An independent engineering report, commissioned by the Portland Bureau of Transportation (PBOT), concludes that wind conditions and decorative design likely caused a panel to come loose and fall from the Portland Aerial Tram on Dec. 4, 2018. The panel struck a pedestrian, who declined medical treatment at the scene.
The report, released today by PBOT and OHSU, determined that maintenance was not a factor in the incident. It also makes recommendations regarding operational changes and other safety improvements to prevent a similar incident from recurring.
Many of the recommended safety and operational updates and policies have already been implemented to ensure Tram safety and normal operations. This summer, additional design enhancements, including a robust customized bolt and tether system will be installed.
Report findings:
The Portland Aerial Tram’s design is one-of-a-kind. The underlying structure of the cabins are wrapped in decorative paneling that creates the sleek, modern shape that has made the cabins beloved Portland icons.
The Dec. 4, 2018 incident marked the first and only time any object has fallen from the Tram since it began operations in 2007. According to the engineering report, conducted by Ridgeline Engineering, the panel - one of four nonessential, visual design elements per cabin - dislodged when its latch and tether system failed.
Each panel is affixed to the cabin by eight metal latches and a secondary security system of carabiners and wire tethers designed to prevent displacement.
While maintenance reports indicate two previous latch malfunctions in the previous eleven years of Tram operations, the secondary system kept the panel attached to the Tram. The December incident was the first time one of the carabiners holding the panel to the cabin broke.
Ridgeline’s report notes the combination of the panel’s design and the specific speed and direction of the wind that day likely caused the secondary system to fail and the panel to come loose. Specifically, the wind traveling over a panel of this shape produces an uplift force, the report says. This uplift force causes the panel to flex upward, which in turn causes the sides of the panel to pull inward. As the sides move inward, the contact area between the latches and their attachment to the cabin goes down until the point where they completely lose contact and the panel is pulled from the cabin.
Air vents that provide ventilation to improve the comfort of Tram passengers can also contribute to the uplift force, the report says.
“While it is impossible to conclusively say why the panel was pulled from the cabin, based on the information that was observed during our visit, it seems that the most likely reason the panel came loose was from lack of stiffness of the panel,” the report says.
Maintenance procedures require the panels to be removed twice a month, so a “design failure, not inadequate maintenance” caused the panel to be too flexible.
Safety improvements made:
In accordance with the report’s recommendations, PBOT and OHSU have completed the following updates to the Tram’s safety tether and panel latch system and operational policies:
• Immediately following the incident on Dec. 4, Tram crews replaced the existing carabiners and wire tethers on all panels with enhanced versions.
• In sustained wind speeds higher than 20 mph, the tram will not operate with cabin roof air vents open. The tram will not operate in sustained winds of more than 30 mph.
Once the customized bolt and tether system is designed, engineered and installed in summer 2019, the Tram will operate according to the existing Operations Manual.
Source: Portland Bureau of Transportation