# Calculating Forces in Truss Members using Method of Section

## Homework Statement

a) Find the reactions provided by the foundations at A and B.
b) Use the method of Resolution at the Joints to calculate the forces in all the members.
c) Which member will fail first and why ?

ƩFx = 0
ƩFy = 0
ƩM = 0

## The Attempt at a Solution

I am really just concerned with whether I have got the following correct. If I have, then I have grasped the concept if not then if someone could point me in the right direction and advise me on where to go next of what I am doing wrong I would be very grateful.

Horizontal Reaction Forces in the X direction at point A = 0
Vertical Reaction Forces in the Y direction at A = 4000N Upwards
Reaction Forces about the moment A at point B = 6000 Newtons Upwards

Forces in Member AC = 0N (0 + AC + AF = 0)
Forces in Member AF = 4000N in Compression (4000 + AC + AF = 0)
Forces in Member BK = 6000N in Compression (6000 + FBK.Sin(90) + FBE = 0)
Forces in Member BE = 0N (-FBE + FBK = 0)

Thanks

#### Attachments

• 6312270865_79022f0ff8_z.jpg
19.8 KB · Views: 547

PhanthomJay
Homework Helper
Gold Member
This is all good, you got them all correct   Thanks very much!!! One last thing. I am now finding the forces in Members FC and FG. Am I right in saying:

Forces in direction:
-(-4000) - FC x sin(53.1) = 0 and then I just rearrange for FC = 5000 Newtons

and now I know FC I can do the following:

Force in direction:
0 + (FC x cos(53.1)) + FG = 0 and again rearrange for FG = -3002 Newtons

PhanthomJay
Homework Helper
Gold Member
Thanks very much!!! One last thing. I am now finding the forces in Members FC and FG. Am I right in saying:

Forces in direction:
-(-4000) - FC x sin(53.1) = 0 and then I just rearrange for FC = 5000 Newtons
yes , compression or tension?
and now I know FC I can do the following:

Force in direction:
0 + (FC x cos(53.1)) + FG = 0 and again rearrange for FG = -3002 Newtons
You have a round off error, these are 3-4-5 right triangles, FG = -3000 N. What does the minus sign mean?

Sorry a negative number means it is compression and a positive number means it is in tension.

PhanthomJay