- Jan 17, 2013
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I'm hardly an expert, but i have some background in structural engineering, so I'll give you my take on it.Probably a stupid question but I'll ask, anyway. Why are the first two tiers made of concrete and the third of meccano? Is there a varying trade-off between stability and weight at different heights in a superstructure, or is there some other reason?
The most basic thing is stability. If you stack coins, you want the heavier coins at the bottom and the lighter at the top.
Secondly, there's a difference between compressive and tensile stress. My assumption is that there's a higher share of compressive stress at the bottom than at the top, and concrete has an incredible compressive capacity. Its tensile strength, on the other hand, is next to nothing, which is why reinforcements are needed.
Now reinforced concrete is, generally speaking, the best, cheapest and most applicable construction material there is. That's why it's so widely used. But it's also very heavy, which means that the dead load from the concrete contributes a lot to the total load on the structure. So if there's a high share of tensile loads in a part of the construction, the use of concrete might actually make things worse. This is why you rarely see concrete in cantilever beams that support a roof. Steel, on the other hand, is equally strong both ways.
Thirdly, there's the cost point of view. You basically want a construction to be as close to its capacity as possible. The loads go all the way from the top, through the construction and down to the foundation, so quite simply the loads will be higher at the bottom than at the top. Which means two things:
1. You want to reduce the cumulative loads at the bottom as much as possible.
2. The upper part of the construction can be weaker than the bottom.
Based on that, it makes a lot of sense to have a heavier, more solid construction at the bottom, and a lighter construction at the top.