Quick question about a zero-force member problem

I really thought I understood these ZFM by inspection problems. But I ask this question because my friend's and my answer don't match the answer provided. No explanation was provided, just a list of the ZFMs in the answer. I won't say what our answer is because I want to see what you guys come up with, but I'll say this: we came up with 4 ZFMs and the answer showed 5.

Can you guys please help identify all the ZFMs, by segment?

Thanks

I count 5. But I'm a water guy, not a structural.

I believe the question is flawed.

You get the 5th one if all three bays have the same length; otherwise 4 zero-force members sounds right.

Thanks guys, but, again, per my original question please:

"Can you guys please help identify all the ZFMs, by segment?"

Thanks.

Ok, I'll play. But remember, I'm not structural, and I haven't studied this yet for the civil PE.

Based on what I remember from school and studying for the fe, I get:

AB
AH
BG
HG
GF

:dunno:

AB

AH

BG

EF

BF if the three bays have equal lengths.

I was close.

Thanks matt.

Yes, SEHQ, that is exactly right! My friend and I got the 4 you mentioned and couldn't get any further. But the provided solution/answer also includes member BF as a ZFM. How is it that you knew that BF had potential to be a ZFM? Neither joint B nor F fit the "3 Rules", do they?

Thanks!

3 Guidelines for determining zero-force members:

1. If a joint only has two members and no external load and/or no support, then those two members are zero-force members.

2. 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 collinear with one of the members then the other member is a zero-force member. If the resultant force at the joint is not collinear with either member, then both members are not zero-force members.

3. If a joint has three members and no external load and/or support, then if two of members are collinear then the non-collinear member is a zero-force member.

I didn't follow the instruction which says to find them "by inspection".

Instead, I did a quick static check to find the forces in all members assuming equal bays (e.g. Let P = 24. Then, reactions will be P. Then, trace the loads and find that horizontal forces are P and diagonal forces are 1.41P).

Doing that, I found BF = 0 which seemed a little strange based on intuition so I did another check using unequal bays which results in BF not equal to zero.

So... this appears to be a trick question since the picture does not show equal bays; therefore, technically, I think your answer is correct.

During the exam, I would suggest that you just do what do but circle the question so that you know to come back later for a thorough check.

Much appreciated Andy.

Oh, one more question about this topic.... are these ZFM by Inspection problems even PE-level problems? Or only FE-level? I see them from time to time in PE practice/review material but I don't recall seeing them on the exam.

If the three bays are equal, then the load-structure system is symmetric. This results in zero PANEL SHEAR in the middle bay (imaging this as a beam with two 24 k loads at the third points - the shear in the middle third would be zero. In that panel, BF is the only one that can contribute to panel shear (the other two are horizontal), so if the panel shear is zero, that member force is zero.