Seismic fragility assessment of high-rise tunnel-form buildings using a bespoke damage scale

Tunnel-form buildings represent the main typology used in mass housing projects in Türkiye, with their heights continually increasing to meet the rising demand for land. Research has shown that low- and mid-rise tunnel-form buildings performed satisfactorily even under earthquake ground motions exceeding their design intensity. However, only a limited number of studies examined the performance of high-rise tunnel-form buildings. These structures are characterised by inherent vulnerabilities due to the use of lightly reinforced slender shear walls and conventionally reinforced squat coupling beams as primary structural members. The present study numerically examines the seismic performance of such structures and offers recommendations to enhance their design. A 14-storey tunnel-form building, representative of a large percentage of mass housing projects across Istanbul, is selected for case study purposes. A state-of-the-art three-dimensional non-linear finite element model is created in OpenSeesPY. Prominent failure modes of the components are incorporated into the model. The modelling strategy is validated at the component level using experimental results and at the system level using the results of ambient vibration tests conducted on an existing building. Standard and multi-mode adaptive pushover analyses are used to provide insights into the evolution of damage and define a damage scale. The seismic performance is evaluated through a Multiple Stripe Analysis procedure, and fragility functions are derived at both component- and system-levels. The fragility analysis shows that high-rise tunnel-form buildings have a very high probability of providing life safety even in very rare, high-intensity earthquakes. However, immediate occupancy of the building is likely to be jeopardised due to the severity of the incurred damage. The study offers several insights into the seismic performance assessment of such structures and provides guidance on how to improve their design.