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Expert Insight

Vertical median barrier extension enhances aerodynamics

A Whitestone Bridge Study

Co-authored by T. Zoli, S. Stoyanoff, +

Gavin Daly Sr Bridge Engineer at McLaren Engineering Group
Gavin Daly, PE
Senior Bridge Engineer

Senior Bridge Engineer, Gavin Daly, PE presented his whitepaper, “Bronx-Whitestone Bridge: Vertical Median Barrier Extension Enhances Aerodynamics”, at the Bridge Engineering Association’s 2023 NYC Bridge Conference.

Gavin provided insights on the use of a vertical extension of an existing median barrier as an innovative, cost-effective solution to enhance the aerodynamic performance of existing cable-supported bridges. The extension can reduce the risk of instability issues such as vortex shedding or flutter during extreme wind events. His paper overviews the timeline of this retrofit project, from wind tunnel testing and conceptualization to detailed design, crashworthiness analysis, and construction, emphasizing the need to adapt new technologies to improve bridge resiliency and performance by exploring opportunities in the field of wind engineering.

Bridge Median Enhances
Aerodynamics

The project team found that adding a 3-foot solid screen on top of the median barrier disrupted vortex patterns and significantly increased the critical flutter speed. This led to the concept of the Median Barrier Extension (MBE), consisting of transparent acrylic panels attached to the median barrier posts with a tubular steel frame.

To ensure the MBE met safety standards, crash analysis studies using non-linear finite element models were conducted to iterate the design details. The final design then underwent full-scale physical crash testing at MASH Test Level 4. The project demonstrated that raising the median barrier by 3 feet significantly improved the bridge’s aerodynamic stability, surpassing modern design criteria. Construction of the Bronx-Whitestone Bridge MBE was completed in 2020.

Abstract

The Bronx-Whitestone Bridge was designed during the 1930s in an era of suspension bridges with decks stiffened by shallow plate girders, many of which were subsequently found to be vulnerable to aerodynamic instabilities such as vortex shedding and flutter. Following the occurrence of mild and benign wind-induced oscillations in the first several years after opening in 1939, the bridge has undergone a series of retrofits, from structural solutions such as stay cables, stiffening trusses, and a steel orthotropic deck, to aerodynamic enhancements such as a tuned mass damper and wind fairings. Wind tunnel studies in 2015 confirmed the improved aerodynamic performance due to the recently installed wind fairing system and stiffer orthotropic deck. A subsequent rehabilitation project gave the opportunity to assess measures to further improve the aerodynamic performance of the bridge. A 3ft tall solid screen added on top of the median barrier was found to act as an above-deck vertical baffle plate, disrupting the alternating pattern of vortices, reducing the susceptibility of the bridge to instabilities. This led to the conceptual design of a Median Barrier Extension (MBE) comprised of 3ft solid transparent acrylic panels fixed to the top of the existing median barrier posts, supported by a tubular steel frame. To ensure this unique barrier modification met current industry safety standards, the MBE design was iterated through a crash analysis study using non-linear finite element models before the final design proceeded to a full-scale physical crash testing program to MASH Test Level 4. This paper presents the full timeline of this innovative retrofit project, from conception during wind tunnel testing, through to design, crashworthiness studies and final construction in 2020. This project has demonstrated that a vertical extension to a median barrier can act as a simple and cost-effective enhancement to the aerodynamic performance of existing bridges.

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