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

Kiedy belka jest poddawana obciążeniu, doświadcza sił wewnętrznych, które można podzielić na momenty normalne, ścinające i zginające.
Siły wewnętrzne można wyznaczyć za pomocą metody przekrojów. Stosowana jest specjalna konwencja znaków, ponieważ występują one w parach, a ich kierunek zależy od tego, która strona przekroju ma być analizowana.
Zakłada się, że siła normalna jest dodatnia, jeśli wytwarza rozciąganie, podczas gdy jest uważana za ujemną, jeśli generuje ściskanie w obiekcie.
Załóżmy, że siły działające stycznie do przekroju są skierowane w dół i w górę odpowiednio po lewej i prawej stronie przekroju. W takim przypadku siła tnąca jest uważana za dodatnią, co powoduje obrót zgodnie z ruchem wskazówek zegara.
Podobnie, jeśli kierunek sił po każdej stronie jest odwrócony, powstaje ujemne ścinanie, co powoduje obrót w kierunku przeciwnym do ruchu wskazówek zegara.
Kiedy siła zewnętrzna działa na belkę, doświadcza ona momentu zginającego, tworząc wklęsłą krzywiznę w górę lub w dół. Moment zginający jest dodatni dla krzywizny w górę i ujemny dla krzywizny w dół.

<ol>
	<li>Ferdinand Beer, E. Johnston, David Mazurek, Phillip Cornwell i Brian Self (2019). Mechanika wektorowa dla inżynierów: statyka i dynamikas. McGraw Hill. Strony 380–391.</li>
	<li>Russella C. Hibbelera (2016). Mechanika inżynierska: statyka i dynamika. Pearson. str. 343-345.</li>
</ol>

konwencja znaku

Podczas analizy belki poddanej różnym obciążeniom, kluczowe jest zrozumienie sił i momentów wewnętrznych generowanych wewnątrz struktury. Te siły wewnętrzne można szeroko sklasyfikować jako siły normalne, siły tnące i momenty zginające. Aby określić te siły i momenty, używamy metody przekrojów i stosujemy określoną konwencję znaków na podstawie ich kierunku oraz strony analizowanego przekroju.

Siła normalna działa prostopadle do przekroju poprzecznego belki i może powodować naprężenia lub ściskanie wewnątrz obiektu. Gdy siła normalna powoduje napięcie, przyjmuje się, że jest dodatnia, natomiast gdy generuje ściskanie, uważa się ją za ujemną.

Siła tnąca odnosi się do sił działających stycznie do przekroju poprzecznego belki. Te siły zazwyczaj występują parami, z jedną siłą działającą po lewej stronie przekroju i drugą po prawej stronie. Jeśli siła po lewej stronie skierowana jest w dół, a siła po prawej stronie skierowana w górę, siła tnąca jest uważana za dodatnią, powodującą obrót zgodny z ruchem wskazówek zegara. Jeśli jednak kierunek sił po obu stronach zostanie odwrócony, powstaje ujemna siła tnąca, prowadząca do obrotów przeciwnie do ruchu wskazówek zegara.

Moment zginający to efekt obrotowy spowodowany działaniem siły zewnętrznej na belkę. Pod wpływem takiej siły belka doświadcza krzywizny, która może być albo w górę (wklęsła), albo w dół (wypukła). Konwencja znaku dla momentów zginających mówi, że dodatni moment zginający odpowiada krzywiźnie w górę (wklęsłej), podczas gdy ujemny moment zginający odpowiada krzywiźnie w dół (wypukłej). Ta konwencja odgrywa ważną rolę w zrozumieniu wzoru odkształcenia belki i projektowaniu odpowiednich elementów konstrukcyjnych do oporu zginania.

Podsumowując, konwencje znaków dla sił normalnych, sił tnących i momentów zginających są kluczowymi narzędziami w analizie konstrukcji. Pomagają inżynierom i projektantom zrozumieć zachowanie struktur pod różnymi obciążeniami, przewidzieć potencjalne tryby uszkodzenia oraz projektować wytrzymałe i efektywne elementy konstrukcyjne.

Konwencja znaku

Podczas analizy belki poddawanej różnym obciążeniom kluczowe jest zrozumienie sił wewnętrznych i momentów generowanych w konstrukcji. Te...

Education

JoVE Core

Mechanical Engineering

Inżynieria mechaniczna

Unit 1

Jednostka 1

Internal Forces

Siły wewnętrzne

KEY TERMS AND CONCEPTS

Kluczowe terminy i pojęcia

Sign Convention

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...

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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...

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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...

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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...

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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...

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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.

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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

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

Beam Forces: Tension, Shear, and Bending

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