# Truss problem

1. Jan 31, 2014

### raymanmusic

1. The problem statement, all variables and given/known data
This crane lifts a 125 kg heavey motor. What is the member forces of BF and BD?
Use 10 m/s^2 as accleration of gravity.

https://dl.dropboxusercontent.com/u/11241083/problemtruss.png [Broken]

2. Relevant equations
F=ma
Sum Fx=0
Sum Fy=0
Sum M=0

3. The attempt at a solution
BF force: -1939 N
BD force: -2596 N

Last edited by a moderator: May 6, 2017
2. Jan 31, 2014

### SteamKing

Staff Emeritus
Drawing a free body diagram? Writing equations of equilibrium? Bueller? Bueller?

3. Jan 31, 2014

### raymanmusic

Force in G is F=125 kg * 10 m/s^2=1250 N
I found reaction force in A: 1250 N and reaction force in C: 0 N

4. Jan 31, 2014

### SteamKing

Staff Emeritus
Yeah, but that's not what the problem solution requires. It also ignores the weight of the lifting mechanism, which is not given, in any event. You are asked to find the forces in two members BD and BF while the motor is suspended.

Again, draw a FBD and write equations of equilibrium.

5. Jan 31, 2014

### raymanmusic

At what joint do I sum moments? Do I sum x and y forces including the BD and BF forces? Can I use method of joints? I'm unable to solve this. https://dl.dropboxusercontent.com/u/11241083/New%20Document20140131230411543.pdf [Broken]

Last edited by a moderator: May 6, 2017
6. Jan 31, 2014

### SteamKing

Staff Emeritus
You've got to split the hoist into several parts, and solve each individually.

For the force in member BF, start with isolating member BF and the top piece EFG. You know the load applied at point G and you know this member is supported by BF and member CDE. You can use the geometry of the hoist to supply the unknown reactions which keep EFG in equilibrium and then solve for their magnitudes.

Once you have worked the top member, take a similar approach to analyze the forces in member BD.

7. Feb 1, 2014

### raymanmusic

I got the right answer for BF. How do I find BD? Do I isolate member EDC and BD and use equations of equilibrium? How do I calculate reaction force in C

8. Feb 1, 2014

### SteamKing

Staff Emeritus
You repeat the process used to solve member EFG. Use the reactions from that member and isolate EDC. Draw its FBD and write a new set of equilibrium equations. Remember, the joint at C is pinned, so it can't support/resist a moment.

9. Feb 1, 2014

### raymanmusic

Do I solve sum of moments about joint C is equal to zero?
https://dl.dropboxusercontent.com/u/11241083/New%20Document20140201213045922.pdf [Broken]

Last edited by a moderator: May 6, 2017
10. Feb 1, 2014

### SteamKing

Staff Emeritus
Yes. The joint at C is pinned and therefore the moment there must be zero.

11. Feb 2, 2014

### raymanmusic

I'm unable to find the force in member BD. Can you please help me draw a FBD?

12. Feb 2, 2014

### nvn

Last edited: Feb 2, 2014
13. Feb 2, 2014

### raymanmusic

BF = -1938.87 N
BD = -2601.27 N
My attempt at solution:https://dl.dropboxusercontent.com/u/11241083/New%20Document20140202212736894.pdf [Broken]

Do I only draw member CDE or do I draw EFG and CDE and then solve sum of moments about joint C equal to zero?

Last edited by a moderator: May 6, 2017
14. Feb 2, 2014

### nvn

Post 1 says use g = 10 m/s^2.

Last edited: Feb 2, 2014
15. Feb 2, 2014

### raymanmusic

I'm sorry that is a mistake. Use g=9.81 m/s^2

16. Feb 2, 2014