we dont have to determine c or t in links at first, we can solve it by assuming its a t type or a c type and if the amount of force was calculated as a negative num , we'll know we have assumed it in a wrong way
That's what I've been taught as well. His distances aren't going straight to joint he's making a moment about which is not what I've been taught in class.
Nioz Ysmael Look at the "wall mount" ball joints. You see 4, right? The one on the bottom constrains in x, y, z directions. (thus, 3 reaction: Ax, Ay, Az). However, see the other 3 ball joints? They do not constrain in xyz? Why? They got little floppy sticks connecting to the truss frame. That effectively invalidates the xyz constraint, and the rule one stick = one reaction, applies. Otherwise, if you took out the sticks at points B and C you will have 3 reactions at those points. Total becomes 9 reactions. (if you took out those sticks). BTW notice joint D is just floating....thus, the rationale for the zero force member CD.
In real terms, how "short" must the end members be for us to make that assumption you made? So about what ratio would the length of a 'short member' have to be to the main members before you define them as short members?
if the y-comp is zero then the force in the member must equal zero. If the member was perpandicular to the y-axis this would not be the case. Try projecting the force in the y-direction onto the member using dot product or cosinus/sinus, you will find that the force in the member is zero
when you analyze joint D the only force acting on the z axis is CD it means that there will be no other member that will oppose CD, to have equilibrium CD must be zero
