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Engineering Terms • Engineers design all structures with enough strength to withstand the force and load that will be placed upon them. Generally loads are masses resting on a structure, but may also be forces such as wind, impacts, vibrations, bending or internal twisting distortions transferred through the structure. There are two types of loads: • Live Loads • Dead Loads
Dead Loads • Dead loads are static. They do not change or move. Engineers also refer to these as static loads. The weight of the bridge is a dead load
Live Load • Live loads are those that move and change. Take the example of a bridge, the weight of the vehicles, wind, snow/ice build up, thermal expanding and contracting are all examples of live loads. Live loads that change in value quickly (i.e. wind gusts)are called dynamic loads. In the picture above both the weight and momentum of the train represent a live load.
Dynamic Loads • Dynamic loads are the most difficult to predict and prepare for. Here is a video image of the Tacoma Narrows Bridge (1940) that wasn’t designed well enough to support wind gusts. Click on the above picture to learn more about the collapse of this bridge.
Stress and Strain • Loads stress a structure by pulling or pushing which can result in bending or twisting. Stress is the measure of the amount of force placed on an object. As you witnessed in the previous video, the stress from the load was too much for the bridge to handle and therefore collapsed.
Forces are always pushing or pulling on structures. Five types of stress on structures are: Types of Stress Compression Tension Bending Torsion Shearing
Compression • Compression means to push or squash a material. Compression tends to make a material shorter or more compact. i.e. Garbage Truck Compression is pushing
Tension • Tension is a pulling force. It tends to make a material longer. i.e. A sling shot. A spring The weight of the water jug is causing tension on the rope supporting it. It is a pulling force.
Shear • Shear occurs when a material is divided by two opposing forces. These forces are usually parallel. One part of the material slides past the other part. i.e. scissors Shear strength is the ability of a material to resist being fractured by opposing forces acting of a straight line but not in the same plane, or the ability of a metal to resist being fractured by opposing forces not acting in a straight line. Think of scissors, which is easier to cut, Paper? Cardboard? Metal? Which of those would have more shear strength? The thicker the material the more shear strength it has.
Bending Bending occurs when a load is placed on or near the center of a horizontal beam. When bending occurs, the top of the beam is in compression and the bottom in tension.
Torsion • Torsion is a twisting force. Torsion is usually caused by dynamic forces such as exhibited in the Tacoma Narrows Bridge video.
Forces • Now that we have learned about the forces that cause stress and strain on structures, we have to apply design and construction techniques to combat those forces. To do that we need to study shapes and the effect that they have in supporting structure.
Structural Shapes • Rectangles and Squares • Columns and braces are joined at 90 degrees • Easily deform under pressure • Need diagonal bracing to support loads and to withstand stress
Structural Shapes • Triangles • Make for strong towers • The more triangles, the stronger the tower. • Triangles strengthen towers by keeping them rigid • The diagonal bracing prevents buckling or deformation of the shape.
Structural Shapes • Arches • Arches support loads at any point along their curve. Triangles only support at the edges. • Arches are more likely to push out at their base than triangles.
Trusses are beams (top and bottom chord) that are made of two horizontal pieces joined in the middle by a series of triangles. The shape and design of the triangles can vary. Trusses are lighter, stronger, and more cost effective than a solid beam of the same dimensions. Trusses
Earthquake Engineering • After the infamous 1906 earthquake in San Francisco, California, it was realized that wood and steel framed construction was superior to that of buildings made of brick and mortar
Rigid Structures • An earthquake produces a series of waves that move horizontally across the ground causing buildings to sway from side to side. Therefore a rigid structure can only withstand minimal shaking
Flexible Engineering • Therefore some flexibility in a building is necessary. Steel and wood construction offer this flexibility.
Earthquake Resistant Structures • Buildings between 5 and 20 stories are at the most risk to earthquake damage. Taller buildings tend to be more flexible because they are designed to withstand high winds. However, non-structural material may fall and cause damage to those below.
Earthquake Resistant Structures • The base on which a structure is built also plays an integral role in it’s safety. Dry sandy soil tends to settle causing the building to settle. Wet soil tends to liquefy. In both instances buildings may stay intact but may tilt or even fall over. To prevent this from happening, buildings are built onto “piles”
Piles • Steel enforced concrete piles are built deep into the earth’s surface to help transmit the load of the building into more solid dry ground. Even in Canada, due to frost heaving the soil, buildings and decks are built onto piles.
