(adsbygoogle = window.adsbygoogle || []).push({}); [PLEASE HELP!!] Forces in members of a truss

1. The problem statement, all variables and given/known data

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 [Broken]

2. Relevant 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]

3. 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 [Broken]

[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.

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# Homework Help: [PLEASE HELP ] Forces in members of a truss

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