Name: beam forces: tension, shear, and bending
Uploaded: 2025-05-22T09:06:03.000Z
Duration: 1 min 30 s
Channel: JoVE

Quando uma viga é submetida a uma carga, ela experimenta forças internas que podem ser categorizadas como momentos normais, de cisalhamento e de flexão.
As forças internas podem ser determinadas usando o método das seções. Uma convenção de sinais especial é aplicada à medida que ocorrem em pares, e sua direção depende de qual lado da seção deve ser analisado.
A força normal é considerada positiva se criar tensão, enquanto é considerada negativa se gerar compressão no objeto.
Suponha que as forças que atuam tangencialmente à seção transversal sejam direcionadas para baixo e para cima nos lados esquerdo e direito da seção, respectivamente. Nesse caso, a força de cisalhamento é considerada positiva, resultando em uma rotação no sentido horário.
Da mesma forma, se a direção das forças de cada lado for invertida, um cisalhamento negativo é produzido, resultando em uma rotação no sentido anti-horário.
Quando uma força externa atua na viga, ela experimenta um momento de flexão, formando uma curvatura côncava para cima ou para baixo. O momento fletor é positivo para a curvatura ascendente e negativo para a curvatura descendente.

<ol>
	<li>Ferdinand Beer, E. Johnston, David Mazurek, Phillip Cornwell e Brian Self (2019). Mecânica Vetorial para Engenheiros: Estática e Dinâmicas. McGraw Hill. Pág. 380-391.</li>
	<li>Russell C. Hibbeler. (2016). Engenharia Mecânica: Estática e Dinâmica. Pearson. pág. 343-345.</li>
</ol>

convenção de sinais

Sign Convention

Ao analisar uma viga sujeita a várias cargas, é crucial entender as forças internas e os momentos gerados dentro da estrutura. Essas forças internas podem ser amplamente classificadas em forças normais, forças cortantes e momentos fletores. Para determinar essas forças e momentos, utilizamos o método das seções e aplicamos uma convenção específica de sinais com base em sua direção e no lado da seção sendo analisado.

A força normal age perpendicularmente à seção transversal da viga e pode causar tração ou compressão no objeto. Quando a força normal cria tração, é assumido como positivo, enquanto quando gera compressão, é considerado negativo.

A força cortante refere-se às forças que atuam tangencialmente à seção transversal da viga. Essas forças geralmente ocorrem em pares, com uma força atuando no lado esquerdo da seção e outra no lado direito. Se a força no lado esquerdo estiver direcionada para baixo e a força no lado direito estiver direcionada para cima, a força cortante é considerada positiva, resultando em uma rotação no sentido horário. Por outro lado, se a direção das forças em cada lado for invertida, uma força cortante negativa é produzida, levando a uma rotação no sentido anti-horário.

O momento fletor é o efeito rotacional causado por uma força externa atuando na viga. Quando submetida a tal força, a viga experimenta uma curvatura, que pode ser para cima (côncava) ou para baixo (convexa). A convenção de sinais para momentos fletores determina que um momento fletor positivo corresponde a uma curvatura para cima (côncava), enquanto um momento fletor negativo corresponde a uma curvatura para baixo (convexa). Essa convenção desempenha um papel vital na compreensão do padrão de deformação da viga e no projeto de elementos estruturais apropriados para resistir à flexão.

Em resumo, as convenções de sinais para força normal, força cortante e momento fletor são ferramentas essenciais na análise estrutural. Elas ajudam engenheiros e projetistas a entender o comportamento das estruturas sob várias cargas, prever possíveis modos de falha e projetar elementos estruturais robustos e eficientes.

Convenção de Sinais

Ao analisar uma viga submetida a várias cargas, é crucial entender as forças internas e os momentos gerados dentro da estrutura. Estes...

Education

JoVE Core

Mechanical Engineering

Engenharia Mecânica

Unit 1

Unidade 1

Internal Forces

Forças Internas

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

beam forces: tension, shear, and bending

Beam analysis depends on the internal forces and moments inside a structure. These include normal force, shear force, and bending moment. Engineers use the method of sections to cut the beam and find these values. A sign convention helps show whether each result is positive or negative.

Normal force acts perpendicular to the beam’s cross-section. It can pull the material in tension or push it in compression. Tension is taken as positive, while compression is taken as negative. This makes it easier to read and compare force results during structural analysis.

Shear force acts tangentially along the cross-section of the beam. It often appears as a pair of forces, with one side of the cut carrying the left-side force and the other side carrying the right-side force. A downward force on the left side and an upward force on the right side represent positive shear and a clockwise rotation. If the directions are reversed, the shear force is negative and the rotation is counterclockwise.

Bending moment describes the turning effect created by an external force on a beam. It changes the beam’s shape into either an upward, concave curve or a downward, convex curve. A positive bending moment gives upward, concave curvature. A negative bending moment gives downward, convex curvature. These conventions help predict how beams deform under load and support better structural design.

Beam Forces: Tension, Shear, and Bending

sign convention

Beam analysis depends on the internal forces and moments inside a structure. These include normal force, shear force, and bending moment. Engineers use the method of sections to cut the beam and find these values. A sign convention helps show whether each result is positive or negative. Normal force acts perpendicular to the beam’s cross-section. It can pull the material in tension or push it in compression. Tension is taken as positive, while compression is taken as negative. This makes it...

normal and shear force

Beam Loads and Internal Forces

Beam loads create internal forces inside a structure. Weight, pressure, and other external forces can all act on a beam. These internal forces affect the beam’s stability and strength. Engineers study these forces to find their size and direction. A common approach is the method of sections. In this method, an imaginary cut is made at a chosen point on the beam, and the forces at that section are analyzed. One internal force is the normal force, also called the axial force. It acts...

bending and torsional moments

Forces That Bend and Twist Materials

Bending and torsional moments describe how materials respond to force. These ideas are important in structural engineering because they help explain how beams, rulers, and screws change shape under load. A moment depends on both the size of the force and the distance from the point of interest to the force’s line of action. A bending moment forms when an external force makes a structural element bend. If the element bends upward, the top side is in compression and the bottom side is in tension.

internal loadings in structural members: problem solving

internal-loadings-in-structural-members-problem-solving

Beam Internal Force Analysis at Point A

Beam internal force analysis shows how normal force, shear force, and bending moment act inside a structural member. These internal loadings help engineers check whether a beam can support the external forces applied to it. The example uses a beam OC that weighs 5 kN and is inclined at 53.13° with the horizontal. The beam is supported at both ends. The goal is to find the internal loadings at point A. The first step is to find the reaction force at point C. This is done by taking the moment...

beams

Beam Types and Support Conditions

Beams are key parts of structural engineering and construction. They support loads that act at different points along their length. Engineers use beam shape and support conditions to understand how a beam will perform. Beams can be classified by geometry. Straight beams are the most common and have a constant cross-section along their length. Tapered beams change in cross-section from one end to the other, while curved beams have a curved shape. Beam cross-section also matters. Common shapes...

shear diagram

Beam Shear Force Diagram

Beam shear force diagrams show how shear changes along the length of a supported beam. In beam mechanics, they help students track the internal shear force caused by perpendicular loads. The beam AB in this example is supported at both ends, so the first step is to draw a free-body diagram that shows the external forces and internal reactions. The reaction forces at supports A and B are found with equilibrium equations for force and moment. For the beam shown, the vertical reaction at A is 24...

bending moment diagram

Beam Load Analysis with Moment Diagrams

Beam load analysis uses a bending moment diagram to show how bending changes along the length of a beam. This diagram helps engineers and designers check whether a structure can handle applied forces. It is also useful for parts such as shelving arms, which must resist forces and moments from a load. To build the diagram, start by finding the reactive forces and any couple moments on the beam. In some cases, especially when a beam is inclined, these forces must be resolved into components that...

relation between the distributed load and shear

relation-between-the-distributed-load-and-shear

Shear Change Under Distributed Loading

Shear force changes along a beam under a distributed load. In structural analysis, this relationship helps explain how beams respond to loading conditions that also include concentrated loads and a couple moment. To study the link, consider a small beam section that has no concentrated load or couple moment on it. Draw a free-body diagram for that elemental section. It includes the distributed load along the beam, the shear force V(x) on the right-hand side of the cut, and a small added shear...

relation between the shear and bending moment

relation-between-the-shear-and-bending-moment

Shear and Moment Diagrams in Beam Loads

Shear and moment diagrams help show how a beam responds to loads. A beam can carry a distributed load, concentrated loads, and a couple moment. These loads create internal shear forces and bending moments. The beam is examined by isolating a small elemental section and drawing a free-body diagram. For that section to stay in equilibrium, the moment on the right side must be slightly larger than the moment on the left side. The distributed load also has a resultant force that acts at a...

shear and bending moment diagram: problem solving

shear-and-bending-moment-diagram-problem-solving

Beam Load Analysis with Shear Diagrams

Beam load analysis uses shear and bending moment diagrams to track the internal forces in a beam. These diagrams are important when a beam carries concentrated loads and a distributed load. They help show how the beam responds so structures can be designed safely and efficiently. The first step is to draw a free-body diagram of the whole beam. A free-body diagram shows the concentrated loads, the distributed load, and the reaction forces at the supports. Use the equilibrium equations, where...

cable subjected to concentrated loads

Finding Tension in a Loaded Cable

Flexible cables can carry vertical point loads between two fixed supports. The cable is treated as flexible, inextensible, and nearly weightless. Under these conditions, it changes shape to form straight-line segments, and each segment carries a constant tensile force. To analyze the cable, a free-body diagram is drawn for the whole cable. This helps identify the reaction forces at the supports. In many cases, the number of unknown reaction components is greater than the available equilibrium...

cable subjected to a distributed load

Finding Cable Shape Under Distributed Load

Cable shape under a distributed load is found by analyzing the forces along a small piece of the cable. This method is important in suspension bridge design because the main cables must carry changing loads safely. The load causes the tensile force in the cable to change, and it also creates cable deformation. The analysis starts with a free-body diagram of a short cable segment. The diagram shows the distributed load and the change in tensile force across that segment. Equilibrium equations...

cable subjected to its own weight

Cable Sag Under Its Own Weight

Overhead power transmission cables sag under their own weight, and that shape affects how they carry electricity across long distances. To study this behavior, the cable is analyzed with a generalized loading function. This function treats the load along the cable's arc length instead of its projected length, which gives a more accurate picture of the cable's response. The analysis then focuses on a small cable segment. A free-body diagram is drawn to show the forces acting on that segment.

cable: problem solving

Cable Tension Under Uniform Loading

Cable tension under uniform loading is found by analyzing a cable fixed between two supports. The problem starts with the support conditions and the load on the cable. From there, the cable’s shape must be described with an equation that matches the curve formed by the load. The next step is to integrate the shape equation to get the cable’s shape function. This makes it possible to solve for constants in the equation. The boundary condition at the origin gives one constant. The slope, which...

key terms and concepts

Learn how normal force, shear force, and bending moment use sign conventions in beam analysis, and how tension, compression, and curvature are labeled.

Beam Forces: Tension, Shear, and Bending | JoVE K12

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mechanical engineering

Beam Forces: Tension, Shear, and Bending

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