Method of joints for bridge truss

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.
  • #1
Comfy
12
0

Homework Statement


Calculate the forces for each member.

Homework Equations



Sum of forces in x=0
Sum of forces in y=0
Sumof moments about D=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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  • #2
Comfy said:

Homework Statement


Calculate the forces for each member.

Homework Equations



Sum of forces in x=0
Sum of forces in y=0
Sumof moments about D=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
 
  • #3
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:
  • #4
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?
 
  • #5
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.
 
  • #6
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.
 
  • #7
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.
 

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  • #8
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
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.
 
  • #10
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.
 
  • #11
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. :frown:
 

What is the "Method of joints" for bridge truss?

The Method of joints is a framework for analyzing the forces acting on each joint in a bridge truss. It involves breaking down the truss into individual joints and using the principles of statics to calculate the forces at each joint.

What is the purpose of using the Method of joints for bridge truss analysis?

The purpose of using the Method of joints is to determine the internal forces (tension and compression) acting on each member of the bridge truss. This information is crucial for ensuring the structural integrity and safety of the bridge.

How is the Method of joints used in bridge truss design?

The Method of joints is used in the design process of a bridge truss by providing a systematic way to calculate the forces at each joint, which in turn helps engineers determine the appropriate size and shape of the truss members to withstand the expected loads.

What are the assumptions made in the Method of joints for bridge truss analysis?

The Method of joints assumes that the truss is composed of straight, rigid members that are connected only at the joints. It also assumes that the joints are frictionless and can only support forces acting along their axes.

Are there any limitations to using the Method of joints for bridge truss analysis?

Yes, there are a few limitations to using the Method of joints. It is only applicable to determinate trusses, meaning that the number of unknown forces can be determined using the equations of statics. It also does not take into account the effects of shear and bending forces, which may be significant in some truss designs.

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