Observations on Building Damage

As a result of the Chilean 8.8 magnitude earthquake on February 27, 2010 and the several strong aftershocks, and the ensuing damage experienced in cities like Viña del Mar, Santiago, Rancahua, Talca, Chillán, Los Ángeles, Concepción and Talcahuano, there are several buildings to be demolished: 8 in Concepción, 2 in Talcahuano, 5 in Santiago, 5 in Viña del Mar. Most of these buildings have been built in the last few years, some not even completely occupied. The stakeholders are diverse, and it is anticipated that there will be a number of forensic analyses to be requested. In particular, the Alto Río building in Concepción that failed collapsing on its side and resulting in 8 casualties is currently cordoned off as a crime scene. In addition, all coastal areas from Coronel in the south to Iloca in the north were seriously affected by the earthquakes and the tsunami following the main shock. Aside from building damage, transportation systems (roads, rails, airports) were affected in Santiago and between Santiago and Concepción. In the same area, several industrial facilities were also affected and many silos collapsed.
This preliminary and brief report addresses the damage to two types of buildings that have become popular in Chile in the last few years: (1) high-rise reinforced concrete shear wall buildings with longitudinal sides devoid of exterior shear walls to make room for parking in the first floor and open views above and (2) mostly low-rise, reinforced concrete and one- to-three story steel, three-dimensionally framed buildings exhibiting open and flexible floors, preferred by office complexes, commercial and some industrial facilities. The earthquake affected several of the type (1) buildings causing irreparable structural damage to few and repairable structural damage to many. On the other hand, several of the type (2) buildings suffered little or no structural damage, but severe damage to the contents and some of the non-structural components of the buildings.

Preliminary observations on building damage

I am attaching a few photos of some of these buildings (scroll to bottom of this post). These are, in general high-rise, reinforced concrete buildings. These buildings were configured with shear walls, probably calculated assuming rigid diaphragms and using ACI-like steel details for the reinforced concrete. I can venture that the materials had been calculated to the wire to produce the most economical design using computer programs allowing for unrealistic simplifications for the building configurations undergoing strong motions such as the one experienced by these buildings. The shear walls in the longitudinal direction are located side by side along the central corridor of the building leaving open spaces at the exterior sides of the buildings for parking spaces on the bottom floor and open spaces for glass windows above the first floor. This configuration leaves the transversal shear walls vulnerable to be responsible for all stresses coming from the rocking that the slender transverse direction exhibits. The transversal shear walls are usually located at regular intervals on each side of the corridor, except for the exterior shear walls that cover the full side. All the walls have openings to allow for doors, located where needed, and leaving the walls even more vulnerable. To reduce the use of concrete and reinforcing steel some of these transversal walls are shorter than others, down to the size of a column, again to produce the most economical design, but also reducing structural redundancies and ignoring the reality of the dynamic seismic behavior of the structure in case of a strong earthquake, like the one experienced in Chile on February 27, 2010. It is telling that older and shorter buildings exhibit thicker walls and are mostly undamaged. For the most part in the damaged walls, boundary steel is not wrapped with stirrups and the horizontal bars end at 90-degree angles at the edge of the walls. Their failure is the same: the transversal walls overstressed at their ends yielding dramatic failures with total loss of structural integrity, and shear diagonal cracks at various locations. In cases like Concepción and Viña del Mar, which have soft soil conditions that amplify the waves and allow for significant soil-structure interaction, these configuration did not help. The absence of exterior shear walls in the longitudinal direction practically condemned these buildings. Furthermore, some of the records so far available show spectra with accelerations higher than those prescribed by the Chilean code in the long period range. In most Chilean cities, including Santiago, Concepción and Viña del Mar, the streets are laid out in a North-South, East-West configuration. Consequently, the buildings and thus their walls are also oriented in the same direction. The tectonic plates forming the subduction zone under the region where Chile is located run parallel to the Chilean coastal boundary, roughly in a North-South direction, and move roughly from West to East. This suggests that the strong motions generated by a seismic event maybe highly directional, especially is areas close to the source, creating stronger accelerations in one direction. Thus, if this is the direction where this type of buildings are structurally weaker (exhibiting transversal walls without exterior longitudinal walls), as it may very well be, it would further increase their seismic vulnerability.
There are two or three cases of failure due to pure first soft-story (the two four-story buildings in Maipú, west of Santiago). In addition, a number of fancy buildings had their contents partially to totally destroyed in Huechuraba ́s Ciudad Empresarial, just North of Santiago where a number of reinforced concrete tridimensional frame buildings have been built to provide open spaces for offices to foreign corporations such as Xerox, Agfa, CSC, LowePorta, and other Chilean corporations. Also auto dealerships such as Volvo and Porsche, and the Radisson Hotel were damaged. Embarrassed, some corporations have cover or removed their signs. Although there are a number of failure modes, the most common is the damage to the contents of the frame buildings whose structures exhibit no apparent damage. The relatively flexible behavior of the structures, triggered a pendulum behavior on the hung ceilings (hung with simple wires) that probably swung wildly, sometimes braking water pipes or other non-structural elements, which further augmented the damage to the contents of the buildings. Furniture collapsed, windows were broken.

Below there is part of a brief preliminary report I wrote to help the Municipality of Concepción make an assessment of three buildings condemned to be demolished. This is one of the buildings I include in the pictures, which is a 15-story, reinforced concrete building structured with shear walls.

EDIFICIO CENTRO MAYOR (Freire 1165)

La estructura de 15 pisos, de uso residencial, está configurada en base a muros. En la dirección longitudinal existen dos muros a los costados del pasillo interior. En la dirección transversal un conjunto de muros exteriores, y en algunos casos columnas exteriores, terminan definiendo una estructuración de piso semi-blando que permite la utilización del piso inferior como estacionamiento. Ambos sistemas de muros presentan vanos en diferentes lugares para permitir el paso de los ocupantes. Se observó que la mayoría de los muros y columnas orientadas en dirección transversal en el primer piso fallaron. Las fallas son mayores, con pérdida de material e integridad estructural, haciendo peligrosa su operación. Debido a la alta peligrosidad del edificio, sólo se lo observó desde el exterior. Las fallas se produjeron principalmente debido a la interacción dinámica entre los muros transversales, la losa que posiblemente se modeló como losa rígida, pero que no lo es, y el suelo que permitió el giro de la estructura en su base en torno a un eje longitudinal. Los esfuerzos inducidos en los muros y columnas transversales exteriores no pudieron ser resistidos y estos fallaron después de algunos ciclos de carga sísmica.
En resumen, el Edificio Centro Mayor presenta daños irreparables que han comprometido la integridad y estabilidad estructural, y que finalmente hacen su operación inaceptable. Debido a esto último y a que el edificio presenta fallas de diseño fundamentales hacen su uso inviable en una zona de características sísmicas y geotécnicas como las que se encuentran en el sitio que se emplaza.
Este edificio debería ser demolido a la brevedad ya que existe peligro de colapso. El mero soporte con alzaprimas no soluciona el problema de la estabilidad y peligrosidad sísmica en el corto plazo.

Figure 1. Torre O’Higgins in Concepcion.	Figure 2. Edificio Emerald in Santiago.	Figure 3. Edificio Emerald in Santiago.
Figure 4. Edificio Emerald in Santiago.	Figure 5. Edificio Emerald in Santiago.	Figure 6. Edificio Emerald in Santiago.
Figure 7. Edificio Toledo in Viña del Mar.	Figure 9. Edificio Toledo in Viña del Mar.	Figure 10. Edificio Toledo in Viña del Mar.
Figure 11. Edificio Toledo in Viña del Mar.	Figure 12. Torre O’Higgins in Concepcion.	Figure 13. Torre O’Higgins in Concepcion.
Figure 14. Torre O’Higgins in Concepcion.	Figure 15. Centro Mayor in Concepcion.	Figure 16. Centro Mayor in Concepcion.

Figure 17. Centro Mayor in Concepcion.	Figure 18. Centro Mayor in Concepcion.