Trouble Working Out Internal Forces in Truss - Need Help!

[liamh05]

hi all,

im having trouble working out the internal forces in this question, i tried before and got stuck working out the forces at joint C. i have been to see my tutor however he is on holiday till after the deadline for this assignment. handy...

i was working out the forces by the method of joints when it occurred to me that i had not taken into account any zero force members, having looked into what constitutes a zero force member i have concluded that there are none in this truss?

obviously i cannot continue with the project until i have calculated the interna forces..

i have some other questions regarding the project so far:

1) am i correct in thinking that there are no zero force members?
2) i don't understand the difference between the frist 2 questions; a) the internal forces in each member and b) the total forces at each joint.

i t14hink once i get started it shouldn't be too bad.

i worked out the force DC to be 19620N in compression
the force DE to be 27746.87N in tension.

i think that is right but i hit a problem by the time i get to joint DCB i wondered if it was because i didnt realize take into account zero force members :S

what is more confusing is that although i think there is no zero force members the equation m> 2j-3
14>15
is not true therefore is it true that there is one zero force member and i am incorrect?

im really confused about this please help.

 

[liamh05]

can nobody help ?

have i missed some information ?

i have comsulted some people at work and they say the structure has no zero force members :/

anyone?

 

[pongo38]

"the equation m> 2j-3" is an interesting equation that is true for many structures but also can be false (but not in this case). Personally, I cannot see any zero loaded members. This is statically determinate and therefore, the problem is self-checking, if you first of all determine the reactions and then work through joint by joint, usually in the order in which you would build it (but not in this case). At the final joint analysis, you should find equilibrium is maintained. I prefer to check reaction calculations by drawing the triangle of forces for the external actions. Drawn to scale, this triangle should give a good agreement with the calcs, and act as a useful check before launching into detailed analysis of all the joints. Joint at C is probably the last one to be considered because it has so many unknowns.