Engineering Question - Statics

[Darwin18]

Evening guys, I thought I might have some luck posting a statics question that I'm having a difficult time with. There certainly seems to be a large number of engineers on the site! This question might seem easy but I'm struggling with it.

I was able to solve the 2nd question, but I'm stuck on the first question (the one at the very top)

The reactions forces at F (RFx = -10 kips and RFy = -56.7 kips) and G (RGy = 86.7 kips). I need to calculate the forces in the trusses by using the method of joints.

I started at Joint F, made my free body diagram, and came up with:

FD = 56.7 kips (tension)
FG = 10.0 kips (tension)

I then move to Joint G, as it has two unknowns and that's where I get stuck. Looking ahead in the problem, I not sure where I would move to after Joint G. D and E both have more than two unknowns....

It's a long shot but I'd figure I'd ask you all.

 

[anteup]

How quickly you need the answer? Give me a few minutes and i'll pull out my old engineering text.

I can't read the dimensions.

 

[Darwin18]

By Wednesday...I literally spent seven hours today on these two problems. I was feeling pretty good about this class until today...

 

[anteup]

I should be able to help. Problem is I haven't done engineering for about 11yrs. However, I did get my BSCE in 1989 and work as a structural engineer for 10yrs. I'll give a shot. Just got to remember.

 

[TXCrash]

You'll need to post a higher res image to get any help. Text isn't readable.

 

[Darwin18]

How is this?

 

[Carter5112]

I'll take a stab at it. Though these sorts of problems are more easily explained with numbers on papers than words! Indeed you have two unknowns at Joint G, but you still have enough information to solve it. Member GD will take 4/5 of the 86.7 kip Y force and 3/5 of the 10 kip X force. Use you Pythagorean theorem to get the force in the direction of GD. Then member GE is just the rest of what's left of the 86.7 kip Y Force. Does that make sense? More plainly, just use your equilibrium equations and summing forces in Y direction gets you your answer. Just be sure to break the diagonal members into their resultant forces. Similarly you can solve members CE and CB right out the gate since they apply that 30 kip force at C and also member AB and AD since there's a force applied at A. Again, just sum forces in X and Y with your unknowns.

Did that make any sense?? Let me know and I'd be glad to try and help more!

 

[Carter5112]

Alright, I re-read that and think I could make that easier. For method of Joints, just take your given Joint then apply your Sum Forces in Y equation and your Sum Forces in X equation.

At Joint G, forces summed in Y are:

86.7-(4/5)GD-GE=0

You can't solve that one by itself. For sign convention I assume up is positive for Y, right is positive for X. I drew my unknowns in compression.

Now, summing forces at G in X:

-10-(3/5)GD=0

From that you can get GD and then solve the equation you got in Y. Again, be careful of sign convention for compression and tension.

 

[Darwin18]

Hmm that does make sense. So I would have:

DF = 56.7 kips (Tension)
FG = 10.0 kips (Tension?)
GD = 75.36 kips (Compression)
EG = 17.34 kips (Compression)

What joint would I move to next? D and E both have 3 unknowns. I think I would need to move to joint A, but segment AD is throwing me off. Unless AD = 0...

 

[Carter5112]

Yes, AD is 0. Just look at the joint, it has to be. I know you probably think since you don't know joint D yet, then maybe DA has force, but from the Method of Joints each joint must be in equilibrium on its own.

You can solve Joint C very easily as well. The 30 kip force is resisted by the CE component in that direction and then the X force that generates in member CE is resisted by CB. From there you can solve Joint B, E then D.

 

[Darwin18]

Thanks Carter. I think I'm just overanalyzing these problems. I just need to stick with Fx=0 and Fy=0.

 

[Carter5112]

No problem and I hear ya! Unfortunately, I had to take the course twice till I figured it out!

 

[Guidry]

Hey Darwin, just curious, which text are you using for your class?

 

[Stick]

Ouch, this thread brings back some painful memories!

 

[Darwin18]

Hey Darwin, just curious, which text are you using for your class?

We're using Applied Statics and Strength of Materials, 5th edition, by Limbrunner and Spiegal. The text hasn't been too bad up until this chapter - just doesn't seem like there were any good examples to help with my assignment.

 

[anteup]

just remember in statics things stay stationary. all forces must equal out. wait until dynamics.