Structural Engineering Fundamentals: Forces, Stresses, and Design Principles

Understanding Structures: Definition and Types

A structure is the set of elements intended to resist the forces acting upon it, providing rigidity and maintaining its shape.

Examples of Structures

Natural Structures

• Trees
• Bones
• Spiderwebs

Artificial Structures

• Buildings
• Bridges
• Furniture

Types of Artificial Structures

Framework Structures

These structures form the skeletal framework of an object, providing its primary support and shape.

Laminar or Plate Structures

These structures surround and protect an object, typically composed of thin sheets of metal, plastic, or similar materials.

Forces and Stresses on Structures

Key Concepts: Force, Load, and Stress

Defining Force

A force is any influence that can deform a body (a static effect) or alter its state of motion or rest (a dynamic effect).

Understanding Load

A load is the external force that acts on a structure. A structure would collapse if it is not sufficiently rigid to withstand the applied load.

Internal Stress Explained

Stress refers to the internal resistance or tension caused within an object by external loads or by the object's own weight.

Main Types of Structural Stresses

Compression Stress

Compression forces attempt to flatten or compress a body. Opposing forces move inwards towards one another along the same trajectory and in opposite directions. The body tends to become shorter under compression.

Traction Stress

Traction forces attempt to stretch the object they are acting upon. Opposing forces move outwards away from the body along the same trajectory but in opposite directions. The body tends to become longer under traction.

Bending Stress

Bending forces attempt to bend a body. This stress acts on elements that are supported at their extremities and have a weight applied to them. It is a combination of both compression and traction.

Shear or Cutting Stress

Shear or cutting forces act like two scissor blades: they are very close together, moving in opposite directions, as they try to cut an object. This stress is exemplified when an object is placed over the edge of a table and struck to break it.

Torsion Stress

Torsion forces attempt to twist a body. They try to turn the ends of the body in opposite directions.

Triangulated Structures: Stability and Design

Advantages of Triangulated Structures

• Highly rigid and strong (offering increased resistance)
• Remarkably lightweight

Why Triangles are Stable

The triangle is the only polygon that is not deformed when a force acts upon it. The stress triangle shows a perfect balance (a steady state) between the forces of compression and traction.

Applying Triangulation in Design

To design a polygon-shaped structure that cannot be deformed, we use triangulation. This principle dictates that all polygons formed by the structural bars must be based on the triangle.

Bracing for Structural Integrity

Diagonal supports are commonly used for triangulation. This prevents square frames from being deformed. This technique is known as bracing.