HandsFreeCookieDunk wrote:You know, I think we can all say that thanks to Balsa Man, all of us know more about towers than we knew we wanted to know. But in all the discussions we've had about what makes a good tower and in looking at the forces that go into it, I've noticed that it seems as though the actual geometry of the tower doesn't matter all that much as long as it's symmetrical. Normally the trope you hear in these building events is "Always use triangles!", but they never really came into play.
Good observation; symmetry is critical for a….high performance tower (or any other structure). Remember that engineering is about approximations that work. If all 4 legs are exactly the same length, lean in at exactly the same angle, are all symmetrical to the vertical centerline, we can assume the load is equally distributed, and (closely) approximate the forces under load, and with testing and calculation, identify sticks that are as light as possible to carry our “design loads.” This analysis, the basis for design and wood selection that we’ve been discussing is a “static” analysis- a perfect structure that doesn’t move. You learn about the relationships and principles we’ve been using in ‘statics’ engineering courses. Its fairly simple and straightforward once you understand it. The more precisely you build, the closer to an ideal, static structure you get, and the closer to ‘the limits’ (of the strength and weight of wood and strength of joints) you can get.
But we all know that any real tower never gets ‘perfect.’ Little variations exist; a slight bow in a leg stick (even though with bracing in-place, its held very close to straight, a slight difference in leg length, either top or bottom leg ends slightly off….design angle, so the load going through the leg is not perfectly axial (i.e., some amount of bending is being induced under load).
A real tower under load is … a dynamic situation/system. If you watch very closely, at some load, little movements occur. In towers that are not precisely built, these distortions come in with relatively little load; overall twisting of the structure starts, and failure happens quickly. If you’ve ever taken a tower that only has ladder bracing in place, put a plate on top so you can put a little down pressure on, you’ve seen how it is easy to ….rotate the plate, or ‘translate’ the plate (moving side to side without rotation), and the whole structure either twists or leans. But what happens when X (or Z) braces are put in? The whole structure…stiffens dramatically- the bracing stops (up to some loading level) twisting/leaning. These X or Z braces form…..triangles. So, yes, triangles come into play, and they are critical to dynamic structural rigidity.