Material Selection by Stress State — Tension, Compression & Shear

Material Selection by Stress State — Tension, Compression & Shear

Outline

  1. Introduction: Aligning physical material properties with intrinsic stress states.

  2. Compression Masters: Stone, concrete, and masonry under gravity loads.

  3. Tension Specialists: Steel cables, wrought iron, and straight-grain timber.

  4. Shear & Torsion Alignment: Wood grain orientation and isotropic vs. anisotropic behavior.

  5. The Hybrid Principle: Combining complementary materials in unified structural assemblies.

Content Draft

Page 9: Material Selection by Stress State — Tension, Compression & Shear

Structural integrity fails when a material is forced to act against its natural mechanical strengths. Material honesty requires matching specific elements to the primary stress states they excel at carrying: Compression, Tension, and Shear.

  COMPRESSION                  TENSION                   SHEAR
 (Pushing In)               (Pulling Apart)           (Sliding Past)

    │      │                   ▲      ▲                   ┌───┐►
    ▼      ▼                   │      │                   └───┘
 ┌────────────┐             ┌────────────┐             ┌───┐
 │   Block    │             │   Cable    │             │   │◄
 └────────────┘             └────────────┘             └───┘
    ▲      ▲                   │      │
    │      │                   ▼      ▼
 (Stone/Concrete)           (Steel/Fiber)             (Fasteners/Pins)

1. Compression Specialists

Materials with high mass and crystalline or granular structures excel in compression but perform poorly in tension.

  • Granite / Cast Stone / Unreinforced Concrete: Outstanding compressive strength (often exceeding $30\text{–}100\text{ MPa}$), but virtually zero tensile strength ($2\text{–}5\text{ MPa}$).

  • Structural Truth: Stone should always act as a post, arch, or gravity mass—never as an unreinforced horizontal lintel across long spans.

2. Tension Specialists

Materials made of linear fibers or drawn metallic grains handle being pulled apart across long distances without necking or snapping.

  • Structural Steel Cables & Rods: Ultimate tensile strength reaching $400\text{–}1800\text{ MPa}$.

  • Straight-Grain Timber: Exceptional strength parallel to the grain.

  • Structural Truth: Use thin steel rods or tension cables to suspend loads, keeping the structural element as light as physically possible.

3. The Hybrid Assembly: Reinforced Concrete & Post-Tensioned Timber

Combining materials allows each component to handle its optimal stress state within a single system:

  • Reinforced Concrete: The concrete mass handles $100\%$ of the downward compression forces, while embedded deformed steel rebar handles $100\%$ of the tensile forces generated at the bottom of the span under bending loads.

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