Forces, external and internal

• Vector


  A quantity (force or velocity) with direction and magnitude
• Distribution


  • Total area of which the load is applied
  • Related to pressure, where: P = Force/Area
  • Uniform load or point load
  • Vertical elements carry the loads directly into the ground and foundations, and horizontal and inclined elements carry their loads to the vertical elements
  • The connections between elements then become critically important in both load transfer, and the directional change of the load path
• Point loads


  • Can result in the bowing of the material, and the force being placed on the edge of the support rather than shared across the surface resulting in a horizontal, as well as vertical force
  • The edge could be under extreme pressure
• The higher up the structure, the less force the components have to be able to withstand
• Components at fixing points

  • Where directions of change, have multiple types of loads
  • Vertical compression as well as shearing force
• All joints must remain stable, particularly against horizonal forces such as wind loads
• Initial design task


  1. Identify a system of structural members, that when joined together, provide a stable structure against the loads that will be applied

  2. The load path must be traced through the structure

  3. Each element must be capable of resisting the accumulation of loads
• Stress


  When materials are stressed (normal, to a degree), the undergo dimensional changes known as strains and will appear to distort the structure
• Too much stress and the element will break
• General approach to the analysis of the physical behaviour

  1. The loads define the conditions under which the structure has to perform

  2. Failure or loss of performance can occur if the loads: Disturb the structure making it unstable, Distort to far so it cracks finishes, transfers loads to a non-load bearing members, or looks/and or feels unsafe, Destroy part of a structure by breaking members or connections at the joints

  3. The proposal or suggestion is for an arrangement of structural members with choice of materials and approximate sizes to support the load

  4. An evaluation of the response of the proposed members to the loads is by analysis of the distortions and the distribution and size of the forces within the structural members to see if they will maintain the load without collapse
• Magnitude


  The total weight of the force to be taken by the structure
• Distribution


  • Distributions are often seen as a single point, uniformally distributed, or possibly moving and impact loads
• Direction


  Completes the specification. Most act vertically downwards, some at other angles (thrust at the bearing of an arch). Some such as wind acting horizontally or even upwards
• Permanent loads


  • Large machinery or heavy fixings/storage must be treated with the characteristics of dead loads
• Wind distribution and direction

  • Can be problematic when calculating the loading. Flowing over and around can cause low pressure areas
• Another allowance must be accidental loading that can cause a local structure failure that can lead to a more general failure of a major part of the building
• Performance specification


  Defining the conditions that should be designed for either as the limits considered as normal or in terms of extreme yet potentially disastrous conditions
• Majority designed for dead, imposed, and wind loads, plus measures against disproportionate collapse
• Foundations


  • Depending on the quality of the ground, the vertical forces acting on the foundations can either cause a compressive stress in the soil (Bulb of pressure) which leads to settlement, or shear stress in the soil (Slip (shear) plane) where the soil moves and travels upwards causing a heave at the surface and resulting in collapse