To determine these unknowns, we have two methods available: the method of joints, and the method of sections. Because each member of the truss is a two force member, we simply need to identify the magnitude of the force on each member, and determine if each member is in tension or compression. When we talk about analyzing a truss, we are usually looking to identify not only the external forces acting on the truss structure, but also the forces acting on each member internally in the truss. This means that if the truss was separated from its connection points, no one part would be able to move independently with respect to the rest of the truss. In addition, statically determinate trusses (trusses that can be analyzed completely using the equilibrium equations), must be independently rigid. 5.1.2 TrussesĪdapted from image by ToddC4176 CC-BY-SA 3.0Ī truss is an engineering structure that is made entirely of two force members. Source: Engineering Mechanics, Jacob Moore, et al. The only way to achieve this is to have the forces be collinear. Because the moment exerted by the two forces must be equal to zero, the perpendicular distance between the forces (d) must be equal to zero. This couple would exert a moment on the beam when there are no other moments to counteract the couple. If the forces were not collinear, then the two equal and opposite forces would form a couple. In order to have the sum of the moments equal to zero, the forces must be collinear. This is the only way to ensure that the sum of the forces is equal to zero with only two forces. In order to have the sum of the forces equal to zero, the force vector on the other side of the beam must be equal in magnitude and opposite in direction. If this body is in equilibrium, then we know two things: 1) the sum of the forces must be equal to zero, and 2) the sum of the moments must be equal to zero. The body has some non-zero force acting at one end of the beam, which we can draw as a force vector. Imagine a beam where forces are only exerted at each end of the beam (a two force member). This will result in all two force members being in either tension or compression as shown in the diagram below. In order to have a two force member in static equilibrium, the net force at each location must be equal, opposite, and collinear. 5.1.1 Two Force Membersīefore we discuss the structure of trusses, we must begin with defining two force members:Ī two force member is a body that has forces (and only forces, no moments) acting on it in only two locations. Method of sections is more like a rigid body analysis where you can also include the moment equilibrium equation. Method of joints is more like a particle analysis wherein you use only x and y equilibrium equations. You’ll analyze these structures more in your Structures course, but for Statics you will need to know how to calculate the force in each member, using two methods: method of joints and method of sections. – Transferred from en.wikipedia to Commons., CC BY-SA 3.0, Image Source: Billbeee at English Wikipedia. Trusses are commonly found in the frame of a roof and the sides of a bridge: Source:Engineering Mechanics, Jacob Moore, et al. Trusses are rigid structures made up of two-force members, which are objects with exactly two forces/connections.
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