Kurtulus Atasever

Substandard reinforced concrete (RC) building stocks continue to pose a major seismic risk in earthquake-prone regions. While fiber-reinforced polymer (FRP) jacketing has been widely demonstrated as an effective retrofitting technique at the member level, its rational application at a large scale requires rapid yet reliable decision-making tools. In this study, the accuracy and applicability of a rapid FRP retrofitting framework integrated into the PERA2019 performance-based seismic assessment methodology are evaluated through a detailed investigation of five actual representative existing RC buildings. For each building, seismic performance before and after FRP retrofitting was evaluated using both the PERA2019 procedure and detailed nonlinear pushover analyses, which served as benchmark solutions. In addition, the framework was validated against the results of a full-scale experimental study reported in the literature. The results indicate that PERA2019 provides seismic safety ratio estimates that are in satisfactory agreement with detailed analysis outcomes. Buildings classified as A class by PERA2019 were consistently shown to satisfy the corresponding controlled damage performance target in detailed analyses, whereas buildings failing to reach A class did not meet this target. A comparison with the experimental results further demonstrated that the framework accurately captured the relative performance improvement achieved through FRP retrofitting and remained on the safe side of the observed structural response. Overall, the findings confirm that the proposed methodology can reliably distinguish buildings that can be effectively upgraded using FRP jacketing from those requiring alternative interventions. The framework provides a computationally efficient and practically applicable tool for large-scale seismic risk mitigation and prioritization of FRP retrofitting strategies.

The seismic assessment of existing building stock, particularly in megacities such as Istanbul, is constrained by insufficient data and the time-intensive nature of data collection. This bottleneck persists despite the prevalence of aging structures that no longer comply with current seismic design codes. This paper proposes a mobile application–based framework for collecting building data to improve seismic performance and strengthen public awareness. The framework comprises six sequential steps—seismic hazard and casualties, building risk prioritization, damage evaluation, rapid/preliminary assessment, detailed assessment, and project planning—and provides progressively more accurate information as more data are collected. A mobile application was developed to test the framework and has been downloaded more than 8,000 times. Across Steps 1–5, a total of 3381, 393, 2, 5 and 2 reports were generated. Application of the methodology to sample buildings showed that 87% of the structures fell within the expected performance range, while the remainder exhibited better-than-predicted performance.

The 2023 Kahramanmaras earthquakes highlighted the urgency of assessing and intervening in old, seismically deficient buildings. However, not all old buildings collapse during earthquakes despite noncompliance with current codes. Prioritizing buildings based on seismic risk offers a reasonable approach. This study analyzed 17 242 1–10 story pre-2000 RC buildings in Kahramanmaras, considering factors such as soil conditions, number of stories, construction year, fault proximity, and seismic demand, and their effects on structural damage distribution. Risk class distributions of these buildings are estimated by a pre-earthquake regional seismic risk estimation methodology developed based on the outcomes of performance-based rapid seismic risk assessment of pre-2000 buildings in Istanbul using PERA2019 methodology, and compared with actual earthquake damage. Results showed a strong correlation between “very high risk” buildings and those heavily damaged or collapsed. Finally, based on the results, a regional-scale prioritization strategy was proposed to efficiently identify high-risk buildings prone to excessive damage.

Summary: There has been an increasing interest in using residual deformation as a seismic performance indicator for earthquake resistant building design. Self‐centering braced structural systems are viable candidates for minimizing residual deformations following a major earthquake. Hence, this study proposes an alternative type of buckling restrained brace (BRB) with externally attached posttensioned (PT‐BRB) carbon fiber composite cables (CFCCs). The steel core of the brace is used as an energy dissipator, whereas the CFCCs provide the self‐centering force for minimizing residual story drifts. Three proof‐of‐concept specimens are designed, fabricated, and cyclically tested at different posttensioning force levels. The CFCC behavior to obtain cyclic response, including the anchorage system, is examined closely. A parametric study is also conducted to show the effect of the different configurations of PT‐BRBs on the inelastic response. Furthermore, optimal brace parameters are discussed to realize design recommendations. The results indicated that the implementation of partially self‐centering BRBs in building frames can lead to the target residual displacements. A stable behavior is obtained for the proposed PT‐BRBs when subjected to the loading protocol specified in the American Institute of Steel Construction (AISC) 2016 Seismic Provisions.

Buckling-restrained braces (BRBs) pose predictable and stable hysteretic behavior with excellent energy dissipation capacity. However, when such BRBs are used in steel frames with pinned connections, damage concentration and large residual deformations may occur due to their low post-yield stiffness. In overseas, post-tensioned cables are added on the BRBs to provide the self-centering force and eliminate residual deformations following a strong earthquake. Hysteretic curves of BRBs with post-tensioned cables can be defined with two parameters, namely post-yield stiffness ratio (α) and energy dissipation ratio (β). Previous studies have aimed to change β from the bilinear (β=2.0) to flag-shaped (β=1.0) hysteresis to have zero residual deformations. However, the flag-shaped behavior may require a significant post-tensioning force due to strain and compressive hardening. As such, limited energy dissipation capacity increase the peak displacement responses. Although previous numerical studies have found that residual deformations are small enough even hysteresis is not flag-shaped, an optimal range of α and β is not well studied. A newly developed BRB named as PT-BRB is introduced in this paper to provide self-centering force by adding post-tensioned cables while the BRB core dissipates input energy. Carbon Fiber Composite Cables (CFCCs) are used as post-tensioned cables and performance of the brace is evaluated experimentally. Section 2 presents the developed PT-BRB configuration and mechanism, and discusses the appropriate α and β ranges by conducting some numerical analysis. In Section 3, a 1/3 scale specimen of the PT-BRBs are actually manufactured, and cyclic loading tests with various post-tensioning forces are performed to confirm the hysteretic properties and deformation capacities. A numerical model that reproduces the behavior of each part of PT-BRB for application to the 3D model is proposed in Section 4. It is found that numerical values agree very well with the experimental results. Note this paper presents the results of a joint research between Japan and Turkey, and the contents conform to the US standards and design guidelines. In summary, the following results were obtained: 1) In the SDOF model, the increase of α is effective in reducing residual deformations in the range of α<10%, but the slope of decrease is small when α is higher than 10%. Peak acceleration and brace axial force increases when α increases. Additionally, when the energy dissipation ratio β is decreased from β=2.0 to β=1.5, the residual deformations decrease significantly. 2) In the 6-story MDOF model, it was shown that partially self-centering (α=5.6%, β=1.8) can minimize the residual deformation even though the maximum deformation and brace axial force are the same as bilinear case. It was also confirmed that a uniform story drift distribution can be expected by increasing the restoring force for this model. 3) Partially self-centering behaviors (1.0<β<2.0) are obtained from the cyclic loading tests and, all PT-BRBs showed stable hysteresis. PT-BRB-16 with the smallest post-tensioning force reached up to 3% story drift. In all specimens, CFCCs remained elastic until the core plate has fractured. 4) A numerical model of PT-BRB is constructed by connecting the Core, Inner Tube, Outer Tube and CFCC as individual element with contact element. This model accurately captures the experimental results and track the actual hysteretic behavior.

Conventional approach to earthquake resistant building design relies upon strength, stiffness, and inelastic deformation capacity, which are great enough to withstand a given level of design earthquake effects. However, modern approach in today's designs aims to mitigate seismic energy before the input energy reaches the structural elements. Dissipating seismic energy through the inelastic deformation of metallic dampers is one of the cost effective solutions. After appearing concept of dissipating energy through the inelastic deformation (mainly yielding and post yielding) of metallic dampers, numerous types of metallic dampers, such as X‐shaped, J‐shaped, U‐shaped, shear panel, triangular plate dampers have been developed and their effectivenesses have been proved both theoretically and experimentally. Inelastic hysteretic behaviour of U‐shaped devices is somewhat complex and varies with geometry of the damper. To address this issue and possibly attain some practical results, this present paper focuses on modelling hysteretic behaviour of U‐shaped dampers. ABAQUS has been used as the computational tool in which a finite element model made of the C3DR8 solid element is adopted. Nonlinear kinematic and isotropic hardening material assumptions are considered to determine cyclic behaviour at 0º, 45º and 90º loading directions. For each analysis, effective stiffness, effective damping ratio, and maximum reaction forces (i.e. horizontal strengths) of the damper are calculated to evaluate performance. As an additional parameter, two different loading protocols are taken into consideration. Hysteretic curves and deformed shapes of a selected damper type taken from an existing experimental work and this numerical study show very good agreement proving that the modelling assumptions made during the analyses are appropriate and sufficient for a better prediction of behaviour. Additional numerical analyses on several dampers under various loading protocols reveal that effective damping ratio of the damper is more than ξ_eff =40% at the maximum displacement level due to significant energy dissipation through plastic deformation of the damper. A numerical example of mixed use of U‐shaped steel dampers and rubber isolation bearings on a selected two story steel framed building is also discussed.