Engineering Principles

We depend on engineers for safe transportation, safe shelter, and safe energy. These tend to be taken for granted. Failures do not occur often.

When failures do occur, as with the Tacoma Narrows Bridge, the Challenger Space Shuttle, or the Chernobyl nuclear reactor they make big headlines. Many people can be killed when engineered structures fail.

When we think of structures: things such as houses, high rise buildings, airplanes, and bridges usually come to mind.

A little more thought would reveal that almost everything is a structure of some kind. Humans, plants, and animals are all structures.

Galileo (1564 – 1642) is considered to have been the first modern engineer because of his research into the strengths of materials. Prior to Galileo, the size and shape of most structures was determined by the traditions and rules of highly skilled craftsmen.

Newly constructed structures tended to look like successful structures of the past. New methods were not tried very often. There was no scientific way of predicting their safety.

Often new designs fell down in the construction process or shortly after. For example, as late as the 1870’s and 1880’s, 25 bridges a year collapsed on the American railways.

Structural designs advanced by trial and error until modern engineers were able to anticipate the characteristics of new buildings, bridges and other structures. Although Engineers apply science and mathematics to the problem of designing safe structures, many engineering principles are based on common sense. In this series of lessons you will learn some of the factors engineers must consider when designing structures.

Live and Dead Loads Before looking at ways a structure can be designed, it is necessary to look at what forces could cause the structure to fall down. There are many forces that pull, push, twist or bend structures. These are caused by forces or loads on the structure.

A structure must contend with two types of loads – live loads and dead loads. Dead Loads are those loads which are considered to act permanently; they are "dead," stationary, and unable to be removed. The weight of the building is a dead load. The weight of the structure itself is an important factor in its design and sometimes it is the major load on the structure.

The strength and weight of the structure and its individual components can be calculated mathematically. The size, shape and type of material used determines the weight of the structure. For example, a steel beam is 16 times heavier than a wood beam of equal size.

Live loads are the variable loads and they are comparatively hard to determine. Their size has to be estimated. Obvious examples of live loads are the number of people in a building, the weight of the furniture, and the number of vehicles on a bridge. Other less obvious live loads are caused by forces in nature, such as the weight of snow on the roof, and strong winds.

Strong winds exert a large force on the sides of tall buildings and long bridges. Designers must always consider the force of wind when designing a structure. Wind may exert this pressure by direct pressure on the windward side of the structure or by suction on the opposite leeward side. A strong wind could blow in the windows on one side or suck the windows out of the opposite side of a building.

Scale models of large buildings and bridges are tested in wind tunnels to determine the way they react to wind loads. The 1940 failure of the Tacoma Narrows Bridge proved the need for wind tunnel tests. The bridge failed in a wind of only 42 mph. The bridge was not designed with adequate torsional stiffness to withstand a gusting wind.

All materials change shape as they expand and contract with temperature changes. In large buildings or bridges the change in dimensions can be very substantial. If expansion joints are not used, enormous destructive forces can be created within the structure.

The accumulation of snow can be a significant factor in determining live loads. The type and design of the roof will vary from one part of the country to another. Areas that receive heavy snowfalls will require stronger roofs.

Earthquakes can also pose structural problems in many area of the world. Structures must be designed to withstand the strong vibrations caused by earthquakes.

The final design of the structure must take into consideration both the dead and live loads. These will vary with the type of load and purpose of the structure, as well as the local climate and geography.

Equilibrium Equilibrium means balance. In a “tug of war” if the flag does not move, the teams are in equilibrium. If the forces on the right do not equal the forces on the left, one team loses.

Structures would crumble or tip over if the forces acting on them were not in equilibrium. Every force on the structure is opposed by an equal force.

 There are three types of equilibrium to be considered namely; vertical, horizontal, and rotational equilibrium.

The foundation prevents the structure from sinking into the ground The foundation prevents the structure from sinking into the ground. This is vertical equilibrium. A structure not in horizontal equilibrium would slide sideways. A structure which tipped over would not have rotational equilibrium.

Elasticity All materials have a property called elasticity. Some materials are more elastic than others. Elasticity is the ability to be squashed or stretched and then return to the original shape and size. This is the property of materials that enables a structure to push or pull back against forces produced by loads.

Stress When external forces act upon a structure it will change its shape by stretching or contracting itself. However, there are limits to the amount of force structures can withstand before breaking. Stress is a measurement of the intensity of the forces trying to push or pull a solid material apart. Stress is measured in Newton's or pounds per square inch.

The amount of stress can be raised or lowered by changing the area to which the forces are applied. A women’s high heel shoe places more stress on the ground than a flat shoe because the women’s weight is concentrated over a smaller area.

Strain When a material is subject to stress it will change its shape because all materials are elastic. The amount or distance a material deforms under stress is called strain or deflection. The greater the stress the greater the strain.

Materials vary in their ability to withstand stress and strain Materials vary in their ability to withstand stress and strain. When the limits of stress and strain are surpassed the material will break.

Strain is measured as a ratio or percent of lengths so it has no units Strain is measured as a ratio or percent of lengths so it has no units. For example, if a 500-cm rope stretches 1 cm under load then the strain equals 1/500 or 0.002 or 0.2 percent.

There is a constant conflict between the forces that are trying to destroy the structure and the forces that are trying to hold it up. Occasionally, structures are not able to support the loads placed upon them and they collapse. This is known as structural failure.

When structures of the same design always break in the same location, it is called a design flaw. Bridge collapse blamed on a design flaw.

Ensuring that the limits of strain are not exceeded by the stress acting on the structure is the task of an engineer. Engineers calculate the risk of failure by understanding how the structure may fail and then adding a factor of safety. For example, if the structure must hold a load of one ton, engineers design it to hold two tons. This gives a safety factor of two.

The factor of safety can be increased further by basing the calculations on only 1/6 of the material strength. This gives a total safety factor of six. In other words, it should take six times the expected load to break the member. This should account for unexpected loads.

Last card on the Engineering Principles tour. You have completed the Engineering Principles tour. You can go over the information again and study it some more or if you feel you know the information well enough you can go and get the Engineering Principles Quiz from Mr. Leidl.