Loads cause stresses, deformations, and displacements in structures. Assessment of their effects is carried out by the methods of structural analysis. Excess load or overloading may cause structural failure, and hence such possibility should be either considered in the design or strictly controlled.
Mechanical structures, such as aerospace vehicles (e.g. aircraft, satellites, rockets, space stations, etc...), marine craft (e.g. boats, submarines, etc.), and material handling machinery have their own particular structural loads and actions.
Types of loads
Dead loads are static forces that are relatively constant for an extended time. They can be in tension or compression. The term can refer to a laboratory test method or to the normal usage of a material or structure.
Live loads are usually unstable or moving loads. These dynamic loads may involve considerations such as impact, momentum, vibration, slosh dynamics of fluids, etc. An impact load is one whose time of application on a material is less than one-third of the natural period of vibration of that material.
Loads on Architectural and Civil Engineering Structures
Building codes require that structures be designed and built to safely resist all actions that they are likely to face during their service life, while remaining fit for use. Minimum loads or actions are specified in these building codes for types of structures, geographic locations, usage and materials of construction. Dynamic Loads Structural loads are split into categories by their originating cause. Of course, in terms of the actual load on a structure, there is no difference between dead or live loading, but the split occurs for use in safety calculations or ease of analysis on complex models as follows:
To meet the requirement that design strength be higher than maximum loads, Building codes prescribe that, for structural design, loads are increased by load factors. These load factors are, roughly, a ratio of the theoretical design strength to the maximum load expected in service. They are developed to help achieve the desired level of reliability of a structure based on probabilistic studies that take into account the load's originating cause, recurrence, distribution, and static or dynamic nature.
The dead load includes loads that are relatively constant over time, including the weight of the structure itself, and immovable fixtures such as walls, plasterboard or carpet. Roof is also a dead load. Dead loads are also known as Permanent loads.
The designer can also be relatively sure of the magnitude of dead loads as they are closely linked to density and quantity of the construction materials. These have a low variance, and the designer is normally responsible for specifying these components.
Live loads, or imposed loads, are temporary, of short duration, or a moving load. These dynamic loads may involve considerations such as impact, momentum, vibration, slosh dynamics of fluids, fatigue, etc.
Live loads, sometimes also referred to as probabilistic loads include all the forces that are variable within the object's normal operation cycle not including construction or environmental loads.
Roof and Floor live loads are produced
- during maintenance by workers, equipment and materials, and
- during the life of the structure by movable objects such as planters and by people.
Bridge live loads are produced by vehicles traveling over the deck of the bridge.
These are loads that act as a result of weather, topography and other natural phenomena.
- Wind loads
- Snow, rain and ice loads
- Seismic loads
- Temperature changes leading to thermal expansion cause thermal loads
- Ponding loads
- Lateral pressure of soil, ground water or bulk materials
- Loads from fluids or floods
- Dust loads
Engineers must also be aware of other actions that may affect a structure, such as:
- Support settlement or displacement
- Creep or shrinkage
- Impact from vehicles or machinery vibration
- Loads during construction
A load combination results when more than one load type acts on the structure. Design codes usually specify a variety of load combinations together with Load factors (weightings) for each load type in order to ensure the safety of the structure under different maximum expected loading scenarios. For example, in designing a staircase, a dead load factor may be 1.2 times the weight of the structure, and a live load factor may be 1.6 times the maximum expected live load. These two "factored loads" are combined (added) to determine the "required strength" of the staircase.
The reason for the disparity between factors for dead load and live load, and thus the reason the loads are initially categorized as dead or live is because while it is not unreasonable to expect a large number of people ascending the staircase at once, it is less likely that the structure will experience much change in its permanent load.
Aircraft Structural Loads
For aircraft, loading is divided into two major categories: limit loads and ultimate loads. Limit loads are often just flight loads and are further divided into maneuvering loads and gust loads. Ultimate loads are crash loads. Maneuvering loads are determined based on the performance limits of the aircraft whether imposed by the flight manual or by the actual aerodynamic performance of aircraft. Gust loads are determined statistically are taken from guidelines or requirements given by the applicable regulatory agency. Crash loads are loosely bounded by the ability of humans to survive extreme accelerations and are also typically taken from regulations.
Other loads that may be critical are pressure loads (for pressurized, high-altitude aircraft) and ground loads. Loads on the ground can be from adverse braking or maneuvering during taxi.
Finally, you cannot discuss aircraft loading without hearing about fatigue and damage tolerance. Aircraft are constantly subjected to cyclic loading. These cyclic loads initiate cracks and cause them to grow.
Thermal loading is rarely considered for the analysis of the primary structure of aircraft but it can become critical under extreme operating conditions and should be examined where materials of disparate coefficients of thermal expansion are joined.
- Probabilistic design
- The Hotel New World disaster was caused by a miscalculation of the dead load of the building.
- Influence Lines
- ASCE/SEI 7-05 Minimum Design Loads for Buildings and Other Structures. American Society of Civil Engineers. 2006. p. 1. ISBN 0-7844-0809-2.
- "220.127.116.11". Eurocode 0: Basis of structural design EN 1990. Bruxelles: European Committee for Standarization. 2002.
- Avallone, E.A., and Baumeister, T. (ed.). Mark's Standard Handbook for Mechanical Engineers (10th ed.). McGraw-Hill. pp. 11–42. ISBN 0-07-004997-1.
- "2.2.1(1)". Eurocode 0: Basis of structural design EN 1990. Bruxelles: European Committee for Standarization. 2002.
- "1604.2". International Building Code. USA: International Code Council. 2000. p. 295. ISBN 1-892395-26-6.
- "2.2.5(b)". Eurocode 0: Basis of structural design EN 1990. Bruxelles: European Committee for Standarization. 2002.
- Rao, Singiresu S. (1992). Reliability Based Design. USA: McGraw-Hill. pp. 214–227. ISBN 0-07-051192-6.
- 2006 International Building Code Section 1602.1.
- EN 1990 Euro code – Basis of structural design section 4.1.1
- EN 1991-1-1 Euro code 1: Actions on Structures – Part 1-1: General actions – densities, self-weight, imposed loads for buildings section 3.2