Cremona diagram /Bow's notation- how to determine what is in tension and what is in compression

[GregLocock]

I think I understand which direction the external forces need to be drawn but I can't remember how to figure tension and compression, without looking at each joint. Ignore the horribleness of the truss design itself.

Cheers

Greg Locock


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[BAretired]

If you name the zones and rotate about each joint in a consistent manner, the direction of the force vector tells you which way the force is going, relative to each joint.

In your case force AB is 10 units down, so I would draw line AB as A down to B.

I have always gone clockwise around the truss to name the zones and clockwise around each joint to indicate direction of applied force, but I'm pretty sure it can be done with another convention.

 

[BAretired]

Greg, I think your truss is unstable. Segment H is a quadrilateral, not a triangle. You will need another member to stabilize it. Also the statement "All joints pinned" is wrong; one roller is required.

 

[GregLocock]

Thanks, I'll see if I can get that right. Oh I agree, it is a terrible truss design. My next little puzzle is whether I could have drawn the plot without knowing what Fx is.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[BAretired]

Yes, I wondered about that too. Fx can't be calculated from the diagram. If both supports are pinned, Fx can't go beyond either support, but if one end has a roller, Fx could be considered an applied load and would be resisted at the opposite support which is pinned.