# Method of joints for bridge truss

• Comfy
In summary, the conversation discusses the process of calculating support reactions for a truss structure with four points of support and a centrally located load. The structure has rollers at A, C, and F, and a hinge at D, causing it to be statically indeterminate. The difficulty arises due to having four points of support but only two equations of statics to work with. The conversation also touches on the potential issue of having rollers on top of the cliffs, as this may not be possible for the structure to support.

## Homework Statement

Calculate the forces for each member.

## Homework Equations

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

## The Attempt at a Solution

Work is on attached image of problem. I'm having trouble solving for support reactions. At D there is [/B]
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 I am stuck). So how can I solve these support reactions?

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• 20160507_184022.jpeg
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Comfy said:

## Homework Statement

Calculate the forces for each member.

## Homework Equations

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

## The Attempt at a Solution

Work is on attached image of problem. I'm having trouble solving for support reactions. At D there is [/B]
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 I am stuck). So how can I solve these support reactions?
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

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?

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Comfy said:
That is the right thing to do right? I keep getting Fy and Cy to cancel out during my sum of the moments equation?
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?

SteamKing said:
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?
There is a roller at A, C and F. There is a hinge at D.

Comfy said:
There is a roller at A, C and F. There is a hinge at D.
What's the purpose of the rollers at A and C? Your sketch is not clear on this.

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

#### Attachments

• 20160507_213340.jpeg
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• 20160507_184022.jpeg
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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.

Comfy said:
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.
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.

SteamKing said:
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.
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.

Comfy said:
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.
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.