Calculating Reaction Forces in Truss Structures

[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 don't 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!

 

[SteamKing]

http://[ATTACH=full]199941[/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 don't 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![/QUOTE]
You have to pick a triangle and work out the angles.

The triangles ABF, BFD, BDC, and FED are all the same: isosceles triangles with a base length of 400 cm and identical sides of 250 cm.

Triangle AGF is an oddball, but it still can be solved. Remember the Law of Cosines?

[CENTER][ATTACH=full]199942[/ATTACH] [/CENTER]

 

[hc23881]

thanks, but I am still stuck. So I have figured out all the angles(picture attached) but I still can't 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?

 

[SteamKing]

thanks, but I am still stuck. So I have figured out all the angles(picture attached) but I still can't 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?

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.

 

[hc23881]

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.

even if i change the axis? then it wouldn't be only in the y direction.. but if i keep the axis as shown in the picture i don't know the angle to use for Ay because it cuts the triangle in a way where i don't know the side lengths and therefore can't find the angle

 

[SteamKing]

even if i change the axis? then it wouldn't be only in the y direction.. but if i keep the axis as shown in the picture i don't know the angle to use for Ay because it cuts the triangle in a way where i don't know the side lengths and therefore can't find the angle

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.

 

[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..

 

[SteamKing]

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..

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.