Design and seismic collapse risk assessment of tall steel buildings with outriggers

Abstract

The growing urbanisation has driven the widespread construction of tall steel
buildings globally, necessitating a robust lateral load resistance system capable
of withstanding seismic events. The Hybrid System (HS), which integrates a
mega-braced core with Outrigger Truss (OT) and Belt Truss (BT) systems, has
emerged as one of the most prominent structural configurations for these
buildings due to its effectiveness in distributing lateral forces. However, current
seismic design methodologies for tall steel buildings with HSs demonstrate
certain limitations in predicting dynamic response and capturing the interaction
between structural elements.
This study addresses these gaps through a comprehensive evaluation of
Eurocode seismic design approaches for tall steel buildings with HS using
advanced nonlinear numerical modelling and analysis techniques. The research
is structured through several interconnected work packages beginning with a
literature review to evaluate current design and analysis practices and investigate
their advantages and limitations. It examined current seismic design standards,
response evaluation techniques, and comparative studies between design
methodologies. The literature review identified limitations in Eurocode-prescribed
approaches compared to responses from Nonlinear Time History Analysis,
particularly regarding dynamic response characteristics and collapse resistance.
Key findings established the theoretical foundation for subsequent analytical
investigations.
The methodology section integrates design and analysis approaches to ensure a
comprehensive evaluation of structural performance under varying seismic
intensities. Fatigue properties have a significant impact on the seismic response,
and the validation studies chapter presents the development of an empirical
equation to estimate fatigue properties of steel members, thereby improving the
accuracy of the material model used in the study. Fatigue parameters of twenty
nonlinear numerical models of brace specimens were calibrated against
experimental data from the literature and compared with established benchmarks
to ensure the accuracy of the material model. Based on improved material
models, comprehensive validations were performed on concentrically braced frames to verify the accuracy and reliability of analysis models through
comparison with experimental data.
The investigation developed nonlinear numerical models of 120 to 180-meter
building prototypes and employed sophisticated analytical approaches, including
Nonlinear Time History Analysis (NLTHA), Incremental Dynamic Analysis (IDA),
and Nonlinear Static Pushover Analysis (NLSPA), to evaluate the structural
response under selected seismic events representing various hazard levels.
Collapse risk assessment is carried out by developing fragility curves based on
IDA results and employing a rigorous evaluation following FEMA P695
guidelines. By applying FEMA P695 alongside Eurocode-based design
procedure, this study ensures a more robust, holistic, and credible validation of
the proposed methodology, supporting its practical implementation and future
standardisation in both European and international contexts. This approach
enabled the identification of potential collapse probability and provided a
quantitative measure of structural reliability under increasing seismic intensity.
Through the Seismic Design and Response Evaluation of Prototype Buildings
Chapter, the study systematically evaluated the adequacy of current Eurocode
seismic design methodologies, particularly examining their ability to accurately
capture ductility demand and dynamic behaviour imposed by seismic loading on
tall steel buildings with HS configurations. It revealed deficiencies in current
design practices, particularly the dynamic amplification effects inherent in tall
steel buildings with HS under seismic loads, while simultaneously requiring
extensive and time-consuming nonlinear analysis to satisfy all design
requirements. To address fundamental limitations, this research developed
Dynamic Amplification Factors (DAF) for buildings ranging from 120 to 180
meters in height through parametric analysis and validation against FEMA P695
collapse prevention criteria. An equation is developed to calculate DAF values,
and DAF is proposed as a supplementary factor to be integrated within the
Eurocode framework rather than replacing standard design principles, effectively
bridging the gap between simplified design procedures and actual ductility
demands experienced by HS configuration.
A systematic comparison was conducted between Eurocode-based and DAFenhanced building designs across multiple performance metrics in the Comparative Assessment of Eurocode and DAF Design Approaches Chapter.
The results indicate that the DAF-based design consistently exhibits improved
performance by effectively controlling drift demands, whereas the Eurocodebased design shows instances of performance limit exceedance, highlighting the
benefits of incorporating the DAF for enhanced seismic resilience. A seismic
collapse risk assessment was conducted following the FEMA P-695
methodology, with results detailed in the Seismic Collapse Risk Assessment
chapter. The analysis indicates that structures designed using the DAF satisfy
performance limits corresponding to ACMR 15% - 20%, demonstrating adequate
collapse resistance. In contrast, Eurocode-based design fails to achieve these
targets, indicating the effectiveness of the DAF in improving structural reliability
under extreme seismic events.
The research findings demonstrate improved accuracy of DAF in predicting
seismic response while maintaining practical applicability for engineering
practice. For structural engineers, the proposed methodology offers significant
advantages, including reduced analysis time and simplified assessment
procedures that maintain compliance with current design standards while
improving safety margins. The nonlinear modelling and analysis techniques
proposed in this research contribute substantially to academic knowledge by
providing a validated computational framework and refined material modelling
techniques that can be applied to similar structural systems and extended to other
research applications. The dual impact of this research addresses the immediate
industry need for improved design efficiency while simultaneously advancing the
theoretical understanding and nonlinear numerical modelling, analysis, and
collapse risk assessment of this structural system within the academic field.

Date of Award	15 Oct 2025
Original language	English
Awarding Institution	Teesside University
Supervisor	Christoforos Dimopoulos (Supervisor) & Zubair Syed (Supervisor)