Name: safety factors in structural design
Uploaded: 2024-03-07T13:22:33.000Z
Duration: 1 min 22 s
Channel: JoVE

K12 Standards

Pennsylvania State Academic Standards

9-12

Earth and Space Science

Human Sustainability

3.3.9-12.O

Design Consideration

Lors de la conception d’une structure, les considérations commencent par la résistance ultime d’un matériau, qui est déterminée par des essais, en mesurant les changements sous une charge accrue jusqu’à ce qu’il atteigne sa limite ou se brise.
La charge maximale au-delà de laquelle le matériau se brise est la charge ultime. Le rapport entre la charge ultime et la section transversale d’origine fournit la contrainte normale ultime ou la résistance ultime.
L’autre considération importante est le facteur de sécurité, qui est le rapport entre la charge ultime et la charge admissible, assurant une réserve de capacité de charge pour un fonctionnement sûr.
Le facteur de sélection de la sûreté implique la prise en compte des variations des propriétés des matériaux, des charges, des types de défaillance potentiels, des incertitudes dans les méthodes d’analyse, de la détérioration future, de l’importance structurelle et des risques pour la vie et les biens.
La méthode conventionnelle des contraintes admissibles regroupe toutes les incertitudes en un seul facteur de sécurité, mais une autre méthode appelée méthode de conception des facteurs de charge et de résistance fait la distinction entre les incertitudes associées à une structure et à sa charge et fait la distinction entre les charges vives et permanentes.

considérations relatives à la conception

La conception d'une structure implique une série de considérations, principalement la résistance ultime du matériau, calculée à l’aide d'essais qui mesurent les changements sous l'effet d'une force accrue jusqu'à ce que le matériau atteigne son point de rupture ou sa limite. La charge ultime, là où le matériau se brise, est divisée par sa section transversale d'origine, ce qui donne la contrainte ou résistance normale ultime. La contrainte ultime de cisaillement est un autre facteur important pris en compte.

Le facteur de sécurité est un autre aspect clé qui relie la charge ultime à la charge admissible. Il assure un tampon dans la capacité de charge pour un fonctionnement sûr. La sélection du facteur de sécurité nécessite de prendre en compte les variations des propriétés des matériaux, les différents types de défaillances potentielles, les incertitudes des méthodes d'analyse, la détérioration future, l'importance structurelle et les risques pour la vie et les biens. Un facteur de sécurité trop faible risque d'échouer, tandis qu'un facteur de sécurité trop élevé peut entraîner des conceptions non rentables ou non fonctionnelles.

Traditionnellement, on utilise la méthode des contraintes admissibles, qui consolide toutes les incertitudes en un seul facteur de sécurité. Cependant, une méthode alternative, la méthode de conception par facteur de charge et de résistance, fait la différence entre les incertitudes liées à une structure et à sa charge et fait la distinction entre les charges vives et mortes.

Considérations relatives à la conception

Lors de la conception d’une structure, les considérations commencent par la résistance ultime d’un matériau, qui est déterminée par des essais, en mesurant les changements sous ...

Education

Enseignement

JoVE Core

Mechanical Engineering

Génie mécanique

Unit 1

Unité 1

Concept of Stress

Le concept de stress

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

safety factors in structural design

Safety factors in structural design help engineers choose a material and loading limit that can be used safely. The process begins with a material’s ultimate strength. This value comes from tests that increase force until the material reaches its breaking point or limit.

The ultimate load is then divided by the original cross-sectional area to find the ultimate normal stress, or strength. Ultimate shearing stress is another key value used in design. These limits show how much stress a material can resist before failure.

A factor of safety compares the ultimate load with the allowable load. It creates a buffer in load-carrying capacity for safe operation. Choosing this factor requires attention to changes in material properties, possible failure modes, uncertainty in analysis, future deterioration, structural importance, and the risk to life and property.

If the factor of safety is too small, the structure may fail. If it is too large, the design may become uneconomical or unusable. The traditional allowable-stress method combines the uncertainties into one safety factor. The Load and Resistance Factor Design method separates uncertainties in the structure and in the load, and it also distinguishes between live loads and dead loads.

Safety Factors in Structural Design

normal stress

Normal Stress in Axial Loading

Normal stress acts perpendicular to a material’s cross-sectional area. It appears when a force is applied along the axis of an object, which is called axial loading. Bridge truss members are a practical example of this kind of loading. When a rod carries axial load, internal forces develop across the section. Because these forces act normal to the plane of the section, the stress is called normal stress. Stress here means the average stress over the whole cross-section, not the value at one...

shearing stress

Shear Forces in Bolts and Rivets

Shearing stress is caused by transverse forces acting on an object. These forces create internal forces in the plane where the external forces are applied. The total internal force in that section is called the shear. Average shearing stress is found by dividing the shear by the cross-sectional area. This value is only an average, because shear stress is not spread evenly across the section. It can be zero at the surface and much higher than the average in other areas. Bolts, pins, and rivets...

bearing stress

Bolt-to-Plate Contact Pressure

Bearing stress is the contact pressure between two separate bodies. A common example is a bolt pushed through a plate. The bolt applies a force to the plate, and the plate pushes back with an equal and opposite force. That force is not a single point load. It is made up of many smaller forces spread across the contact surface between the bolt and the plate. Because these microforces are complex, engineers use an average value called bearing stress. Bearing stress is found by dividing the...

applications of stress

Boom and Rod Stress Analysis

Boom and rod stress analysis shows how a pinned support structure carries a load. The boom and the rod are connected by a pin and held in place by brackets and pins. The structure is then separated from its supports so the forces can be studied clearly. A free-body diagram is drawn to identify every force on the boom, the rod, and the load. The reaction forces on both members are found using equilibrium equations. At point P, the reaction causes compression in the boom. At point R, the...

stress on an oblique plane

Axial Load Stress on Inclined Sections

Axial loading creates stress in a material along its central axis. This kind of load can cause compression or elongation, and it produces normal stress. Normal stress acts perpendicular to the material area and can be either tensile or compressive. When the section is not perpendicular to the load, it becomes an oblique plane. On this inclined plane, the stress state includes both normal stress and shearing stress. Shearing stress acts parallel to the area and changes shape without changing...

stress: general loading conditions

Stress Components on a Loaded Body

Stress components on a loaded body can be found by cutting the body through a point with a plane parallel to the xy plane. That section carries the original loads, along with normal forces and shearing forces. The result is a simple way to describe how a structure responds under combined loading. The shearing force in the section can point in different directions within the plane. It is split into two parts that run parallel to the x and y axes. Each part is divided by the area, and as that...

components of stress

Stress Components on a Cube at a Point

Stress components on a small cube help describe how a body behaves under multiple loading. The cube is centered at point O and has stress acting on all six faces, whether the faces are visible or hidden. Normal stress components σx, σy, and σz act perpendicular to the x, y, and z axes. Shearing stress components also act on the cube faces. The visible faces carry shear stresses such as τxy and τxz. The hidden faces carry equal and opposite stresses. This balance keeps the cube in equilibrium.

design consideration

Safety factors in structural design help engineers choose a material and loading limit that can be used safely. The process begins with a material’s ultimate strength. This value comes from tests that increase force until the material reaches its breaking point or limit. The ultimate load is then divided by the original cross-sectional area to find the ultimate normal stress, or strength. Ultimate shearing stress is another key value used in design. These limits show how much stress a material...

key terms and concepts

Learn how ultimate strength, ultimate stress, and factor of safety guide structural design. Compare allowable-stress and Load and Resistance Factor Design methods.

Safety Factors in Structural Design | JoVE K12

JoVE Core Mechanical Engineering offers concise, easy-to-understand animated videos covering key topics such as equilibrium, forces, truss analysis, and structural mechanics. Explore their real-world applications, and witness these concepts in action through scientist-led experiments.

mechanical engineering

3.3.9-12.P

Louisiana Student Standards for Science

High School

Earth Science

HS-ESS3-2

HS-ESS3-4

Chemistry

Matter And Its Interactions

HS-PS1-6

Energy

HS-PS3-3

HS-PS3-6

Texas Essential Knowledge and Skills

Specialised Topic in Science

112.51.c.1.

112.51.c.2.D.

Next Generation Science Standards

Physical Science

motion-and-stability-forces-and-interactions

Motion and Stability: Forces and Interactions

HS-PS2-3

Biology

BIO.2

BIO.2.A

BIO.2.D

Physics

PHYS.2

PHYS.2.A

PHYS.2.D

Chemistry and Physics

IPC.2

IPC.2.A

IPC.2.D

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an-overview-of-bgdgt-biomarker-analysis-for-paleoclimatology

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3.3.9-12.M

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Students who demonstrate understanding can evaluate competing design solutions for developing, managing, and utilizing energy and mineral resources based on cost-benefit ratios. [cite: 30] Clarification Statement: Emphasis is on the conservation, recycling, and reuse of resources (such as minerals and metals) where possible, and on minimizing impacts where it is not. [cite: 31] Examples include developing best practices for agricultural soil use, mining (for coal, tar sands, and oil shales), and pumping (for petroleum and natural gas). [cite: 32] Science knowledge indicates what can happen in natural systems-not what should happen. [cite: 33] Assessment Boundary: N/A [cite: 34]

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3.3.9-12.Q

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3.3.9-12.R

Safety Factors in Structural Design

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