Types of structural load

[edit] Introduction

Structural analysis is a very important part of a design of buildings and other built assets such as bridges and tunnels, as structural loads can cause stress, deformation and displacement that may result in structural problems or even failure.

The building regulations require that structures must be designed and built to be able to withstand all load types that they are likely to face during their lifecycle.

There are a number of different types of load than can act upon a structure, the nature of which will vary according to the design, use, location and materials being used. Design requirements are generally specified in terms of the maximum loads that a structure must be able to withstand.

Loads are generally classified as either dead loads (DL) or live loads (LL):

Dead loads refer to the structure's self weight and generally remain constant during the structure's life.
Live loads, such as traffic loads may vary.

Loads may also be categorised as:

Concentrated loads (or point loads): Single loads that act over a relatively small area, such as column loads.
Line loads: Loads exert a load along a line, such as a partition's weight on the floor.
Distributed (or surface) loads: These exert a load over a surface area, such as the weight of floors and roofing materials.

[edit] Dead loads (DL)

Dead loads, also known as permanent or static loads, are those predominantly associated with the weight of the structure itself, and as such remain stationary and relatively constant over time. Dead loads may include the weight of any structural elements, permanent non-structural partitions, immovable fixtures such as plasterboard, built-in cupboards, and so on.

Dead loads can be calculated by assessing the weights of materials specified and their volume as shown on drawings. This means that in theory, it should be possible to calculate dead loads with a good degree of accuracy. However, structural engineers are sometimes conservative with their estimates, minimising potential deflections, allowing a margin of error and allowing for alterations over time, and so design dead loads often far exceed those experienced in practice.

For more information see: Dead loads.

[edit] Live loads (LL)

Live loads, also known as imposed loads, are usually temporary, changeable and dynamic. These include loads such as vehicle traffic, occupants, furniture and other equipment. The intensity of these loads may vary depending on the time of day, for example an office building may experience increased live loads during week-day work hours but much smaller loads during the night or at weekends.

Live loads may be concentrated or distributed and may involve impact, vibration or acceleration.

For more information see: Live load

[edit] Environmental loads

Environmental loads may act on a structure as a result of topographic and weather conditions.

[edit] Wind load (WL)

Wind loads can be applied by the movement of air relative to a structure, and analysis draws upon an understanding of meteorology and aerodynamics as well as structures. Wind load may not be a significant concern for small, massive, low-level buildings, but it gains importance with height, the use of lighter materials and the use of shapes that my affect the flow of air, typically roof forms. Where the dead weight of a structure is insufficient to resist wind loads, additional structure and fixings may be required.

A building's design wind speed is usually determined from historical records using extreme value theory to predict unusual wind speeds that may occur in the future.

Particular effects that may need to be considered might include:

Corner streams or jets that occur around the corners of buildings.
Vortex shedding that occurs in the wake of a building.
Through-flow, or passage jets, that occurs in a passage through a building or small gap between two buildings.

In complex situations, it may be necessary to undertake wind tunnel testing of building forms to assess the change in air flows caused by the presence of a structure. Increasingly, analysis is also possible using computational fluid dynamics software.

[edit] Snow load (SL)

This is the load that can be imposed by the accumulation of snow and is more of a concern in geographic regions where snowfalls can be heavy and frequent. Significant quantities of snow can accumulate, adding a sizeable load to a structure. The shape of a roof is a particularly important factor in the magnitude of the snow load. Snow falling on a flat roof is likely to accumulate, whereas snow is more likely to fall of a steeper the roof pitch

This may be similar issues in areas of heavy rainfall where ponding may occur.

[edit] Earthquake load

Significant horizontal loads can be imposed on a structure during an earthquake. Buildings in areas of seismic activity need to be carefully analysed and designed to ensure they do not fail if an earthquake should occur.

[edit] Thermal loads

All materials expand or contract with temperature change and this can exert significant loads on a structure. Expansion joints can be provided at points on long sections of structures such as walls and floors so that elements of the structure are physically separated and can expand without causing structural damage.

[edit] Settlement loads

Stresses can occur in buildings if one part settles more than another. A flexible structure will be able to accommodate the small stresses, whereas a stiff structure will need careful design to alleviate the more severe stresses that may be exerted.