Evaluation of Energy Dissiption in Shear Links of High Strength Steel EBFs, with 304 L Steel Links under Successive Earthquakes

Eccentrically braced frame (EBF) is known as a lateral force resisting system in steel structures which dissipate the earthquake energy through the links. The importance of this issue increases when the structure is exposed to successive earthquakes because in the seismic active zones, a large earthquake may consist of numerous successive shocks (foreshock or aftershock) which can lead to permanent displacements and resistance loss in these frames. In a seismic scenario, the most damage will often occur in the link beam – as the structural fuses – and the other members will remain in the elastic region, while the link beams may fail under successive earthquakes in most of the structures and other members such as adjacent beams, columns and braces, will behave non-linearly. This paper evaluates the energy dissiption of links in high strength frames with 304L series steel link beams in an area with a high seismicity risk under different critical scenarios with/without seismic sequence phenomenon. In these links, there will be no risk of corrosion. The highest energy dissipates in the Links of EBF frames, so by examining the energy dissiption in this member, it is possible to judge the energy dissiption of the entire frame. In this regard, 2D eccentrically braced frames with a variety of materials were designed based on the Iranian earthquake design code (Standard No. 2800, 4th version – 2014), modeled in Opensees software. For this purpose, "Force Beam-Column Element" has been selected for the implemension of beam and column elements. Brace elements are modeled using "Trust Element". Moreover, a behavior curve is defined with the Parallel material command to introduce the end elements of the link length. Also, the "Elastic-Perfectly Plastic Material" model has been assigned to the material model of the Parallel material. Link elements are implemented using an element with concentrated plasticity. To define the shear behavior of links, two zero length elements have been used, and the behavior curve of parallel materials has been defined for them based on the study of Eskandari and Vafai in 2015. Based on Hoveidai's proposal in 2019, the Ramberg-Osgood model has been used for the cyclic response of stainless steel materials in Opensees software and the Steel02 model has been used for the st37 and st52 materials. In the following, seismic scenarios with and without seismic sequence are selected based on effective peak acceleration (EPA) which has been suggesteb by Rajabi and Ghodrati Amiri in 2020. In order to nonlinear dynamic analysis of the studied eccentrically braced frames, single and successive earthquakes should be scaled based on the design spectrum. For this purpose, the suggested spectrums in Standard No. 2800 and proposed methodology by Abdollahzade in 2019 have been used, respectively. The results indicate that the amount of energy dissiption through links increases between 2 and 3 times after the consecutive earthquakes compared to the main shocks while after the seismic sequence, a lower average energy dissiption ratio has been reported for this frame. Also among the studied frames, the high-strength frame with stainless steel links has a higher energy loss of 10.4% compared to normal frames and 31.6% higher than frames with stainless links under seismic sequence. Therefore, the using of this type of material can be suggested for all or some members of normal eccentrically braced frames, especially in active zones with high seismic risk.