Earthquake Forces

|
1
Earthquake Forces

The tallest Jenga tower on record was 40 levels, reached using the original Jenga set designed by Leslie Scott herself [source: Museum of Childhood]. Most players are lucky if they can get more than 30 levels before the whole thing comes crashing down. The reason the tower becomes increasingly unstable as is grows is due to uneven weight distribution. When too much weight is located at the top of the structure, it begins to act like a reverse pendulum, swaying back and forth on its narrow connection to the earth [source: FEMA]. In Jenga, the result is a two-minute cleanup. In real life, you'd have a catastrophe.

When structural engineers choose to build in a seismically active region, they need to consider the effects of lateral vibrations on their building. When seismic waves ripple through the earth, they jostle buildings both up and down and back and forth. The up and down bumps aren't as dangerous as the lateral movements, which are more likely to lead to collapse [source: Association of Bay Area Governments].

These side-to-side vibrations are experienced differently at different distances from the ground. The higher you travel up a building, the more pronounced the vibrations. When you throw weight into the equation, the effects can be disastrous. According to the seminal text, "Why Buildings Fall Down," earthquake forces grow in proportion to the weight of the structure and the square of its height [source: Levy].

A top-heavy structure vibrates with a much longer period -- the time it takes to cycle through one complete vibration -- than a bottom-heavy building. A longer period also means a larger physical displacement. Take the example of a two-story building. When an earthquake strikes, the building sways 2 inches (51 millimeters) off center. When you add weight to the top of the same building (even if it's something simple like a heavy tiled roof), the sway increases to 3 inches (76 millimeters) off center [source: Association of Bay Area Governments].

We hope you've learned a few things about why buildings fall -- and what you can do to finally beat your sister at Jenga. For lots more information about family games and everyday science, jump over to the links on the next page.

|