# Mr. Williams – Grade 7 Science

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Mr. Williams – Grade 7 Science
Form and Function: Part 6 – Making Structures Strong: The Truss, Ach and Dome Mr. Williams – Grade 7 Science

Making Structures Strong
Sometimes designers want to use materials in shapes other than beams to make their structures stronger and more interesting. They can do this by using: Trusses Arches Domes

Trusses A truss can be used as a bridge or a cantilever, and for many other applications Truss – A network of beams that form triangles Trusses are often used in the roofs of home construction projects. Trusses can also be bent or curved and still retain their strength Trusses take advantage of triangles to make the structure strong as the forces are distributed between the vertices.

Arches Arch – Is a curved structure often used to support loads
Arches are used in spaces where supporting beams are not practical. For example: windows, doorways, places of worship and bridges. The arch’s curved design transfers compression forces downward into the structure. Many arches that were built by the Romans over 2000 years ago are still standing today. The arch relies on the “Keystone” to stay upright. This is the stone that is placed at the top of the arch.

Domes Dome – A shell structure that looks like the top half of a sphere or an egg. Similar to an arch, a dome directs compression forces downward. In an arch, the downward force only happens in one plane of application, in a dome however, the compression force is directed downward in many places at once.

Structural Failure Structural Failure – The failure of a structure as result of the structure, or part of the structure losing the ability to support a load Once a structure loses its load-carrying ability it can crack, deform, or collapse completely. For example: An umbrella bending backward in the wind, a glass breaking when it falls. There are four main reasons for structural failure: Bad Design Faulty Construction Extraordinary Loads Foundation Failure

Bad Design Approximately 40-60% of all structural failures occur due to bad design. This can be attributed to incorrect materials, not considering stress, and failure to account for different loads. In 1986, the space shuttle Challenger exploded after liftoff because of a faulty design in an O-ring which caused a gas leak and ignited.

Roofing nails have a larger head to hold down shingles
Faulty Construction This is the second most common form of failure. This is caused by the use of poor materials, poor installation or sloppiness on behalf of the workers, or a combination of both. For example: Shingles blowing off of a roof in a strong wind storm. What would happen if a roof was secured using finishing nails? Roofing nails have a larger head to hold down shingles Finishing nails have a smaller head and a smoother surface, which are less visible around doors and frames.

Extraordinary Loads Extreme conditions can result in structural failure. This is not because of flaws in the design, but because of unexpected events that place extraordinary loads on structures. Ice Storms, like this seen here are examples of extraordinary loads. The weight of the ice build up causes the power lines to collapse.

Foundation Failure Failure of a structures foundation is less common than bad design and faulty construction Foundation failure can be caused by poor soil conditions, poor installation, foundation that is not large enough or even earthquakes. The Leaning Tower of Pisa is a very common design of a structure with a poor foundation. The tower is built on sandy soil and started to lean almost immediately!