How is it that concrete floors in stadiums move/wiggle when jumped on without collapsing?

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Basically, I went to a concert, thought we were all going to die, then we did not die.

In: Engineering

Anything built to be absolutely rigid will eventually fail, so all structures allow for movement – better to bend than break.

These structures aren’t solid concrete – for all its strength, concrete is also brittle and inflexible and makes a poor structural material when used alone. Imagine a very thin sheet of concrete – the stuff is basically crumbly glass.

Instead, the concrete walls and floors are reinforced with steel rods and much of the structural strength for the building comes from steel beams and pillars inside or behind the concrete.

The steel is more flexible than the concrete, and can absorb all that energy and vibration without shattering.

Concrete in building construction has re enforcement steel bars laid in as the concrete is poured. This creates a super strong structure.

I’ll Explain it by first explaining 4 concepts so you can piece more clearly everything together it’s a bit long but probably as easy as i can make it.

#1

Grab a stick break it in half easy right? Put both halves next to each other and try to break it again as you keep going you won’t be able to break the short sticks. Because your muscles won’t be able to exert enough force to spread on all of them that’s more cross sectional area that you are trying to tear through that’s the first concept and the second concept the length of the sticks is shorter which makes it even harder to break.

#2

The reason shorter is harder to break is a concept in physics called moment. The simplistic explanation when an object like take a door for example. If you open and close the door from handle it’s easy because the joint is furthest away but try pushing the door from near the joint it requires a lot more force and at the joint itself it’s no longer feasible.

#3

Now back to our first concept the force exerted on an area is just stress. The deformation caused by force exerted on an area is called strain the third concept i want you to understand. For all materials they behave cute some stress applied to it once removed the object returns to it’s previous state this means the strain it undergoes is reversible like for example holding a ruler at the edge of the table and bending it slightly the ruler doesn’t break and vibrates returning to what it was before. In mechanical engineering they simply grab a piece of metal and try to stretch it and measure the stress vs strain the phenomena and what they see is it’s a straight slope then it curves into a fat arc and plummets. The straight part of that slope is the elastic deformation this is were the material still acts like rubber bouncing back to where it was because little stress is applied each material is different this way. Past that the molecules undergo slipping which causes the curves and this is where when you remove the stress the material restores itself but it’s not back to 100% the original basically like stretching a rubber band more than you should have not enough to snap it but you notice it doesn’t look like the same rubber band as before it restores itself but somewhat more stretched on rest. Different materials have things differently when it comes to their behavior regarding stress vs strain but to give you a run down metals bend and undergo slipping after elastic deformation and their region of elasticity is just large and can handle high stresses but also it takes a while to shatter this has a name and it’s called ductile. Rocks on the other hand have a high stress with a tall elastic region then nothing it’s simply taking on stress then bam shatters into cracks this is useful because less vibrating and more being solid as well a rock.

#4

The last concept is manipulation what happens when you have both steel and rock together you basically get something in between being a rock and being steel the best of both worlds when it comes to stress and strain.

By making the fibers of steel in the matrix of concrete form a higher cross sectional area while causing tougher moment to break through you get a vibration absorbing reinforced composite which makes the ground a trampoline of stone.

Another point I didn’t see mentioned before. They literally design in some give and sway so the crowd can feel their own impact. It gives the otherwise brutal hard shell some warmth.