Introduction to Structures

Posted on Sep 11, 2024 in Technology

What is a Structure?

A structure is something that supports an object or a load.

Requirements of a Structure

A structure must:

  • Be strong enough to support its own weight and any load that is put on it.
  • Be stable (not topple over easily when a force acts on it).
  • Resist deformation without losing its shape more than expected.

Types of Structures

There are three types of structures:

  • Mass Structures: Solid structures that rely on their own weight to resist loads. Examples include Roman bridges, aqueducts, walls, and domes of ancient stone churches.
  • Frame Structures: Made from many small parts (called members) joined together. Examples include electric towers, some bridges (e.g., suspension bridges), cranes, buildings, and scaffolding. There are three types of frame structures:
    • Triangular Structures: Made of metal or wooden beams joined together to form triangles, which are the only geometric figures that don’t deform under pressure.
    • Post and Beam Structures: Composed of vertical and horizontal shafts (posts and beams) firmly joined to produce three-dimensional shapes. Modern buildings, housing, and office blocks are frame structures. Commonly used materials include steel, reinforced concrete, and wood.
    • Suspended Structures: Use cables called suspenders from which the structure hangs. Cables are used to fasten bridges, marquees, antennas, and towers. These structures can cover large spaces like stadiums and pavilions without columns or pillars.
    The term “triangulation” describes the use of triangles arranged together to form a frame, which is one of the most economical ways of building structures.
  • Shell Structures: Made or assembled from shaped panels to make one piece. Examples include tin cans, bottles, car bodies, and aeroplane bodies.

Natural Structures

Structures are not new. Nature produced the first structures before humans. A tree is a natural structure that carries the weight of its branches and resists strong winds.

Manufactured Structures

A manufactured structure is a structure built by human beings.

Many of Nature’s structures have been copied by humans. For example, the shell of a snail and the body of a modern car are both shell structures designed to protect their occupants.

Manufactured structures include bridges, electricity pylons, and tall buildings, but some common structures are quite simple. Most objects resist loads, and most objects are also structures.

Functions of Structures

All structures perform one or more of these functions:

  • Support: To hold something upright and steady.
  • Span: To reach across a distance or a gap.
  • Contain: To protect something.

Forces Acting on Structures

A successful structure must resist all forces acting on it without collapsing.

Types of Forces

  • Static Forces: Structure weight and any load permanently attached to it.
  • Dynamic Forces: Produced by wind, sea waves, vehicles, people, etc. These forces are usually much greater than static forces, difficult to predict, and are the most common reason for structural failures.

Effects of Forces

External forces or loads cause internal stresses. Forces can:

  • Bend
  • Pull
  • Press
  • Twist

Types of Stress

Stress is the internal tension that bodies experience when forces are applied. There are five types of stress, depending on the external force or load:

  • Tension: A force that tries to pull something apart. A structural member in tension is called a tie, which resists tensile stresses.
  • Compression: A force that tries to squash something together. A structural member in compression is called a strut, which resists compressive stresses.
  • Torsion: A turning or twisting force.
  • Shear: Created when two opposite forces try to cut something in two, causing one part of the structure to slide over another.
  • Bending: Forces acting at an angle to a member, tending to make it bend.

Members in a Structure

The different parts of a frame structure are called members.

Different members within a structure resist different kinds of forces. Choosing the most suitable member for the job is important.

Factors Affecting Member Resistance

How well a structural member resists forces depends on:

  • The material it is made of
  • Its shape
  • Its size

Member Shapes and Forces

  • Flat strips, cables, or wires resist tension forces well but are poor in compression.
  • Angle girders, I-girders, and similar sections are used for compression forces.
  • Beams resist bending forces and are used in bridges and buildings to span gaps and carry loads without deflecting. Beams are named after their shapes (e.g., I-beam).
  • A cantilever is a beam supported at one end only (e.g., a diving board).

Structural Conditions

There are three conditions for a structure to work well:

  • Stability: The capacity of a structure to remain upright and not fall over. A structure’s center of gravity must be centered over its base and close to the ground for greater stability.
  • Resistance: The capacity of a structure to bear the tensions it’s subjected to without breaking. Resistance depends on the structure’s shape and the material used.
  • Rigidity: All bodies deform slightly under force, but this deformation must not hinder the structure’s function. Triangular structures or cross-bracing with cables can prevent deformation.

Ensuring Good Structural Conditions

  1. Triangulation: Used in cranes, electricity pylons, and roof supports to make them rigid. Triangulation prevents a structure from being forced out of shape, making it rigid. The triangle is the most rigid frame structure.
  2. Gusset Plates: Pieces of material used to brace and join members in a structure. They can be triangular or corner gusset plates.
  3. Support Brackets: Can be used instead of triangulation.
  4. Straps or Tensioners: Used to prevent the structure from collapsing.