# Truss structures and forces

1. Oct 16, 2015

### hc23881

http://[ATTACH=full]199940[/ATTACH] I am having trouble with this truss problem. I have attached my work so far. I used the entire structure as a system first and did the sum of the forces equals zero but I don't know how I can finish the moment at a because I dont know the x and y distances for the 500N force. and I have no idea where to go from there and have been working on it for a long time. I get so lost with the angles in problems like these. help would be appreciated. thanks!

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Last edited by a moderator: May 7, 2017
2. Oct 16, 2015

### SteamKing

Staff Emeritus

Last edited by a moderator: May 7, 2017
3. Oct 16, 2015

### hc23881

thanks, but I am still stuck. So I have figured out all the angles(picture attached) but I still cant figure it out. if i make my coordinates line up with Ax and Ay what degrees I would use for the x and y component of the 500N force. (when I am taking the moments about A and the sum of the x and y forces of the big triangle as the system. or is this not necessary and can i start at some other triangle to get the answer?

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4. Oct 16, 2015

### SteamKing

Staff Emeritus
It looks to me like the 500 N force is a gravity load, like something suspended from this truss. The 500 N has only a vertical component.

5. Oct 16, 2015

### hc23881

even if i change the axis? then it wouldnt be only in the y direction.. but if i keep the axis as shown in the picture i dont know the angle to use for Ay because it cuts the triangle in a way where i dont know the side lengths and therefore cant find the angle

6. Oct 16, 2015

### SteamKing

Staff Emeritus
You've lost me, here, and I think you are making this analysis more complicated than it needs to be.

From the picture, it looks like A is a pinned connection, and you can assume the center of the pin and point A and the vertex of the triangles sharing that point all coincide.

Why would you want to change the axis? To create even more calculation work?

You can use the equations of static equilibrium to find the reactions at A and G due to the load at E. The structure can rotate about point A, thus it can't sustain a moment at that location. You'll need a force at G to keep the structure from rotating about A.

7. Oct 16, 2015

### hc23881

but when i have the x and y components of the forces at A. they arnt directly on the x and y axis given , therefore i have to find both the x and y components of those forces. that is what i am having trouble with..

8. Oct 16, 2015

### SteamKing

Staff Emeritus
The x-y axis shown on the diagram is just there as a reminder.

You can make the origin of your coordinate axis at a convenient location, say point A. The positive x-axis can be taken running along the line AFE, and the positive y-axis only needs to start at A and be perpendicular to the x-axis. It won't matter as far as calculating the reaction forces which keep the truss in equilibrium are concerned.