ELI5: How do engineers calculate if a dam will withstand decades of fluctuating water pressure?

Engineers use advanced computer simulations to test how a dam handles years of changing water pushes, like flexing a metal paperclip back and forth to make sure it will never snap.

Imagine holding your hand against a friend who is gently leaning on you. It is easy at first, but if your friend constantly leans hard, then steps back, and then shoves you over and over for hours, your muscles get tired. Water does the very same thing to a dam. When it rains a lot, the water pushes with immense force; during dry summers, the push becomes much lighter.

To ensure the dam survives this endless cycle for decades, engineers treat the problem like a giant puzzle using simple, everyday ideas:

• Checking the strength of the ingredients: Just like bakers know how stiff dough will be, engineers test chunks of concrete in labs by squeezing them with huge machines to find out exactly how much pushing and relaxing the material can take before it gets tiny cracks.
• Creating a virtual twin: Engineers build a 3D video-game version of the dam inside a computer. They program the computer with decades of weather history, simulating massive storms, dry spells, and even earthquakes. The computer simulates the water pushing, letting go, and pushing again millions of times in just a few minutes.
• Studying material fatigue: If you bend a plastic spoon once, it bends right back, but if you wiggle it fifty times, it turns white and breaks. This is called fatigue. Engineers calculate the exact point where concrete gets "tired" and design the dam to stay well below that danger zone.
• Adding a huge safety cushion: If engineers calculate that the biggest flood will push with the force of ten elephants, they build the dam strong enough to hold back thirty elephants. This extra strength is called a factor of safety.