Abstract
This paper presents the observations during EEFIT’s post-earthquake reconnaissance missions to the September 20, 2009 Padang (Mw7.6), March 11, 2011 Tōhoku (Mw9.0) and April 16, 2016 Muisne (Mw7.8) earthquakes. The performance of buildings and geotechnical structures within the affected regions were investigated to gain insights on their design and construction deficiencies. Findings on these damage observed are compared along with the characteristics of the earthquake and nature of building codes in these countries. They include building damage caused by resonance, deficiencies in reinforcement detailing, vulnerability to soft storey collapse, ground settlement, soil liquefaction and landslides. It was demonstrated that buildings which were severely damaged had natural building frequencies coinciding with the dominant frequencies of the ground shaking. The locations of damage of several such buildings showed insufficient confining reinforcements and lapping of stirrup links. Soft storey collapses were also observed in the three earthquakes, although many were attributed to old building codes that were less effective. In areas affected by the Muisne earthquake, soft storey collapses were mainly found at mid height of the building rather than at the ground floor as observed in the Padang and Tōhoku earthquakes, likely due to extension of building long after the bottom floors were completed. In the aspect of geotechnical failure, foundations of buildings found on piles performed reasonably well, except for areas affected by soil liquefaction. Landslides occurred following these earthquakes led to large concentration of casualties and property losses, motivating the EEFIT teams to invest efforts in hazard mapping and ground-truthing exercises using satellite images at Padang and Muisne earthquakes respectively. Such geospatial tools applied in these three earthquakes were reviewed and demonstrated to be capable of identifying landslide sites and producing reliable landslide hazard map.
Highlights
The UK-based Earthquake Engineering Field Investigation Team (EEFIT) has been deploying multi-disciplinary teams following major earthquakes in the world and reporting their findings to the engineering community since 1982
Following the analysis of the field missions to the Mw7.6 2009 Padang earthquake in Indonesia, Mw9.0 2011 Tohoku earthquake in Japan and Mw7.8 2016 Muisne earthquake in Ecuador, the following are the lessons learnt from these events: 1. Soft-story collapse was by far the major structural failure mode observed in engineered buildings in the three earthquake events
The insufficient and poor reinforcement detailing such as use of smooth bars for links and main reinforcement, large spacing between confining links, and insufficient hook length and angles further exacerbate the susceptibility to severe damage of these buildings following older version of building codes
Summary
In areas affected by the Muisne earthquake, soft story collapses were mainly found at mid height of the building rather than at the ground floor as observed in the Padang and Tohoku earthquakes, likely due to extension of building long after the bottom floors were completed Such extension of building can either lead to local reduction in capacity due to weaker concrete-rebar bonding, possibly insufficient lapping of reinforcement, as well as increased axial loads. In the aspect of geotechnical failure, foundations of buildings found on piles performed reasonably well, except for areas affected by soil liquefaction Landslides occurred following these earthquakes led to large concentration of casualties and property losses, Padang, Tohoku, and Muisne Earthquakes motivating the EEFIT teams to invest efforts in hazard mapping and ground-truthing exercises using satellite images at Padang and Muisne earthquakes respectively.
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