The Optimal Percentage of Shape Memory Alloy in Steel Structures Equipped with Buckling Restrained Brace (BRB) under Successive Earthquakes

Document Type : Original Research

Authors
D. Saberi
E. Rajabi
Department of Civil Engineering, Tafresh Univesity, 39518-79611 Tafresh, Iran
10.22034/25.2.21
Abstract
In the seismic active areas, strong ground motions usually consist of the numerous successive shocks (Foreshock-mainshock or mainshock-aftershock), which have the significant potential to increase the structural response and cumulative damage. This phenomenon (as called seismic sequence) can affect on the behavior of structures, control the seismic performace of buildings. Multiple earthquakes which have been recorded in all parts of the world are proven that the structures located in the mentioned areas are not only experienced a single event, but also they withstand a series of shocks. Due to the high importance of consecutive earthquakes, application of buckling restrained braces (BRB) and shape memory alloy (SMA) materials as smart materials in engineering sciences in the past decades, this paper tries to evaluate the seismic performance of steel frames equipped with buckling restrained brace by determination of the optimal percentage of shape memory alloy under successive earthquakes. Because SMA has unique advantages and characteristics such as no need to replace after an earthquake, high resistance to corrosion and fatigue, the ability to absorb high energy, the ability to return to the original state by applying temperature, tolerating strain up to about 10% without leaving residual strain after an earthquake, and tolerating multiple cycles of loading and unloading, various applications can be found separately and combined in controlling the behavior of structures. It should be noted that despite the high damage potential of successive earthquakes, they are neglected in the seismic codes and design earthquake is still proposed without successive shocks.

Hence, the acceptance of new methods for improving the seismic performance of structures under consecutive shocks seems necessary by the engineering community. Therefore, in this regard, 4 and 7 story steel frames with diagonal buckling restrained braces representing short and mid rise structures were designed based on Iranian codes in ETABS software and then implemented in OpenSees software. After selecting the reference model, the performance of the studied models is verified for the linear and non-linear region through comparison of periods and pushover curve of reference and implemented model. In the following, different percentages of shape memory alloys including 20, 40, 60, 80 and 100% for the 4 story steel frames and 5, 10, 15, 20 and 25% for 7 story steel structure has been considered. The studied models are analyzed with/without shape memory alloys under seismic scenarios with and without seismic sequence in Opensees software. For this purpose, critical successive shocks are selected based on effective peak acceleration (EPA) from PEER center. For compatibility aspects between the seismic analysis and seismic design, the selected records should be scaled by designing spectrum for each fundamental period of studied structure in order to have identical spectral acceleration. The results of nonlinear dynamic analysis show that with the increase in the percentage of shape memory alloy in the 4 story steel frame, the response ratio of steel frames under single and consecutive earthquakes increased, but in the 7 story steel frame, it almost decreased, and this reduction is better felt in the higher stories under the single earthquake. Finally, the optimal percentage of shape memory alloy among the selected percentages in the present study is suggested to be 20% for 4 story steel frame and 15% for 7 story steel frame.

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Modares Civil Engineering journal
Volume 25, Issue 2 - Serial Number 82
May and June 2025
Pages 21-34