# Chapter6 - TRUSSES METHODS OF JOINTS APPLICATIONS Trusses...

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TRUSSES – METHODS OF JOINTS

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APPLICATIONS Trusses are commonly used to support a roof. For a given truss geometry and load, how can we determine the forces in the truss members and select their sizes? A more challenging question is that for a given load, how can we design the trusses’ geometry to minimize cost?
APPLICATIONS (continued) Trusses are also used in a variety of structures like cranes and the frames of aircraft or space stations. How can we design a light weight structure that will meet load, safety, and cost specifications?

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DEFINING A SIMPLE TRUSS (Section 6.1) A truss is a structure composed of slender members joined together at their end points. If a truss, along with the imposed load, lies in a single plane (as shown at the top right), then it is called a planar truss . A simple truss is a planar truss which begins with a a triangular element and can be expanded by adding two members and a joint.
ANALYSIS and DESIGN ASSUMPTIONS When designing both the member and the joints of a truss, first it is necessary to determine the forces in each truss member. This is called the force analysis of a truss. When doing this, two assumptions are made: 1. All loads are applied at the joints. The weight of the truss members is often neglected as the weight is usually small as compared to the forces supported by the members. 2. The members are joined together by smooth pins. This assumption is satisfied in most practical cases where the joints are formed by bolting or welding. With these two assumptions, the members act as two-force members. They are loaded in either tension or compression . Often compressive members are made thicker to prevent buckling .

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THE METHOD OF JOINTS (Section 6.2) In this method of solving for the forces in truss members, the equilibrium of a joint (pin) is considered. All forces acting at the joint are shown in a FBD. This includes all external forces (including support reactions) as well as the forces acting in the members. Equations of equilibrium ( F X = 0 and F Y = 0) are used to solve for the unknown forces acting at the joints.
STEPS FOR ANALYSIS 1. If the support reactions are not given, draw a FBD of the entire truss and determine all the support reactions using the equations of equilibrium.

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## This note was uploaded on 04/20/2011 for the course ENG 1333 taught by Professor Brr during the Spring '10 term at American College of Computer & Information Sciences.

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Chapter6 - TRUSSES METHODS OF JOINTS APPLICATIONS Trusses...

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