Need help with NCEES problem 118
Posted 21 September 2011 - 10:27 AM
The solution says "By inspection, Member b=0 kips, and Member c=100kips". This is not intuitive to me and I cannot do it "by inspection". Can someone give me some pointers?
I tried finding the reactions first... I assume each support has one vertical and one horizontal reaction. If I sum moments about the bottom support, I can find each horizontal reaction... and then I'm stuck.
Thanks in advance!
Posted 21 September 2011 - 01:27 PM
zero force member will occur on a member that is perpendicular to bottom chord with two adjacent diagonals with the same slope, these adjacent members have equal members loads, so the applied vertical load 100kips at that joint is resisted by the two diagonal members and not member "b"
c=100, isolate this joint, and use method of joint, the equation in equilibrium is summation of Forces vertical, the applied vertical load =100 balance by c=100
Posted 21 September 2011 - 01:35 PM
The CERM discusses zero-force members in trusses in chapter 41. Good luck!
Posted 22 September 2011 - 04:20 PM
Identification of Zero-Force Members:
For an ideal truss (nodes are pinned, loads act only at nodes), each node is a system of concurrent forces. Some of these forces are collinear, i.e., they share the same line of
action. If there are nodes where all forces can be grouped into only two lines of action, we may say the following:
1. If one line of action consists of a single force, that force must be zero.
2. If one line of action consists of a pair of forces, they must be equal and opposite.
NOTE These rules apply if and only if the forces can be grouped into only two lines of action. THESE TWO LINES OF ACTION NEED NOT BE PERPENDICULAR.
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