Engineer Boards

Need help with NCEES problem 118

Recommended Posts

Can somebody help me with NCEES problem 118?? (2011 blue book edition)

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.

Share on other sites

i dont have this reviewer but anyways, i would assume this is a truss, if it is then just follow the concepts of truss analysis there you will get the theories.

Share on other sites

See attached problem.

Share on other sites

thats right these are the theories, in this case no need to calculate reactions.

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

good luck

Share on other sites
Ble_PE    2,020

^Good explanation. Another way to look at member b is to notice that the joint at the top of b only has one vertical member framing into it with no vertical load applied. This means that the force in b has to be 0. The same could be said for member d as well.

The CERM discusses zero-force members in trusses in chapter 41. Good luck!

Share on other sites

Thank you so much for your responses. Very helpful! I appreciate it.

Share on other sites
Thank you so much for your responses. Very helpful! I appreciate it.

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.

Share on other sites

Zero force members are very important, you should do a few more examples and really get it down, it's pretty simple once you get the hang of it, and its a good chance you may get one.