Humans are natural-born structural engineers. If you find that hard to believe, watch a small child play with a set of simple wooden blocks. With no outside instruction -- and through lots of energetic trial and error -- he or she will eventually learn that the most stable way to build upward is to place a horizontal beam across two vertical columns.
The child's intuitive logic is the same that inspired the Mycenaean architects of the 13th century B.C.E. to construct the famous Lion Gate out of two stone columns and a slightly arched beam. It's the same structural savvy that told the ancient Egyptians that if you want to build something tall out of stone, you need to start wide at the base. And it's that same natural-born engineer in all of us that says, "Dude, if you want to win at Jenga, don't leave a single support at the bottom of the tower!"
Jenga is one of the most popular games in the world, third only to Monopoly and Scrabble in the number of units sold [source: Little]. The object of the game is simple: You start with a stack of 54 blocks -- three blocks across, 18 levels high. Each level of blocks should be perpendicular to the level below it. Each player must remove a block from near the bottom of the tower and place it on the top using only one hand at a time. Eventually the tower becomes dangerously unstable. If you're the one who finally knocks it over, you lose.
Jenga was invented by Leslie Scott, a British citizen born and raised in Kenya and Tanzania. (Jenga means "build" in Swahili.) Scott played the game with her family in Africa for years; she eventually left a job with Intel to launch Jenga at a 1983 toy fair, where it became an instant gaming phenomenon [source: Little].
Part of Jenga's charm is its simplicity; nothing but wooden blocks and gravity. But even this simple game can teach us a lot about the more complex world of structural engineering. Buildings, after all, are vulnerable to the same forces that can topple a Jenga tower -- forces like loads, tension, compression, torsion and more. An accidental bump of the game table is an excellent scale version of a catastrophic earthquake.
Let's start our exploration of the structural engineering of Jenga with a look at loads.
One of the most important principles of structural engineering is loading. Have you heard of a load-bearing wall? It's usually an internal wall (like the one that divides your kitchen and living room) that also serves as a column that holds up the second floor or the roof. If you remove a load-bearing wall, the structure might not be able to support its own weight -- and that spells trouble.
In Jenga, no two wooden blocks are cut to exactly the same dimensions, which means that the blocks rest on each other unevenly [source: Smith]. One of the main tricks of Jenga is locating the "loose" pieces, which are easier to remove without disturbing the integrity of the tower. If a piece is loose, then you know it can't be load-bearing.
So what does this teach us about structural engineering? When designing a building, engineers need to consider the load path from the top of the building to the foundation. Each level of the structure needs to support the forces applied downward from the levels above. There are three kinds of loads that occur in a building:
- Dead loads -- The forces applied by all of the static components of the structure, like beams, columns, rivets, concrete and dry wall.
- Live loads -- The forces applied by all of the "moving" elements that can affect a structure, including people, furniture, cars, and normal weather events like rain, snow and wind.
- Dynamic loads -- Dynamic loads are live loads that occur suddenly with great force. Examples are earthquakes, tornados, hurricanes and airplane crashes [source: Yes Mag].
Engineers need to do careful calculations to ensure that load-bearing walls, ceilings and roofs can support dead, live and even dynamic loads, particularly when building in seismically active zones.
The next important principle that Jenga teaches about structural engineering is the importance of a foundation.
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.
Tension and Compression
In structural engineering, there are two basic forces at work in any structural element: compression and tension. Compression is the force applied when two objects are pushed together. Think of a stack of heavy stones. The force crushing down on the bottom stone is compression. Tension is the force applied when an object is pulled or stretched. A good example is the surface of a trampoline. When someone jumps down on the trampoline, the material stretches.
Engineers talk about the tensile strength of materials. This is the maximum force that can be applied to a material without pulling it apart. Bundles of steel cables have an incredibly high tensile strength, which is why they're used in the world's longest and heaviest suspension bridges. Even a single steel cable only 1 centimeter in diameter can hold the weight of two full-grown elephants [source: Yes Mag].
Now let's think about a typical structure in Jenga. If you remove the center piece in a row, then you create two simple beam-and-column structures on either side of the tower. A beam laid across two columns experiences both compression and tension at the same time. The weight bearing down on the top of the beam compresses it inward toward the center of the beam. And even though you can't see it with your naked eye, the underside of the beam is being stretched outward.
Imagine if the beam was made of rubber. The weight would stretch it into a "U" shape. That's why rubber makes such a lousy construction material. Structural engineers choose (and sometimes design) materials with the best compression and tension characteristics for the job. Stone is excellent under compression, but remarkably easy to pull apart. That's why a stone arch lasts a lot longer than a stone beam. Reinforced concrete is an ideal building material, because the concrete gives it compression strength and the embedded steel rods give it tensile strength.
Jenga towers don't get tall enough or heavy enough to apply serious compression or tension on the wooden pieces, so there's very little concern of splitting a beam. But in real construction projects, engineers need to carefully consider each element's strengths and weaknesses.
Now we'll explain why it's always better to leave two supports at the bottom of the Jenga tower.
Experienced Jenga players know that the quickest way to a falling tower is to pull away the two outside pieces of the bottom row, leaving the whole structure balancing on a single narrow wooden block. With only one support at the bottom, every bump and nudge of the tower is magnified, causing it to sway precariously from side to side. But what exactly are the forces that act upon a structure with such a narrow support? And what makes them so dangerous?
Structural engineers don't talk about keeping a building "balanced." They talk about maintaining rotational equilibrium. Imagine a tall building as a long lever arm with the majority of the arm above ground and a smaller section (the foundation) below ground. The point where the building meets the ground is the fulcrum of the lever. Now picture the building tipping slightly to the right or the left. Instead of merely falling over, you can think of it as rotating around the fulcrum. Engineers and physicists have two names for this rotational force: the moment or torque.
A basic tenet of structural engineering is that the longer your lever arm (or the further it is away from the fulcrum), the greater the moment. To decrease the moment of a very tall building, you need to build wide supports. The wider the supports, the lower the moment. To understand this, try standing with your feet spread wide apart and have a friend try to push you over sideways. It requires a lot of force. Put your heels together and try the same thing. Your friend barely has to touch you and you tip right over. A structure with a nice wide base is inherently more stable that a building with a narrow base.
For the last structural engineering lesson learned from Jenga, we'll talk about earthquakes.
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.
Despite what the nursery rhyme says, London Bridge is not falling down and never has. But the bridge spanning the Thames has been rebuilt over and over.
- Association of Bay Area Governments. Earthquake and Hazards Program. "Height/Weight Relationship" (Sept. 12, 2011) http://www.abag.ca.gov/bayarea/eqmaps/fixit/ch2/sld011.htm
- Association of Bay Area Governments. Earthquake and Hazards Program. "Period of Vibration" (Sept. 12, 2011) http://www.abag.ca.gov/bayarea/eqmaps/fixit/ch2/sld012.htm
- Federal Emergency Management Agency. Earthquake Handbook. "An Introduction to Structural Concepts in Seismic Upgrade Design" (Sept. 10, 2011) http://www.conservationtech.com/FEMA-WEB/FEMA-subweb-EQ/02-02-EARTHQUAKE/1-BUILDINGS/C~-Structures-Intro.htm
- Levy, Matthys; Salvadori, Mario. Why Buildings Fall Down: How Structures Fail. W.W. Norton & Company. 1994 http://books.google.com/books?id=Bwd-MHINMGsC&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false
- Little, Rg. The Oxford Times. "Demolishing the Jenga Myth." November 12, 2009 (Sept. 12, 2011) http://www.oxfordtimes.co.uk/news/features/4728039.Demolishing_the_Jenga_myth/
- Smith, Dan. Wired UK. "How to Beat Anyone at Jenga." June 10, 2011 (Sept. 10, 2011) http://www.wired.co.uk/magazine/archive/2011/07/how-to/how-to-beat-anyone-at-jenga
- V&A Museum of Childhood. "Jenga" (Sept. 12, 2011) http://www.vam.ac.uk/moc/collections/games/jenga/index.html
- Yes Mag. "The Science of Structures" http://www.yesmag.ca/focus/structures/structure_science.html