Why do buildings collapse during earthquakes? This is a question that often arises in our minds whenever a strong earthquake strikes and wreaks havoc on cities and towns, as buildings, churches, and houses fall like a card castle. Some urban structures are reduced to a heap of rubble, with many people being trapped underneath. Buildings fall during strong tremors due to the 'lever' effect on tall structure; this effect is caused by the horizontal movement or displacement of the ground below, which puts the columns and walls out of plumb, making the mass (weight) of the upper part of the building pull and push laterally in a manner of a swinging lever. Thus, the taller the building is, the longer the lever will be and the more powerful the lateral swaying movement of the weight on top will become.
During tectonic plate earthquakes, the ground level columns become the fulcrum on which the lever effect is actuated as they are subjected to powerful lateral forces (the effort of the lever), with the building swaying from side to side. The columns eventually break at the base on ground level and, at the same time, on the first, second and third story level, at the junctures or unions of a column and a beam, which are joined together at a 90-degree angle. The lateral forces pull these junctures out of their 90-degree right angle. The columns, which are rigidly embedded in its foundations, and the beams on the lower portion of building eventually break and the whole structure collapse. Thus, to avoid the break of the juncture, an extra slanting concrete strut must be used to reinforce the union of a column and a beam and keep the 90-degree junction intact during an earthquake.
Poorly-constructed vs earthquake-resistant buildings
When we see that the amount of damages and devastation, and that the number of casualties vary widely from one country to another that has been struck by the same earthquake intensity, a second question arises: why do some buildings and infrastructures collapse and others remain standing, with their integrity intact? There are two answers to this question: 1) construction quality, and 2) corruption in engineers and company's CEOs (in private buildings construction) and corruption in government officials in public building construction and inspection.
For example, during the powerful 2010 earthquake that struck Chile, every building, church, and house remained standing, with no casualties, in the capital of the country, Santiago, where the earthquake intensity reached 8.5 on the Richter scale, while only four buildings fell sideways, keeping their integrity (without crumbling into rubble) and two bridges caved in Concepción where the tremor reached the apocalyptic magnitude of 9.3 during the same earthquake. However, a 7.0-magnitude earthquake had killed 300,000 people in Haiti a month before (in January), with 75% of buildings and houses crumbling down to smithereens. In Chile, there were only 500 casualties in Valparaiso, and most of them drowned in the ensuing tsunami. Chile and Japan have the best earthquake-resistant technology in the world.
Exact technical cause of a building collapse
A building collapses during an earthquake because the reinforced concrete fails. And it fails for two reasons: 1) the longitudinal steel rods/bars, which are essential, integral parts of both columns and beams are not enough in quantity and girth (thickness) as they also do not have enough stirrups, which are the steel rod loops that hold the longitudinal steel rods together with the concrete inside; 2) the concrete is of poor quality, lacking enough cement and aggregate (small stones), with too much sand. Thus, we can say that getting the concrete mix ratio right is the single most important factor in determining the strength, durability, and resistance of a building's structural member, such as a column.
How an earthquake-resistant skyscraper should be made
In a seismic zone, such as one neighboring the site of two tectonic plate collision, every building taller than 15 stories, should be built on a two-layer foundations: 1) an extra-reinforced concrete base plate deep below ground level, and 2) a structural foundation plate from which the building's mother columns arise. This structural foundation plate must sit on either steel balls that gentle rolls in concrete bowl-like concavities built in the primeval base plate; thus, the energy of the lateral/horizontal movement is absorbed by the rolling balls. In Chile, engineers make the structural foundations sit on either seismic rubber base isolators or large steel springs, which absorb the energy of the wobbling of the lever effect. In this way, the whole building slides sideways, from bottom to top, avoiding and preventing the destructive lever effect caused by the action of gravity and the swinging lateral force (effort) that break the lower columns, which happens in a building that has a rigid structure at the base, with its upper portion wobbling and swaying from side to side, like a tree being subjected to strong gusts of wind in a powerful storm.
Buildings moving on foundational rubber isolators during an earthquake in Tokyo. (Video)
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| Above, a reinforced concrete column got cracked during an earthquake due to the reduced number of stirrups, which are too thin. Stirrups must be set only 7 cm apart, using thicker steel rods. |
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| Structural failure at the junctions of columns and beams. |
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| Insufficient number of stirrups, whose steel rods are too thin. |