Structural principles

[edit] Introduction

Within the context of the built environment, the term ‘structure’ refers to anything that is constructed or built from different interrelated parts with a fixed location on the ground. The structure is responsible for maintaining the shape and form under the influence of subjected forces.

[edit] Forces

It is important that the strength and stability of a structure and its individual components must be considered. Structural analysis is used to calculate the effects of the forces acting on any component and on the structure overall.

Three properties of forces that should be considered are:

• Magnitude: The size of the force.
• Direction: The direction in which the force is acting.
• Position: The position on which the force acts.

Isaac Newton developed three laws of motion:

• First law: An object will remain at rest or in uniform motion unless compelled to do otherwise by some external force acting on it.
• Second law: A force is caused by an acceleration acting on an object.
• Third law: Action and reaction are equal and opposite.

One of the main structural principles is that elements such as the roof, floor and walls must remain stationary. For this to happen, there needs to be an equilibrium of forces – when the forces acting on them are equal and opposite. Under loading, some deflection and deformation – in the form of bending and buckling – may occur, and if this movement is not allowed for then structural failure may be the result. Therefore, a principle of structures is that they be designed to maintain a state of equilibrium; resisting external loads without moving.

The study of the causes and effects of stationary forces acting on rigid objects is statics. When a structure is stationary or in equilibrium, it is a ‘static body’. For a structure to remain static, three basic equations must hold true:

• Sum of all vertical forces must be zero.
• Sum of all horizontal forces must be zero.
• Sum of all bending forces, or moments, must be zero.

[edit] Loads

Another principle is that the structure should be capable of withstanding the most severe combination of forces that are likely to be applied. This is determined by the geolocation relevant to the structure, such as in places where strong winds or heavy rain are common weather conditions.

The main types of load which a structure must be able to resist are:

• Dead loads: Such as the fixtures and structural elements.
• Live loads: Such as occupants, furniture, traffic.
• Environmental loads: Such as wind, snow, earthquake, settlement.

For more information, see Types of structural load.

[edit] Materials

The effectiveness of a structure depends on the mechanical properties of the materials from which it is constructed. These properties include:

• Strength.
• Toughness.
• Elasticity.
• Plasticity.
• Ductility.
• Malleability.
• Brittleness.
• Hardness.

For more information, see Construction materials.

[edit] Structural members

Structural members are the primary load bearing components of a building, and each have their own structural properties which need to be considered. Such members include:

• Beams: Horizontal members which transfer loads to supports.
• Columns: Vertical members which transfer compressive loads to the ground.
• Bracing: Members that interconnect and stiffen columns and beams.
• Roof trusses: Load-bearing frames constructed of connected triangular shapes.
• Retaining walls: Support soil where a sloping site requires excavation.
• Concrete slabs: Span horizontally between supports, used as floors and sometimes as roof systems.
• Footings: Transfer load from the structure to the foundations.

[edit] Find out more

[edit] Related articles on Designing Buildings Wiki

• Bearing capacity.
• Bending moment.
• Building science.
• Building technology.
• Concept structural design of buildings.
• Construction materials.
• Detailed structural design.
• Elements of structure in buildings.
• Multi-storey structure.
• Shear force.
• Stiffness.
• Structural engineer.
• Structural engineering codes.
• Structural steelwork.
• Structural vibration.
• Structure definition.
• Substructure.
• Superstructure.
• The design of temporary structures and wind adjacent to tall buildings.
• Tube structural system.
• Types of structural load.
• Types of structure.
• Vibrations.