Downbursts, referring to downward high intensity winds associated with thunderstorms, pose a major threat to power transmission grids in many parts of the United States. Considering the distributed size of the power transmission infrastructure, significant investments will be needed in order to upgrade the existing systems and properly design new infrastructure to resist downburst wind forces. The goal of this research is to develop an integrative experimental and numerical framework that is capable of characterizing the extent of vulnerability of the power transmission infrastructure against downbursts, and identify the most critical components. The framework will provide knowledge that can help reduce outage-induced societal disruptions caused by downbursts, and thus enable continued national prosperity and welfare following a downburst event. Using this framework, various significant factors for the downburst performance of transmission systems and potential causes of past failures will be investigated. Moreover, this research will develop the first generation of downburst fragility models for transmission tower-line systems (TLSs) using experimentally validated numerical models. These fragility models are crucial for risk-informed decision making for design and management of the transmission grid in order to mitigate future failures and enhance the resiliency of the grid against extreme weather events. The numerical and experimental studies will provide the knowledge needed to enhance design methodologies to include downburst wind loads for transmission line systems. The research findings will be integrated into undergraduate and graduate courses at Florida International University (FIU) and The Ohio State University to better prepare the future generation of infrastructure engineers. Project data will be archived and publicly shared in the NSF-supported Natural Hazards Engineering Research Infrastructure (NHERI) Data Depot (https://www.DesignSafe-ci.org/). This research will develop an integrative experimental and numerical framework to characterize the fragility of transmission line systems against downbursts. The experimental research will involve developing a versatile downburst simulator at the NSF-supported NHERI Wall of Wind (WOW) Experimental Facility (EF) at FIU. Using this simulator, a scaled, aeroelastic multi-span TLS will be tested; the results will be used to experimentally validate high-fidelity finite element models of coupled transmission tower-insulator-conductor-foundation systems. Through these experimental and numerical investigations, new knowledge will be gained with regard to the aerodynamic behavior of conductors and drag and shielding effects on lattice tower sections under non-synoptic downburst wind fields. Moreover, these investigations will reveal extreme nonlinear behaviors of transmission line systems in post-elastic regimes when towers are damaged or conductors fail. The research also will provide new physics-based insights into the role of uncertainties in the downburst performance of TLSs. Failure modes that are unique to or are more likely to occur under non-synoptic downburst loadings, as compared to those under synoptic hurricane loadings, will be identified and characterized. The produced data and models will be integrated to develop the first generation of multi-dimensional demand and fragility surfaces for TLSs under downbursts at component- and system-levels using highly efficient and accurate machine learning techniques. In addition, the experimental downburst simulator at the NHERI WOW EF at FIU will provide the natural hazards community with a unique testbed with dual simulation capabilities to generate both non-synoptic and synoptic winds and analyze the impacts on buildings and other structural systems.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.