Reducing Earthquake Hazard of Existing Steel Buildings Using Innovative C-TnT Braces

Abstract

The study focuses on seismic hazard reduction in existing steel braced framed buildings, particularly Special Concentrically Braced Frames (SCBFs), which resist lateral forces in during earthquake events. Traditional brace design has evolved to incorporate ductility requirements, but previous research has often overlooked the structural implications of brace fractures. This study explores the interaction between brace fracture and beam yielding, highlighting how brace strength loss can lead to beam failures and potential structural collapse, and proposes a novel seismic retrofitting strategy using channel encased tube-in-tube (C-TnT) braces in order to reduce seismic collapse hazards. Initial investigations reveal that brace fractures significantly affect the flexural behavior of beams, particularly at brace-beam intersections. A typical SCBF is evaluated with different beam strengths, and results show that merely increasing beam strength does not prevent brace fracture, and maximum story drift ratios are observed with over 4% or even 6%, posing high risk on seismic collapse. Thus, this study proposes a novel, low-cost retrofitting strategy using channel-encased tube-in-tube brace sections for retrofitting the existing SCBFs designed before modern seismic specifications. Numerical simulations indicate that C-TnT braces significantly improve structural performance, reducing brace ductility demands by more than 100% compared to traditional braces. This reduction prevents brace buckling, ensuring a uniform distribution of ductility demand and nearly eliminating flexural demand on brace-intersected beams. Even with beams designed considering gravity loads only, using C-TnT braces can reduce maximum story drift by more than 100% and material costs by 45% compared to common retrofitting strategies, offering a cost-effective and efficient solution. In conclusion, while increasing beam strength might delay brace fractures, it is impractical for existing braced frames and does not prevent brace fractures. The introduction of C-TnT braces substantially improves seismic performance, providing a viable retrofitting option that enhances the resilience of existing SCBF structures. © 2025 Elsevier B.V., All rights reserved.