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Class 9 - The Engineering Process Using quantification to...

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9/27/2007 ENGR 111 Track A 1 The Engineering Process: Using quantification to make decisions!   Decision making from quantitative data: Will the  truss withstand the necessary load?   Calculation of the Safe Loads in a Truss Class 09
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9/27/2007 ENGR 111 Track A 2 1.  If a joint connects only two truss members  and no support reaction or external force is  applied to the joint, then the members must  be zero-force members. Zero-force Members
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9/27/2007 ENGR 111 Track A 3 1. If a joint connects only three truss members, two  of the members are co-linear, and no support  reaction or external force is applied to the joint,  then the third member is a zero-force member. Zero-force Members
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9/27/2007 ENGR 111 Track A 4 Identification of zero-force members by  inspection: Example from the textbook
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9/27/2007 ENGR 111 Track A 5 Ex_09_01: by inspection, identify the zero-force  members in the truss (from our textbook) A B C D F G
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9/27/2007 ENGR 111 Track A 6 Zero-force Members in your truss OK All the examples are 2-D  trusses. What happens to our actual  truss? Consider the truss shown here: The white links are the side walls  (vertical planar truss) The gray links are the cross- braces Assume a coordinate system with X  along the length of the truss, Y  vertical and Z along the thickness. Let the load (yellow  arrow) be along  the Y axis. WITH THIS SETUP, ALL THE  GRAY LINKS WILL HAVE ZERO  LOADS! z x y P/4 P/4 P/4 P/4
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9/27/2007 ENGR 111 Track A 7 Example: FBD OF A CORNER P/4 Fab Fac Fad Fae z x y 0 0 z F Fae = = Σ z x y P/4 P/4 P/4 P/4
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9/27/2007 ENGR 111 Track A 8 Zero Force Members: Key Concept 1. IF  your truss has two  vertical walls joined with cross braces, and 2. IF  the external forces are also vertical THEN The forces will be transmitted entirely through the side walls, the  cross-braces along the thickness direction will be zero. You need to do only a planar truss analysis for this case.
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