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 tolearn more about the collapse of this bridge.

Stress and StrainLoads 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.

Types of Stress Forces are always pushing or pulling on

structures. Five types of stress on structures are:

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 springThe 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.

TorsionTorsion is a twisting force. Torsion is

usually caused by dynamic forces such as exhibited in the Tacoma Narrows Bridge video.

ForcesNow 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 ShapesRectangles 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 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.

Earthquake EngineeringAfter 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 StructuresAn 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 EngineeringTherefore 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”

PilesSteel 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.