What are structural loads and how do they affect buildings?
A structural load is a force, deformation, or acceleration applied to structural elements. A load causes stress, deformation and displacement in a structure. Structural analysis assesses the effects of loads on structures and structural elements.
The determination of the loads acting on a structure can be a complex problem. The nature of the loads and method of analysis can vary with the: architectural design, the materials, the use of the building, location of the structure etc. The loading conditions on a structure will change during its full life cycle and the design needs to account for and combination of these it may undergo.
The initial loadings are also increase by a predetermined amount as prescribed in the British Standard or Eurocode in order to provide a factor of safety in the design.
Different types of loads
Dead load
Dead loads, also known as permanent or static loads, are those that remain relatively constant over time and comprise of, for example, the weight of a building’s structural elements, such as beams, walls roof and structural flooring components. Dead loads encompass the weight of the structure or other fixed elements before any live loads are taken into consideration.
Live load
Live loads refer to a load that can change over time. Anything in a building that is not fixed to the structure can result in a live load since it is not static. The live load we design for is determined by the intended use of the building.
Thermal loads
All building materials expand or contract with temperature change. Long continuous buildings will expand, and it is necessary to consider the expansion stresses.
Impact loads
Impact loads are sudden or rapid loads applied on a structure over a relatively short period of time compared with other structural loads. They cause larger stresses in structural members than those produced by gradually applied loads of the same magnitude. Examples of impact loads include: moving vehicles, vibrating machinery or dropped weights. Blast loading is also a potential impact from gas explosions etc, depending on the size/height and risk category of the building it may need to be designed for disproportionate collapse and as such blast loading design can be incorporated.
Wind loads
Wind loads are pressures exerted on structures by wind flow. Wind forces have been the cause of many structural failures, especially in coastal regions. The speed and direction of wind flow vary continuously, making it difficult to predict the exact pressure applied by the wind on existing structures.
Application of loads
When a structural member carries a force it will undergo internal stresses and strains. The different applications of load are as follows;
- Shear: This is effectively a cutting force through a structural member.
- Moment: A force multiplied by a distance equates to a moment. It contributes to the bending of a structural member.
- Torsion: A twisting motion on an element.
- Axial: This is a force applied to the primary axis of the beam. The best example of this is a column.
- Deflection: Although not a type of load this is another factor to consider in design. Considered as a ‘serviceability’ requirement a designer must ensure the structure is fit for purpose and avoid excessive deflections. They must also ensure excessive/perceptible vibrations are inherently designed into the element.
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