Structural Strength & Stability: Chapter 11

• Chapter 11 covers structural strength and stability, addressing what makes structures strong and stable.

Looking Ahead: Key Concepts

• A stable structure maintains its shape and position over time.
• Beams, trusses, arches, and domes are used to help structures support loads.
• Scientific inquiry helps investigate factors affecting a structure's load-bearing ability.
• Technological problem-solving determines the most efficient way for a structure to support a load.
• Structural failure occurs when a structure loses its load-bearing ability.
• Scientific inquiry investigates methods to ensure structural safety.

The Ritual of the Calling of an Engineer

• In 1922, Canadian Herbert Haultain created a ceremony for new engineers.
• New engineers recite an Obligation and receive an iron ring symbolizing pride and responsibility for safety.
• The Quebec Bridge disaster, which collapsed in 1907 (killing 75) and again in 1917 (killing 11) due to engineering errors, prompted the need for greater responsibility among engineers.
• Haultain collaborated with Rudyard Kipling, who wrote The Ritual of the Calling of the Engineer.
• The first ceremony was held in 1925, and it continues yearly.
• The words of the Obligation are secret.
• Kipling also wrote poetry on engineering and building.

Stability of Structures

• Stability is the ability of a structure to maintain or regain a balanced position when forces act on it.
• Engineers design structures to be stable for safety.
• Most structures, from toys to buildings, are designed to be stable. Some amusement park rides are intentionally designed to appear unstable.
• Center of gravity is the point around which an object's mass is equally balanced in all directions; it is the point where the mass seems to be concentrated.

The Centre of Gravity of Common Structures

• An object's center of gravity is usually located deep inside the object, not on its surface.
• When standing, the human body's center of gravity is located just below the belly button.
• The center of gravity changes as the body moves and bends. It depends on the shape of the object and mass distribution.

Conditions for Greatest Stability

• A sports car is more stable than a truck due to having a low center of gravity and a wide support base.
• Objects with those two characteristics tend to be stable.
• Maintaining stability requires the center of gravity to lie directly over the support base.
• Stability decreases as the center of gravity rises.
• Standing in a canoe is dangerous because it raises the center of gravity.
• The loon's center of gravity lies ahead of its feet, making it awkward on land.
• Acrobats use a long pole to lower their center of gravity, increasing stability.

Making Structures Strong: The Beam

• A beam is any reasonably level structure designed to support a load.
• One of the oldest beam structures was a log lying across a river, supported by the riverbanks.
• Beams can be strengthened by changing the material they are made of.
• Materials include wood, stone, concrete, or steel, and selection depends on the structure's load requirements.

I-Beams

• An I-beam has a shape like the letter "I" when viewed from the end.
• I-beams are commonly used in building construction, including houses.
• Wooden I-beams are lighter than steel ones but can still support very heavy loads.
• I-beams also use wood chips, reducing the need for solid wood.

Corrugation

• Corrugation involves placing triangular ridges, grooves, or folds in a structure.
• Corrugation provides additional strength.
• It is common in cardboard boxes and is applied to metal or plastic roofing for extra strength.

Rebar

• Concrete beams are often strengthened with steel reinforcing rods (rebar).
• Beams experience compression on top and tension on the bottom.
• Concrete is strong under compression but weak under tension.
• Rebar helps concrete resist tension.
• Concrete with rebar (reinforced concrete) can resist both compression and tension.

The Cantilever

• A cantilever is a beam supported or fixed at only one end.
• Examples include tree branches, diving boards, canopies over entrances, and apartment balconies.
• Cantilevers are useful for spanning great distances without a central support.

Supporting the Beam

• Adding structural support strengthens a beam.
• A tie resists tension forces and is set at an angle between the beam and its support base.
• A strut provides resistance to compression forces and is placed below the beam.
• A gusset is a flat, plate-like device (often triangular) that reinforces the connection between the beam and its support base.

Making Structures Strong: The Truss, Arch, and Dome

• Designers use shapes other than beams to make stronger, more interesting structures.
• These shapes include triangles (trusses), curves (arches), and domes.

The Truss

• A truss is a network of beams that forms triangles
• It can be used as a bridge or cantilever.
• Trusses utilize the strength of triangles: forces are distributed between the points, and thus the structure can support more weight.
• Trusses allow for different structural designs; trusses can be used in applications that beams cannot.

The Arch

• An arch is a curved structure used to support loads.
• Arches are used where supporting beams are impractical (doorways, windows, bridges, places of worship).
• An arch's curved design transfers compression force downward.
• Like the beam and the truss, the arch is a basic component of structures.
• Arches were used by the Romans to build structures.

The Dome

• A dome looks like the top half of a sphere or an egg.
• Domes are used in places where supporting beams are impractical.
• Like an arch, a dome directs compression force downward, but in many planes at once.

Tech Connect: Carbon Nanotubes

• A carbon nanotube is made of a single layer of carbon atoms arranged in a hexagonal shape.
• Carbon nanotubes are 10,000 times thinner than human hair but stronger than steel. Their light weight and extreme strength make them ideal for structures that face extreme conditions.
• They have great potential in designing sports gear, fighting infection in medicine, and in building better and smaller electric circuits.

Preventing Structural Failure

• Structures are designed to certain specifications to fulfill their purpose.
• Safety is an important component in the structure's design and use.
• Engineers use several methods to ensure structural safety.

Structural Failure

• Structural failure occurs when a structure loses its ability to support a load.
• It can crack, deform, or collapse completely.

Bad Design

• Approximately 40% to 60% of structural failures result from bad designs, such as failure to account for load, specifying incorrect materials, or not considering important factors and stresses.

Faulty Construction

• Faulty construction is the second most common cause of structural failure.
• Construction errors can result from poor-quality materials, poor installation, sloppiness, or lack of expertise.

Foundation Failure

• Failure of a structure's base (foundation) is less common than bad design and faulty construction, but it can lead to significant structural problems.
• It can be caused by poor soil conditions, poor installation, a foundation that is not large for the structure's load, or even earthquakes.

Extraordinary Loads

• Extreme conditions can cause structural failures.
• The failures result from unexpected events creating extraordinary loads on structures rather than from poor design.
• In January 1998, parts of North America experienced a massive ice storm, crushing about 130 transmission towers under the ice weight.