Frist we used washers but it didn't work because we had nowhere to put them. So we used the black books and it worked very well. We got ten books on the structure, that is 8 kg. We got a little surprised that our structure can hold that much, if it is that little.
We had no changes in our strong structure.
I learned that cylinder is stronger than the other shapes. I learned that we use more columns than beams. If the structure is too tall it will not hold that much. The structure has to be balanced or it will not stand. The dead load should not be a lot.