High wind has caused deadly consequences for gondolas. This #seismicsaturday we tell the story of the Squaw Valley Tram Car failure.
On April 15, 1978, a blizzard with gale force winds caused the tram car (pictured above), which was heading down the mountain, to unlatch from the first of it’s support “track” cables. Track cables weight of the cable car (which rolls on wheels along them) while a propulsion cable pulls the car along. After detaching from the track cable, the gondola plummeted 75 feet, and then shot back up when the other track cable tightened. As the car bounced back up, the first track cable swung down, slicing through the roof of the cabin and pinning 12 people to the floor. Four of those people tragically died. The rescue operation of the other 40 people on the cable car would continue for 10 hours into the night amidst a blizzard of 60 mph winds (See Frohlich 2008). Figure 1 and 2 show the effects of the cable slicing through the car.
Why did the first cable derail? There is no clear answer – one passenger reported “swinging” and “twisting” of the car in the wind just before the accident, but the clamp should have been been able to handle this movement. Dr. Karl Bittner, a leading expert in cable cars, was called in to investigate the cause of one cable unlatching, but couldn’t figure out the specific cause. Following a State Investigation, the district attorney and Cal-OSHA officially declared the accident ‘an act of god’ meaning that it was due to unknown causes and could not have been prevented with “reasonable” foresight (Renda 2015).
It is likely that something went wrong in the clamping mechanism, causing one of the two cables to detach. The Squaw Valley Gondola was a multi-cable detachable grip system (see fig. 4). In this system, one cable is for propulsion, while the other cables are called “track” cables and are rolled on with wheels to provide stability and take vertical weight. The Gondola can detach from the cable mechanism so that, upon boarding of the gondola in the station, the car can moves slowly. These detachable systems are essential in maintaining a high capacity of modern gondola systems, as the propulsion cable doesn’t have to stop or slow down every time a gondola car enters the station to pick off or drop off passengers. The detachable clamp is designed to only detach in the station when subjected to a special un-clamping mechanism. A benefit of multi-cable systems is they have better stability than their single cable counterparts (where the single cable is the propulsion cable) due to their added stabilizing track cables, and therefore can reach higher speeds (See “Bicable Detachable Gondolas”).
Although the specific cause of the track cable derail was never determined, engineers tried to learn from what they think may have been the cause of failure to build the next cable car better. Dr. Bittner, in his inconclusive investigation, recommended that the new Squaw Valley cable cars system be built with improvements including deeper grooves on the cable hanger, a longer cable guard, and cable clamps on the support towers. These improvements were implemented when the new Squaw cable car system went into effect the next winter season in December of 1978 (Frohlich 2008). Squaw Valley also now conducts daily inspections of the brakes, the counterweights, and other machinery. A new Squaw Valley cable car is shown in fig. 4, with the glistening lake Tahoe in the background. Modern cable car hangers, like the Leitner 3S Carriage, feature innovative lateral damping system to improve wind resistance, a modern system of shock absorption, and an automatic shutdown which immediately notifies the operator when defects in the rolling mechanism are detected.
Note: in my research for this post, I was unable to find any technical report detailing the old clamping mechanism of the Squaw Valley Gondola, or any other technical report on the failure. If you find one, can you please share it below? I wonder if anyone has taken another look at the mystery as to what caused the initial cable derailment since Dr. Bitner’s inconclusive report in 1978 and OSHA’s “Act of God” determination.
Renda, Matthew. “Tram Car Trauma at Squaw Valley.” Tahoe Quarterly. Published winter 2014-2015. https://tahoequarterly.com/winter-2014-2015/tram-car-trauma
Frohlich, Robert. “30 Years Later: The Squaw Valley Cable Car Tragedy.” Moonshine Ink. April 10 2008. https://www.moonshineink.com/tahoe-news/30-years-later/
“Bicable Detachable Gondola.” The Gondola Project. https://www.gondolaproject.com/bdg/