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4
Foundations

Just as you need to find the perfect surface on which to play Jenga, structural engineers need to consider the surfaces upon which they choose to build.

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Every family has their favorite surface on which to play Jenga. The flimsy card table is out of the question because the slightest bump from an errant elbow will send your tower tumbling. The sturdy kitchen table is a solid choice, because it doesn't rumble around as easily as the card table, but nothing beats a good hardwood floor. You can't knock it sideways, it's pretty darn flat and the only threat to stability is the occasional crawling baby or pet.

Structural engineers must also consider the surface onto which they're building their structure. If you plop a 15-story building onto loose soil, the structure might settle unevenly, causing cracks in the walls or even a collapse. Even if a building is constructed atop solid rock, an earthquake could jostle it sideways, causing it to slide down the street a few feet, crushing anything in its path. That's why all modern buildings, small and tall alike, are built upon foundations.

A foundation serves a couple of key purposes. Firstly, it transfers the load of the structure into the ground. (We talked about loads on the last page.) The taller and heavier a building, the more load is driven downward. If the building sits flat on the surface, then the lowest elements in the structure would have to bear the combined load of everything above them. But with a properly engineered foundation, the load of the entire structure passes through the lowest elements and is dispersed into the earth below.

Foundations also serve the purpose of physically anchoring the structure to the ground. This is a crucial role in very tall buildings. Imagine trying to balance a yard stick on one end. You might be able to pull it off on an extremely flat surface, but even an exhale would topple it over. But what happens if you take the yard stick out back and jam one end into the ground a few inches? Now you can tap it, or even kick it, and it won't tip over. A foundation buries a portion of the building in the ground, giving it increased stability against dynamic load changes.

For tall buildings built on loose soils or sand, engineers drive steel piles deep into the earth until they reach bedrock. Then they build a reinforced concrete foundation around the steel piles to create a firm anchor on which to build.

Next we'll look at what wooden Jenga blocks can teach us about building materials.

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