1. Jul 2, 2008

### ttiger2k7

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
$$\sum F_{x} = 0$$

x-components
$$\sum F_{y} = 0$$

Moment
$$\sum M = 0$$

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]

$$\sum F_{x} = 0:$$
$$A_{x} = 0$$

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

$$\sum M_{a} = 0:$$
$$V_{y} (18) = (1.5)(3) + (1.5)(6) + (1)(9) + (1.5)(12) + (1.5)(15) + (1)(18) =$$
$$V_{y} = 4.5 kips$$ ----- (2)

Then plugging equation (2) into equation (1) I get
$$A_{y} = 4.5$$

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.

Last edited by a moderator: May 3, 2017
2. Jul 3, 2008

### 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.

Last edited: Jul 3, 2008
3. Jul 3, 2008

### 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.

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4. Mar 29, 2010