Let's quickly notice what happened

when we moved from the truss with five nodes,

which we knew was statically determinate, to the truss with seven nodes at first.

Well, we added one, two, three, four bars ;

so four, here, on the left part of the equation ;

and then we added two nodes ; so, 2 x 2 = 4,

it is logical that the structure remained statically determinate,

and that when we moved from a truss with seven nodes to nine nodes,

we have added again one, two, three, four bars and two nodes,

so again, it is logical that our structure remained statically determinate.

We could keep going,

making a truss with 11 nodes, with 13 nodes and so forth,

and the solving method would remain the same.

I am not going to solve the equilibrium of this truss here

but I gave it to you in an illustrative way,

it is an another way to represent it, you can see :

we have started again by the node which is on the far-right,

I have called it A in this case, and I have drawn

the Cremona diagram -- here you have, still in grey,

the final diagram.

I have drawn the contribution of this orange node to this first step.

Afterwards, we moved to the node B, where we have discovered two new bars,

which I drew in green since they have been discovered studying the node B,

which is itself drawn in green ; here are their contributions,

the node C has this contribution, with this return that we have already seen

in the truss with 7 nodes ; and so forth for the whole structure.

Here, on the bottom, the internal forces in each of the bars are represented ;

so, again, under a load of 30 N, plus a load of 10 N.

And here, we have this figure which is a bit bigger,

on which we have represented, for the arch-cable, as well as for the truss,

the amplitude of the internal forces using the width of the bars.

We can notice that in the arch-cable, all the elements are essentially subjected to

high amplitudes, they are quite thick.

However, in the truss, we can notice that a certain number of bars,

like these ones, are subjected to low internal forces compared to other bars

which are subjected to much larger internal forces.

What we can also notice is that there is a value

which is absolutely identical in all the structures : the maximum tensile force at the bottom,

which is constant in the arch-cable and has here a value of 31.3N.

Other values, such as this one, 27.2 or even 18.9 are quite similar.

The same phenomenon occurs for the compression. Here, we do not find exactly

the same value : we have 45.1 and 42.3 but it is something

which is quite similar anyway.

Likewise, the compression in the upper part,

has a value of 31.4N in the arch-cable, while it is 29.2N for the truss.

What we can deduce from this is that the larges internal forces

in the truss are similar to the ones in the arch-cable.

Some of you have maybe been surprised by the fact that I consider this truss

as a truss with seven nodes.

This is true : we can see one node,