Soft Story and More – Why Do Building Fail
The large number of fatalities and damage here was greater than anticipated and can be attributed to several factors:
• A large portion of Mexico City is situated on an ancient sea bed and the underlying soil is very soft. This causes the amplification of ground motion to the built infrastructure. So although there are no faults in the city, events from far-away earthquakes are amplified and can generate large forces.
• After the 1985 earthquake, the building codes were modified and include good ductile detailing. However, compliance with the code is not always enforced, especially in outlying areas, and, in many cases, residents make alterations and additions to the buildings that can after alter these structures’ earthquake resistance.
• Damage assessment data from this earthquake is yet not available. However, preliminary data indicates that a large number of buildings with significant damage used either unreinforced masonry (URM) bearing walls or reinforced concrete construction of older vintage. These building types (especially with a soft/weak story or torsion) are most susceptible to earthquake damage and collapse.
• This event had a long duration for strong motion, resulting in buildings being subjected to a large number of shaking cycles.
Read Miyamoto’s official tech report on the Sept. 19 Mexico earthquake that collapsed at least 44 buildings in far-away Mexico City alone.