PE Practice Exam Civil AM, Question 115

bulls2030 · Apr 9, 2016

So I am having some trouble understanding this simple question thats on the NCEES Practice Exam.

Wouldn't member b have a horizontal for pointing up of 100 kips since there is an external load of 100kips pointing down?

The question and solution is attached. Would appreciate anyone's explanation on this.

(FYI, I am taking the transportation PM portion)

View attachment Doc2.pdf

matt267 PE · Apr 10, 2016

I think, member b is a zero force member because if you look at the joint at the top of the truss, you have three members. Two are collinear and there is no external force on that joint. That makes the non-collinear member zero.

Edit:

Here's a quote from my college statics book (Engineering Mechanics Statics, R.C. Hibbeler, 2001): "In general, if three members form a truss joint for which two of the members are collinear, the third member is a zero-force member provided no external force or support reaction is applied to the joint."

dcald86 · Apr 11, 2016

I found these explanations of zero force members helpful:

http://www.ae.msstate.edu/vlsm/truss/analysis_of_statdet_trusses/zfm.htm

https://www.youtube.com/watch?v=LAb_ezELQ_Y

ptatohed · Apr 11, 2016

The three basic zero force member by inspection rules:

If a joint only has two members and no external load and/or no support, then those two members are zero-force members.
If a joint only has two members and is loaded, then if the line of action of resultant force from applied loads at the joint colinear with one of the members then the other member is a zero-force member. If the resultant force at the joint is not colinear with either member, then both members are not zero-force members.
If a joint has three members and no external load and/or support, then if two of members are colinear then the non-colinear member is a zero-force member.

Another useful site:

http://www.tekniksipil.org/civil-engineering/engineering-mechanics/zero-force-members-in-trusses-examples/comment-page-1/

And a practice problem for you:

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