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Structural load
Structural load
Structural load
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A structural load or structural action is a mechanical load (more generally a force) applied to structural elements.[1][2] A load causes stress, deformation, displacement or acceleration in a structure. Structural analysis, a discipline in engineering, analyzes the effects of loads on structures and structural elements. Excess load may cause structural failure, so this should be considered and controlled during the design of a structure. Particular mechanical structures—such as aircraft, satellites, rockets, space stations, ships, and submarines—are subject to their own particular structural loads and actions.[3] Engineers often evaluate structural loads based upon published regulations, contracts, or specifications. Accepted technical standards are used for acceptance testing and inspection.

Types

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In civil engineering, specified loads are the best estimate of the actual loads a structure is expected to carry. These loads come in many different forms, such as people, equipment, vehicles, wind, rain, snow, earthquakes, the building materials themselves, etc. Specified loads also known as characteristic loads in many cases.

Buildings will be subject to loads from various sources. The principal ones can be classified as live loads (loads which are not always present in the structure), dead loads (loads which are permanent and immovable excepting redesign or renovation) and wind load, as described below. In some cases structures may be subject to other loads, such as those due to earthquakes or pressures from retained material. The expected maximum magnitude of each is referred to as the characteristic load.

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 variable or moving loads. These can have a significant dynamic element and 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.

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.

Imposed loads are those associated with occupation and use of the building; their magnitude is less clearly defined and is generally related to the use of the building.

Loads on architectural and civil engineering structures

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Structural loads are an important consideration in the design of buildings. 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.[4] Minimum loads or actions are specified in these building codes for types of structures, geographic locations, usage and building materials.[5] Structural loads are split into categories by their originating cause. 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.

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[6] based on probabilistic studies that take into account the load's originating cause, recurrence, distribution, and static or dynamic nature.[7]

Dead load

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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 carpet. The roof is also a dead load. Dead loads are also known as permanent or static loads. Building materials are not dead loads until constructed in permanent position.[8][9][10] IS875(part 1)-1987 give unit weight of building materials, parts, components.

Live load

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Imposed load (live load)

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 and material fatigue.

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 people.

Bridge live loads are produced by vehicles traveling over the deck of the bridge.

Environmental loads

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Live snow load

Environmental loads are structural loads caused by natural forces such as wind, rain, snow, earthquake or extreme temperatures.

Other loads

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Engineers must also be aware of other actions that may affect a structure, such as:

Load combinations

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A load combination results when more than one load type acts on the structure. Building 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 size of the load factor is based on the probability of exceeding any specified design load. Dead loads have small load factors, such as 1.2, because weight is mostly known and accounted for, such as structural members, architectural elements and finishes, large pieces of mechanical, electrical and plumbing (MEP) equipment, and for buildings, it's common to include a Super Imposed Dead Load (SIDL) of around 5 pounds per square foot (psf) accounting for miscellaneous weight such as bolts and other fasteners, cabling, and various fixtures or small architectural elements. Live loads, on the other hand, can be furniture, moveable equipment, or the people themselves, and may increase beyond normal or expected amounts in some situations, so a larger factor of 1.6 attempts to quantify this extra variability. Snow will also use a maximum factor of 1.6, while lateral loads (earthquakes and wind) are defined such that a 1.0 load factor is practical. Multiple loads may be added together in different ways, such as 1.2*Dead + 1.0*Live + 1.0*Earthquake + 0.2*Snow, or 1.2*Dead + 1.6(Snow, Live(roof), OR Rain) + (1.0*Live OR 0.5*Wind).

Aircraft structural loads

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For aircraft, loading is divided into two major categories: limit loads and ultimate loads.[11] Limit loads are the maximum loads a component or structure may carry safely. Ultimate loads are the limit loads times a factor of 1.5 or the point beyond which the component or structure will fail.[11] Gust loads are determined statistically and are provided by an agency such as the Federal Aviation Administration. Crash loads are loosely bounded by the ability of structures to survive the deceleration of a major ground impact.[12] 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 taxiing. Aircraft are constantly subjected to cyclic loading. These cyclic loads can cause metal fatigue.[13]

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Structural load

View on Grokipedia
from Grokipedia
A structural load is a force, deformation, or acceleration applied to a structure or its components during its intended use, causing stress, deformation, or displacement within the material. In structural engineering, these loads are fundamental to the design and analysis of buildings, bridges, and other infrastructure, ensuring they remain safe, stable, and functional under various conditions. Structural loads are broadly classified into four main categories: dead loads, which are permanent and include the self-weight of the structure and fixed components; live loads, which are temporary and variable, arising from occupancy, furniture, vehicles, or equipment; impact loads, which involve sudden dynamic forces such as those from moving machinery or falling objects; and environmental loads, which encompass natural forces like wind, snow, rain, earthquakes, and floods. Engineers must account for these loads' magnitudes, directions, and durations, often using probabilistic methods to predict maximum probable values over the structure's lifespan. The (ASCE) standard ASCE/SEI 7-22, titled Minimum Design Loads and Associated Criteria for Buildings and Other Structures, establishes the current minimum requirements for determining and combining these loads in the United States, covering dead, live, , , , , , atmospheric ice, seismic, , and fire loads. This standard coordinates with material-specific codes (e.g., from ACI and AISC) and incorporates load factors—such as 1.2 for dead loads and 1.6 for live loads—to address uncertainties and ensure structural integrity against ultimate limit states like or excessive deformation. Proper consideration of structural loads prevents failures, underscoring their role in public safety and economic resilience.

Fundamentals

Definition

A structural load refers to any external force, deformation-inducing action, or applied to a , which generates internal forces and stresses within its components. These loads encompass mechanical actions such as tension, compression, shear, bending, and torsion, arising from external influences like or motion, and are fundamental to analyzing how structures respond to their environment. The concept of structural loads evolved in 18th-century engineering, with foundational contributions from Leonhard Euler and Daniel Bernoulli, who developed the Euler-Bernoulli beam theory around 1750 to model beam deflections under applied forces. This theory provided the initial mathematical framework for understanding load effects on elastic beams, assuming small deflections and plane sections remaining plane. Euler's work in his 1744 publication Methodus Inveniendi Lineas Curvas and subsequent collaborations with Bernoulli established principles still central to modern structural analysis. In the International System of Units (SI), structural loads are quantified using newtons (N) for point or distributed forces, pascals (Pa) for pressure or stress (1 Pa = 1 N/m²), and newton-meters (Nm) for moments or torques. These units derive from base SI measures of mass (kilogram), length (meter), and time (second). At its core, a structural load as a force PP follows Newton's second law of motion, expressed as
P=m×a,P = m \times a,
where mm is the mass of the affected body and aa is the acceleration, providing the basis for both static (where a=0a = 0, so PP balances other forces) and dynamic load calculations in engineering.

Importance in Design

Structural loads play a pivotal role in preventing catastrophic failures by ensuring that designs account for all anticipated forces acting on a . A notable historical example is the collapse of the in 1940, which occurred due to underestimated dynamic wind loads causing aeroelastic flutter and excessive torsional oscillations. This incident, which resulted in the bridge's complete destruction just four months after opening, underscored the necessity of accurately predicting and mitigating dynamic effects to avoid and instability in long-span structures. In modern engineering practice, the consideration of structural loads is deeply integrated into standardized design codes that provide systematic methods for load determination and application. The ASCE 7 standard, titled Minimum Design Loads and Associated Criteria for Buildings and Other Structures, outlines requirements for various loads including dead, live, wind, snow, and seismic forces, ensuring compliance with safety and performance criteria across the United States. Similarly, the Eurocode 1 (EN 1991) series establishes a harmonized framework for actions on structures in Europe, covering permanent, variable, and accidental loads to facilitate consistent design practices across member states. These codes mandate the use of safety factors to amplify design loads or reduce material strengths, introducing a margin against uncertainties such as material variability, construction tolerances, and load exceedances. The (FS), defined as the ratio of a material's ultimate strength to the allowable stress under conditions (FS = ultimate strength / allowable stress), typically ranges from 1.5 to 3 for building structures, depending on the load type and material. This range balances reliability with economic feasibility; for instance, a FS of 2.0 is common for buildings to account for potential overloads while avoiding overdesign. By incorporating such factors, engineers enhance structural resilience, minimizing the risk of or disproportionate damage. Neglecting proper load assessment carries profound economic and societal consequences, with global direct economic losses from disasters—many involving structural failures due to inadequate load —exceeding $202 billion annually as of 2025. These impacts extend beyond immediate repair expenses to include disruptions in transportation, utilities, and , amplifying indirect losses through reduced and heightened premiums. For example, events like bridge failures or building collapses not only endanger lives but also strain public resources, highlighting the imperative for rigorous load-inclusive design to safeguard societal .

Classification

Static and Dynamic Loads

Static loads are forces applied to a structure that remain constant or vary very slowly over time, such that the structure's response reaches equilibrium without significant inertial effects. These loads do not cause appreciable vibrations or accelerations, allowing the structure to deform gradually under the applied force. A representative example is the self-weight of the structural members themselves, which acts continuously and predictably. In contrast, dynamic loads are time-varying forces that induce accelerations, , or oscillations in the due to their rapid changes in magnitude, direction, or point of application. Such loads generate inertial forces proportional to the of the , amplifying the overall response beyond what a static would predict. Representative examples include sudden gusts on tall buildings or the cyclic operation of rotating machinery, both of which can lead to resonant conditions if not properly accounted for in design. The key distinction between static and dynamic loads hinges on the duration of load application relative to the structure's natural period of , which is the time for one complete cycle of free oscillation. A load is classified as static if its application duration exceeds approximately 10 times the natural period, ensuring that dynamic (inertial) effects remain negligible and a quasi-static suffices. This criterion helps engineers determine when full dynamic is unnecessary, avoiding while maintaining safety. For certain impact scenarios, such as those involving sudden deceleration in hoisting systems or vertical drops, the effects of can be quantified using the dynamic load factor (DLF), which scales the equivalent static load to account for amplification. In cases of sudden deceleration, the DLF can be approximated as: DLF=1+ag\text{DLF} = 1 + \frac{a}{g} where aa is the deceleration, and gg is the acceleration due to gravity (approximately 9.81 m/s²). This arises from the superimposed inertial mama on the gravitational mgmg. For general impact loads like drops, standard formulas such as DLF=1+1+2hδst\text{DLF} = 1 + \sqrt{1 + \frac{2h}{\delta_\text{st}}}
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