How Do You Calculate Forces in Truss Members IK, JL, and JM?

[ttiger2k7]

[PLEASE HELP!] Forces in members of a truss

Homework Statement

A barrel vault truss is loaded as shown. Knowing that the length of the bottom chords DF, FH, ..., and QS is 3 ft, determine the force is members IK, JL, and JM.

http://img68.imageshack.us/img68/4817/fyjfhjfgjhfghgx8.jpg

Homework Equations

Sum of y-components
[tex]\sum F_{x} = 0[/tex]

x-components
[tex]\sum F_{y} = 0[/tex]

Moment
[tex]\sum M = 0[/tex]

The Attempt at a Solution

First I did a free body diagram of the entire truss, which gave me three unknowns tosolve for first.

http://img392.imageshack.us/img392/9493/fyjfhjfgjhfgh1ur3.jpg

[tex]\sum F_{x} = 0:[/tex]
[tex]A_{x} = 0[/tex]

[tex]\sum F_{y} = 0:[/tex]
[tex]A_{x} = (1+1.5+1.5+1+1.5+1.5+1) = A_{y} + V_{y}[/tex]
[tex] 9 kips = A_{y} + V_{y} [/tex] ---- (1)

Solving for Moment about A
[tex]\sum M_{a} = 0:[/tex]
[tex]V_{y} (18) = (1.5)(3) + (1.5)(6) + (1)(9) + (1.5)(12) + (1.5)(15) + (1)(18) =[/tex]
[tex]V_{y} = 4.5 kips [/tex] ----- (2)

Then plugging equation (2) into equation (1) I get
[tex]A_{y} = 4.5[/tex]

Then I get stuck. I know I need to pass a section through the truss so I can get two portions, with each including the desired members I am solving for. Then I just draw a FBD and solve for no more than three unknowns.

 

[PhanthomJay]

You can get IK pretty quick using the method of joints. Beyond that, I don't like that roller support at the bottom corner. Nonetheless, try then cutting a section from IK through JM.

 

[loststudent0]

have you considered this.. do you know the reference angle?(i know it, i think you know it too)
Once you figure out IK, and you cut through as PhanthomJay says, you now have 3 unknowns and 3 equations..in other words, solvable.

 

[e_0ne]

help... anyone can help to settle this problem?