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.

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References

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/

How do you build an earthquake-resistant structure that looks like coral? This Seismic Saturday, we travel to Santiago, Chile, to feature the Cruz del Sur Building.

The first thing that the eye catches is the structure’s surprising form, starting thin and expanding outward up the building (figure 2). A thick central shaft, which takes both the base shear as well as the overturning moment, runs from the ground foundation all the way to the roof. Diagonal columns jut out from the central shaft to support the edges of the concrete slab, as well as the vertical columns which travel up the building.

This unconventional design causes an interesting load path under gravity load. The diagonal columns are in compression(shown in red), and the first floor slab is under tension (shown in blue) (figure 3). Concrete is very weak in tension; thus massive cables are routed through the first floor slab to carry the tension. The ends of the cables are capped with steel cylinders to allow for later access (figure 4). A structural detail of the slab is shown in figure 5, in which the the steel post-tension cable system can be seen with three slots for cables, as well as an angled cap to press inward (and slightly upward) against the concrete. Axes (or “eje” en Español) for the vertical column, the diagonal column, and the slab meet at a central point.

For any structure in Santiago, Chile, the million dollar question is, of course, what happens in an earthquake? Most of Chile borders the the Nazca Plate, which is being thrust underneath the south-American plate at a rate of ~80 cm/year, creating a subduction zone. Huge earthquakes happen every decade up and down the coast of Chile. For instance, in the last 12 years, Chile has been jolted by the 2015 Coquimbo Earthquake (8.3 Mw), the 2014 Iquique Earthquake (8.2 Mw), and the 2010 Maule Earthquake (known also as the Concepción Earthquake) (8.8 Mw). Santiago can experience lenghy earthquakes which cause peak ground accelerations in excess of .3 g (Hussain et. al. 2020). Imagine falling sideways 30% of the acceleration in which you fall downwards; that is how fast the ground accelerates. Since acceleration and motion is amplified for higher floors, the top floor of a structure, under these ground accelerations, can easily be jerked back and forth with an acceleration in excess of 1 g.

For structures with thin bases, like the Cruz del Sur Building, withstanding these extreme earthquake forces is challenging. To understand why, think of a person on a metro. When the train accelerates away from a station (and no handhold is in reach), people naturally stand with their legs wide apart, in line with the train’s acceleration. Anyone with their feet together would fall over. The same problem exists for the Cruz Del Sur structure with its slim base – the base has to resist the urge to overturn with “its feet close together” and thus a small lever arm. This overturning scenario which manifests as tensile and compressive stresses induced by the earthquake forces as well as base shear, is shown in figure 6. The solution found by the structural engineers to prevent the overturning is to extend the shaft deep into the ground and design a mat slab beneath. When the building tries to turn, the soil pushes back on the mat slab (green arrows) and on the shaft (pink arrows) to prevent it from doing turning. To deal with the stresses induced by the overturning, which can cause tensile failure or compressive failure as shown in fig. 6, the walls on either side the central shaft incredibly thick (~2 m deep). These walls are loaded with both vertical rebar to take tension, and confining rebar to prevent a spalling compressive failure. Essentially, if the building were a person on the metro, it would have its feet close together, but its feet would be latched to the ground and its legs would be bigger than Dwayne “The Rock” Johnson’s thuder-things.

One of my favorite aspects of the building is that it reminds me of a coral reef (figure 7). The base starts out thin, and then the building expands as it goes upward. It is interesting to think that coral reefs face a similar constraint to buildings in a city environment – there is a high demand for ground-level space, but upon building upward, there is more space available to expand. For coral, only a small anchor point is needed to flourish upward and then outward (and to be able to host the largest number of photosynthetic algae). For the Cruz del Sur building, a smaller anchor point allows for a public Plaza underneath, while expansion as the building travels upwards allows for more rentable office space at higher (and more desirable) elevations.

A fascinating effect, whereby light reflects off of the surface of a small pool onto the column of the structure, reminded me of the moving light patterns on the ocean floor caused by reflection through the surface (figure 8).

Also notable in the structure is the use of circular windows, both in the floor slab above the lobby, and in the 2 meter-thick shaft wall. For me, the circles add to the oceanic aesthetic, as they resemble the windows in a ship’s hull. It is interesting to add that the circular windows are located within the reinforcing wall and slabs, in areas that would be subject to high flexural, shear, and axial demand. The circular form may do a better job at preventing stress concentrations that occur in the corners of square windows. (This is why airplanes have oval windows: /https://aerospaceengineeringblog.com/dehavilland-comet-crash/). Of course, this would also depend on the steel rebar design around the circular windows.

Also of note in the structure is the strategic hiding of circular vents behind the diagonal column groups, to keep the the visible surface of the building cleaner. Additionally, the ~60 cm overhang of the slab above each window which shades the window while the sun is at its highest angle during the Summer. The structure maintains the modern window-facade look from afar, while being more energy efficient than glass-walled office buildings.

The Cruz del Sur’s unconventional shape, seismically resistant technology, and bio-inspired form, make it fascinating from the perspective of both engineers and architects. As I walk down Apoquindo (the Wall Street banking hub of Chile nicknamed San-hattan), I’m drawn towards the structure by its overhanging inverted form, and its audacity to be supported by a thin shaft in one of the Earthquake capitals of the world.

References

Hussain, Ekbal et. al. “Contrasting seismic risk for Santiago, Chile, from near-field and distant earthquake sources.” Natural Hazards and Earth System Sciences 20 (2020): 1533–1555. Published 29 May 2020. https://www.researchgate.net/publication/341738393_Contrasting_seismic_risk_for_Santiago_Chile_from_near-field_and_distant_earthquake_sources

Izquierdo, Luis. Lehmann, Antonia. Edificio Cruz del Sur, Santiago. Revista de la Escuela de Arquitectura de la Pontificia Universidad Católica. ARQ, n. 73 Valparaíso, Santiago. Publicado diciembre 2009, p. 12-19. https://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0717-69962009000300002

“Edificio Cruz del Sur / Izquierdo Lehmann” 20 jul 2012. ArchDaily en Español. Accedido el 16 Ago 2022. Fotos por Cristobal Palma. https://www.archdaily.cl/cl/733942/edificio-cruz-del-sur-izquierdo-lehmann

We travel south of the border this #seismicsaturday to Baja California, Mexico. Featured is an observation platform at “El Mirador” (The Lookout) in San Pedro Mártir Nacional Park, at 9100 ft.

⛰️The platform is made with a cantilever steel truss structure. The main truss extends outward, reducing its depth as it goes (Fig. 2). Secondary truss structures go across, to tie the main trusses together (Fig. 3). Every single steel-to-steel connection is welded (Fig. 4). It is notable that not a single bolt was used on any of the steel-steel connections. Given the extreme temperature variations at 9100 ft, with sub-freezing temperatures in the winter and hot weather in Summer, it is interesting to think about thermal stresses that would develop in this stiff structure.

⛰️One thing that makes this structure unique is the way it is kept in place on the mountainside. A huge pile of rocks, kept in place by steel tubes and mesh (pic 5), serves as a counterweight, keeping the structure on the hillside. Furthermore, a bunch of oddly (and seemingly arbitrarily) angled steel plates are bolted to rocks on the mountain (pic 6). Your Seismic Outreach correspondent has never seen anything like this before.

⛰️In an area battered by rain in the Summer, freeze-thaw cycles in the Winter, and high humidity, painting the structure is important to prevent corrosion. Except for a little section to the right of the joint in pic 4, this structure has been well painted, and as such, doesn’t seem to show signs of corrosion.

From the cantilevered steel tubing, to the rock boulder counterweight, to the wacky steel plates anchored to the ground, engineers building the lookout came up with a creative solution to create a platform over a precipitous drop. The platform gives hikers a gorgeous (and nail-biting) view.