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
Saul Chaplin 
This was a really interesting read, thanks Saul!
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