisited as the demonstration building. The six-story SMABF and SEDRC system are designed through the displacement-based design procedure to obtain similar maximum inter-story drifts to that of BRBF under design basis earthquake (DBE). Second, nonlinear static and dynamic analyses are conducted to study the seismic performance of considered systems. Then, the seismic fragility of the structural and main nonstructural members included in the considered systems are assessed based on the results from incremental dynamic analyses (IDAs) in a probabilistic framework with the consideration of far-field and near-fault ground motions. Finally, some conclusions are summarized based on the analysis results.