An old truss near the Civil Engineering faculty building

One of the structures that fascinated me the most during my first years as a Civil Engineering student was this old truss near the faculty building:

The area covered by this truss is a total mess, but I managed to focus on the truss every time I passed by this structure. Notice how the compressed members of the truss are made of reinforced concrete large cross sections whereas the tension members are thin and slender and they are made of steel.

Ideal trusses are composed of members either in pure tension or pure compression. Since buckling can occur in compressed members but not in tension members, the former need to be made with large cross sections (large moment of inertia). Tension members on the other hand, can be very thin, as long as they are made of a sufficiently strong material (such as steel) with the required cross sectional area without worrying about buckling.

In the following video, I try to illustrate this concept by exaggerating the slenderness of members in tension. Practically, the string I used has no resistance against buckling, so it doesn't work in compression. For this reason, I need to stabilize the truss by applying some load to it:

Here is why the upper cord is in compression and the lower one in tension:

I don't know if the structure I showed earlier is still there, near the faculty. I hope it is, because it is inspiring for first and second year Civil and Architecture students. It was for me, at least.