Chapter 15 Stability & center of gravity Forces & Failure

Levers Levers: Pull tabs, wrenches and crowbars develop large forces because they are levers. The key features of a lever: A long rigid bar that can pivot around a balance point or a fulcrum; a place where an effort or input force is applied and another place where an output force acts on a load.

Shear Shear solid materials always contain tiny cracks or weaknesses. When a solid material is compressed one section may shear (slide over another section along a weakness). This type of failure can happen in the soil beneath a heavy building such as the Leaning Tower of Pisa. The weight of the building compresses the soil causing some of it to slip sideways. The ground beneath the building sinks and the building tilts or collapses.

Buckle Buckle: If you put pressure on a metal can, the thin metal folds and the can buckles (gives way) under the compressive force you apply. The same thing happens to a piece of paper when you push the ends together. All thin panels tend to buckle when they are compressed.

Metal Fatigue Metal Fatigue: Early railway cars often developed a serious problem after they had been used for several years. Their solid metal axles broke and their wheels fell off, even though the parts were designed to be much stronger than necessary. Railway officials identified the problem that metals weaken when they are bent or twisted over and over again. 

Stable Structures Stable Structures: Almost all structures can lean a bit without falling down. A bicycle rider who is moving fast enough can even lean a lot without losing balance. To design stable (less likely to tip) structures, engineers needs to know what features of a leaning object determine whether it will tip over or stay balanced.

Center of Gravity Center of Gravity: It exists in the exact center of a large object, far below the surface. If you can find an object's center of gravity you can predict how gravitational forces will affect it. Example whether they will cause it to tip over.

Thrust Lines Thrust Lines: You will discover that a tower is most stable when its center of gravity is directly above its base. The line passing vertically through the center of gravity is called a thrust line. The thrust line shows how the tower's weight presses straight down from its center of gravity. When the thrust line passes inside the base, then all sides of the base help to support the weight. As long as the material below the structure does not fail under the compression force, the tower will not tip. It is stable.

Principles of Stability 1) Build on firm foundation 2) Balanced forces 3) Keep thrust lines vertical 4) Use rapid rotation