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Common Building Materials

4 questions

All types4Multiple choice2True / False2
Question 1Question →

Plain concrete is a common building material that possesses high compressive strength but very low tensile strength.

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Answer: True

Answer

True
The statement accurately reflects the most fundamental mechanical property of concrete: it resists crushing (compression) exceptionally well but fails easily under stretching or bending forces (tension).

Step-by-Step Solution

1
Evaluate the mechanical properties of plain concrete described in the statement.
The statement claims concrete is strong against compressive forces (squeezing) and weak against tensile forces (pulling).
Verifying basic material properties is necessary to determine the truth value of the statement.
2
Compare the claim against standard structural engineering principles.
The claim matches the real-world behavior of concrete.
Concrete is a ceramic-like material that resists crushing exceptionally well but is brittle and fails under tension.

Key Concept

Compressive vs. Tensile Strength of Building Materials
Question 2Question →

When constructing a building framework in an earthquake-prone region, engineers typically choose structural steel over cast iron. Which of the following best explains why structural steel is the superior building material for this specific application?

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Answer: Structural steel is highly ductile, allowing it to bend and absorb seismic shock without suddenly fracturing.

Answer

Structural steel is highly ductile, allowing it to bend and absorb seismic shock without suddenly fracturing.
Structural steel possesses high ductility, which is a material's ability to undergo significant plastic deformation before rupture. During an earthquake, a building framework experiences extreme and shifting lateral loads. A ductile material like structural steel will bend and yield, safely absorbing the seismic energy without experiencing a sudden, catastrophic fracture. Cast iron, by contrast, is brittle and would crack or shatter under the same dynamic, bending stress.

Step-by-Step Solution

1
Identify the primary forces exerted on a building during an earthquake.
Earthquakes introduce dynamic, unpredictable shocks and vibrations that cause structures to sway, creating rapidly alternating tension, compression, and shear forces.
Understanding the environmental stress is the first step in selecting the appropriate building material.
2
Compare the mechanical properties of structural steel and cast iron under dynamic stress.
Cast iron is hard but brittle, meaning it snaps or shatters under sudden, bending stress. Structural steel is ductile, meaning it can stretch and deform elastically and plastically before failing.
Matching material behavior to the required load type reveals which material will survive the event.
3
Determine which property prevents building collapse during seismic activity.
Ductility is the preferred property because the material absorbs the energy of the earthquake by flexing. A building framework that bends is significantly safer than one that snaps abruptly.
This confirms why structural steel is specifically chosen over brittle alternatives for seismic zones.

Key Concept

Ductility vs. Brittleness in Structural Materials
Estimated Time:45s
Question 3Question →

When used as a structural building material, timber can support a significantly heavier compressive load if the weight is applied perpendicular to its grain rather than parallel to its grain.

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Answer: False

Answer

False. Timber is much stronger in compression when loaded parallel to the grain.
The statement is false because the fibrous nature of timber means it acts like a series of vertical columns. It possesses its maximum compressive strength when loaded parallel to the grain, not perpendicular.

Step-by-Step Solution

1
Analyze the physical structure of wood as a building material.
Wood consists of long, hollow cellulose fibers that are aligned in one primary direction, which forms the wood's grain.
Understanding the cellular structure is required to determine how the material handles mechanical stress.
2
Evaluate the effect of a compressive force relative to the grain orientation.
Compressing wood parallel to the grain pushes against the solid ends of the fibers, providing high load-bearing resistance. Compressing it perpendicular to the grain easily crushes the hollow fiber walls.
This physical behavior proves that the claim of higher strength perpendicular to the grain is incorrect.

Key Concept

Directional strength (anisotropy) and load-bearing properties of timber
Question 4Question →

Reinforced concrete is a widely used composite building material that combines poured concrete with steel reinforcing bars (rebar). While the steel is primarily added to provide tensile strength, what other critical physical property must the steel and concrete share to prevent the structure from cracking or tearing itself apart during seasonal temperature changes?

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Answer: They must have nearly identical coefficients of thermal expansion.

Answer

The steel and concrete must have nearly identical coefficients of thermal expansion so they expand and contract together at the same proportional rate when the temperature changes.
For a composite material like reinforced concrete to remain structurally sound across differing environments, its component materials must expand and contract at the same proportional rate when the temperature changes. Because structural steel and concrete share nearly identical coefficients of thermal expansion, they change size together in harmony. This prevents the creation of internal stresses that would otherwise destroy the bond and crack the brittle concrete.

Step-by-Step Solution

1
Identify the effect of temperature changes on physical materials.
Almost all building materials expand when heated and contract when cooled (thermal expansion).
Temperature changes are an unavoidable environmental stressor on outdoor structures.
2
Analyze the mechanics of a composite material like reinforced concrete.
Steel is embedded tightly inside the concrete, creating a solid, bonded structural unit.
The materials must work together to distribute loads.
3
Evaluate what happens if the two bonded materials react differently to heat.
If steel expanded significantly more than concrete when heated, the internal swelling of the rebar would shatter the brittle concrete from the inside.
Mismatched expansion rates create massive internal shear and tensile stresses.
4
Conclude the necessary shared property.
They must expand and contract by the exact same amount per degree of temperature change, meaning they have the same coefficient of thermal expansion.
This compatibility ensures the composite material maintains structural integrity year-round.

Key Concept

Thermal expansion compatibility in composite building materials
Estimated Time:45s
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