Lecture 8 part III

Lecture 8 part III - PART III Contents SPACE TRUSSES FRAMES...

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31 PART III Contents SPACE TRUSSES FRAMES AND MACHINES
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32 SPACE TRUSSES When the joints in a truss do not all lie in a plane and/or that is loaded and supported out o fits place is called a space truss . The member of a space truss may be treated as a two-force member provided that no member is continuous through a joint, the external load is only applied at the joints. The 3D equivalent of a triangle is a tetrahedron , which is constructed from tetrahedral subunits. Analogous to a simple planar triangle, a simple space truss is formed by adding tetrahedron units to the truss. But now each new joint brings three new members . The relationship between the number of the joints j and the number of members m in a simple space truss is given by 36 mj = (5) For the method of joints, the equilibrium of the members is ensured by the assumption that they are two-force members . Therefore, only the FBD of the pins should be considered by the force equilibrium equation as 0 x = F 0 y = F 0 z = F (6) for each joint. Since each joint consists of a system of concurrent forces in 3D, moment equilibrium needs no consideration.
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33 Fig. 7-9 Simple space truss constructed using tetrahedron subunits For method of sections , similar procedures can be applied to the space truss. However, it is usually difficult to pass a section through a typical space truss that cuts no more than 6 members. For this reason, it is not often used for space truss . Example 7-16 Since at joint E, we have 3 unknown reactions with support E, summing moment about E can be a good choice to start with // / / / D E i EiA EAB EBC ECD ED iA = = ×= ×+ × ∑∑ M r Fr F
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This note was uploaded on 04/20/2009 for the course CVEN 221 taught by Professor - during the Summer '08 term at Texas A&M.

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Lecture 8 part III - PART III Contents SPACE TRUSSES FRAMES...

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