# Method of joints for bridge truss

1. May 7, 2016

### Comfy

1. The problem statement, all variables and given/known data
Calculate the forces for each member.

2. Relevant equations

Sum of forces in x=0
Sum of forces in y=0

3. The attempt at a solution
Work is on attached image of problem. I'm having trouble solving for support reactions. At D there is

a hinge. At A, C, and Fthere is a roller. I know that the hinge provides a horizontal and vertical reaction force where the rollers only provide a vertical reaction force. I know I can't sum the moments about a roller and I'm don't think I can sum the moments about the hinge (did this anyways as Im stuck). So how can I solve these support reactions?

File size:
17.2 KB
Views:
69
2. May 7, 2016

### SteamKing

Staff Emeritus
You can sum moments about any convenient point. Just because a roller doesn't support a moment does not prevent this.

A structure in static equilibrium must have forces and moments sum to zero, regardless of reference

3. May 7, 2016

### Comfy

That is the right thing to do right? I keep getting Fy and Cy to cancel out during my sum of the moments equation? So I have sum of moments about D to be 0=0?

Last edited: May 7, 2016
4. May 7, 2016

### SteamKing

Staff Emeritus
It's not clear what you are talking about here. From the diagram, it looks like the supports are located at D and F.

The structure is symmetrical about the line BE. What do you think the reactions should be?

5. May 7, 2016

### Comfy

There is a roller at A, C and F. There is a hinge at D.

6. May 7, 2016

### SteamKing

Staff Emeritus
What's the purpose of the rollers at A and C? Your sketch is not clear on this.

7. May 7, 2016

### Comfy

We are suppose to design a truss that can handle a load of 12.5 kip at the center of this bridge. There is a roller at the top left just under bridge, there is a roller at top right just under bridge, there is a roller on support underneath bottom right of bridge, and there is a hinge on support underneath bottom left of bridge.

File size:
24 KB
Views:
66
File size:
17.2 KB
Views:
63
8. May 7, 2016

### Comfy

When I calculate support reactions for the truss I made with the 12.5 kip load in center I keep getting zero. That should not happen, unless I am mistaken.

9. May 7, 2016

### SteamKing

Staff Emeritus
If the rollers at A and C can support reactions at the cliffs, and the roller at F and the hinge at D also have support reactions, then your structure is no longer a simple truss, but a statically indeterminate structure with four points of support but only two equations of statics to work with.

10. May 7, 2016

### Comfy

So what you are saying is that there should not be rollers on top of the cliffs? That drawing is from my teacher as a guideline to get us started and such.

11. May 7, 2016

### SteamKing

Staff Emeritus
Well, I don't know all the details of this project.

I'm just saying that you can't analyze this structure as a simple truss when it isn't one.