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anonymous one year ago Determine the stresses in all the members of the hexagonal truss shown. Point O is not a joint.

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1. mathmate

It would be more appropriate if you post the question in Engineering or Physics. Post another question here with a reference to the original post if you wish.

2. anonymous

|dw:1444134442509:dw|

3. mathmate

1. In which direction is the 32kN force? 2. I suppose you need the forces and not stresses, because no member characteristics have been given.

4. mathmate

First step: find the reactions, assuming the force 32kN is to the right. |dw:1444134954850:dw| Since the system is statically determinate, external forces are not altered by the shape of the truss. I drew a rectangle to make it easier to understand. Can you find the reactions Ra and Rb, using $$\sum Fx, \sum Fy$$ and taking moments about a or e?

5. anonymous

|dw:1444135207390:dw|

6. mathmate

Sorry, it's a roller on the right. |dw:1444135374698:dw|

7. anonymous

then? sorry if I can't fully respond to what are you teaching.. because our prof didn't teach that yet..

8. mathmate

Have you learned about ∑Fx,∑Fy and taking moments ?

9. mathmate

You would also need to know about forces at a point, triangle of forces, etc. Has your teacher started talking about these topics? Is it a physics or engineering statics course?

10. anonymous

i know how to get the x and y components and taking moments.. BUT, i'm confused..

11. anonymous

engineering statics course...

12. mathmate

|dw:1444136258079:dw| To help you get started, here are some calculations. The roller at point e means that horizontal reaction at e is zero. So all the horizontal force of 32 kN is resisted at a. By taking moments of 32kN*4 m=128 m-kN, we conclude that the vertical reactions at a and e are 128/8=16 kN, downwards and upwards respectively as shown. Now your next step is to solve for the forces of the truss, given the 4 external forces. Hints: use tools such as triangle of forces, equilibrium of forces at a point, symmetry, etc. to solve for the internal forces. It is not a beginner's problem, so you will need to be quite familiar with the tools.

13. mathmate

Further hints: 1. There are only 3 forces acting at joints c and f. So by geometry symmetry and summing Fx, you will find that Fbc=Fcd=x. 2. similarly, Faf=Ffe=y. (x and y are different because the geometry of c and f are different. 3. knowing x and y, you can solve for z=Fcf in terms of x and y, and consequently set up a relation between x and y. You now have four other joints (a,b,d,e) to solve for the four remaining member forces. 4. You will have to make a reasonable judgment on the direction of forces on the members, and propagate the directions throughout the truss before you can start working joint by joint, because summing forces at each joint requires a direction of the internal members.

14. mathmate

I will be tied up for a while, so work on it as far as you can. Post what you have. Someone else might be able to help you in the mean time. I will be back to check on you probably tomorrow some time.

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