Structural load
Encyclopedia
Structural loads or actions are force
s, deformations or accelerations applied to a structure
or its components.
Loads cause stress
es, deformations and displacement
s 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.
Engineers often evaluate structural loads based upon published regulation
s, contract
s, or specifications. Accepted technical standards are used for acceptance testing and inspection
.
Live loads are usually unstable or moving loads. These dynamic loads may involve considerations such as impact, momentum
, vibration
, slosh dynamics
of fluids, etc.
Cyclic loads on a structure can lead to fatigue
damage, cumulative damage, or failure. These loads can be repeated loadings on a structure or can be due to vibration
.
s 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.
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 code
s 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.
. 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 himself is normally responsible for the specifications of these components.
loads may involve considerations such as impact, momentum
, vibration
, slosh dynamics
of fluids, fatigue
, etc.
Live loads, sometimes 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 live loads are produced
Bridge live loads are produced by vehicles traveling over the deck of the bridge.
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.
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.
Force
In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...
s, deformations or accelerations applied to a structure
Structure
Structure is a fundamental, tangible or intangible notion referring to the recognition, observation, nature, and permanence of patterns and relationships of entities. This notion may itself be an object, such as a built structure, or an attribute, such as the structure of society...
or its components.
Loads cause stress
Stress (physics)
In continuum mechanics, stress is a measure of the internal forces acting within a deformable body. Quantitatively, it is a measure of the average force per unit area of a surface within the body on which internal forces act. These internal forces are a reaction to external forces applied on the body...
es, deformations and displacement
Displacement (vector)
A displacement is the shortest distance from the initial to the final position of a point P. Thus, it is the length of an imaginary straight path, typically distinct from the path actually travelled by P...
s in structures. Assessment of their effects is carried out by the methods of structural analysis
Structural analysis
Structural analysis is the determination of the effects of loads on physical structures and their components. Structures subject to this type of analysis include all that must withstand loads, such as buildings, bridges, vehicles, machinery, furniture, attire, soil strata, prostheses and...
. Excess load or overloading may cause structural failure
Structural failure
Structural failure refers to loss of the load-carrying capacity of a component or member within a structure or of the structure itself. Structural failure is initiated when the material is stressed to its strength limit, thus causing fracture or excessive deformations...
, 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.
Engineers often evaluate structural loads based upon published regulation
Regulation
Regulation is administrative legislation that constitutes or constrains rights and allocates responsibilities. It can be distinguished from primary legislation on the one hand and judge-made law on the other...
s, contract
Contract
A contract is an agreement entered into by two parties or more with the intention of creating a legal obligation, which may have elements in writing. Contracts can be made orally. The remedy for breach of contract can be "damages" or compensation of money. In equity, the remedy can be specific...
s, or specifications. Accepted technical standards are used for acceptance testing and inspection
Inspection
An inspection is, most generally, an organized examination or formal evaluation exercise. In engineering activities inspection involves the measurements, tests, and gauges applied to certain characteristics in regard to an object or activity...
.
Types of loads
Dead loads are static forces that are relatively constant for an extended time. They can be in tensile 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
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
, vibration
Vibration
Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...
, slosh dynamics
Slosh dynamics
In fluid dynamics, slosh refers to the movement of liquid inside another object . Strictly speaking, the liquid must have a free surface to constitute a slosh dynamics problem, where the dynamics of the liquid can interact with the container to alter the system dynamics significantly...
of fluids, etc.
Cyclic loads on a structure can lead to fatigue
Fatigue (material)
'In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.Fatigue occurs...
damage, cumulative damage, or failure. These loads can be repeated loadings on a structure or can be due to vibration
Vibration
Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...
.
Loads on Architectural and Civil Engineering Structures
Building codeBuilding code
A building code, or building control, is a set of rules that specify the minimum acceptable level of safety for constructed objects such as buildings and nonbuilding structures. The main purpose of building codes are to protect public health, safety and general welfare as they relate to the...
s 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.
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 code
Building code
A building code, or building control, is a set of rules that specify the minimum acceptable level of safety for constructed objects such as buildings and nonbuilding structures. The main purpose of building codes are to protect public health, safety and general welfare as they relate to the...
s 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.
Dead load
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 carpetCarpet
A carpet is a textile floor covering consisting of an upper layer of "pile" attached to a backing. The pile is generally either made from wool or a manmade fibre such as polypropylene,nylon or polyester and usually consists of twisted tufts which are often heat-treated to maintain their...
. 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
Variance
In probability theory and statistics, the variance is a measure of how far a set of numbers is spread out. It is one of several descriptors of a probability distribution, describing how far the numbers lie from the mean . In particular, the variance is one of the moments of a distribution...
, and the designer himself is normally responsible for the specifications of these components.
Live loads
Live loads, or imposed loads, are temporary, of short duration, or moving. These dynamicDynamics (mechanics)
In the field of physics, the study of the causes of motion and changes in motion is dynamics. In other words the study of forces and why objects are in motion. Dynamics includes the study of the effect of torques on motion...
loads may involve considerations such as impact, momentum
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
, vibration
Vibration
Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...
, slosh dynamics
Slosh dynamics
In fluid dynamics, slosh refers to the movement of liquid inside another object . Strictly speaking, the liquid must have a free surface to constitute a slosh dynamics problem, where the dynamics of the liquid can interact with the container to alter the system dynamics significantly...
of fluids, fatigue
Fatigue
Fatigue may refer to:*Fatigue , structural damage from repeated loading*Fatigue , a state of physical and/or mental weakness*Fatigue , safety implications of tiredness...
, etc.
Live loads, sometimes 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 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.
Environmental loads
These are loads that act as a result of weather, topography and other natural phenomena.- Wind loadsWind engineeringWind engineering analyzes effects of wind in the natural and the built environment and studies the possible damage, inconvenience or benefits which may result from wind. In the field of structural engineering it includes strong winds, which may cause discomfort, as well as extreme winds, such as in...
- Snow, rain and ice loads
- Seismic loadsSeismic loadingSeismic loading is one of the basic concepts of earthquake engineering which means application of an earthquake-generated agitation to a structure...
- TemperatureTemperatureTemperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
changes leading to thermal expansionThermal expansionThermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...
cause thermal loads - PondingPondingPonding is a term that refers to the unwanted pooling of water, typically on a flat roof.-Cause:Most flat roof systems are designed with a slight pitch to shed water off the sides, usually into a scupper system or into an internal drainage system. When a scupper or drain is clogged or fails for...
loads - Lateral pressure of soil, ground water or bulk materials
- Loads from fluids or floods
- Dust loads
Other loads
Engineers must also be aware of other actions that may affect a structure, such as:- Support settlement or displacement
- Fire
- Corrosion
- Explosion
- CreepCreep (deformation)In materials science, creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material....
or shrinkage - Impact from vehicles or machinery
- Loads during construction
Load combinations
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.
See also
- Probabilistic designProbabilistic designProbabilistic design is a discipline within engineering design. It deals primarily with the consideration of the effects of random variability upon the performance of an engineering system during the design phase. Typically, these effects are related to quality and reliability...
- The Hotel New World disasterHotel New World disasterThe Hotel New World emergency occurred on 15 March 1986, and was Singapore's deadliest civil disaster since the Spyros disaster of 12 October 1978. The six-story building situated at the junction of Serangoon Road and Owen Road rapidly collapsed, trapping 50 people beneath the rubble...
was caused by a miscalculation of the dead load of the building. - Influence Lines
External links
External links
- Luebkeman, Chris H., and Donald Petting "Lecture 17: Primary Loads". University of Oregon. 1996http://darkwing.uoregon.edu/~struct/courseware/461/461_lectures/461_lecture17/461_lecture17.html
- Fisette, Paul, and the American Wood Council. "Understanding Loads and Using Span Tables". 1997.http://www.awc.org/technical/spantables/tutorial.htm