The piano is under tension?

Each string in the piano carries tension. Depending on where the string is located in the piano, the tension can range from around 130 to 220 pounds. The average piano has 230 or so strings. Multiplying it out yields a total string tension in the neighborhood of 16 to 18 tons! That is why every piano has a big, heavy, cast-iron plate - to keep it from collapsing upon itself.