The combined tension of the strings in a grand piano can support the weight of two large school buses.

A typical modern concert grand piano, such as a Steinway Model D, contains approximately 230 individual strings made of high-carbon steel. Each string is stretched to a tension ranging from 160 to 200 pounds. When these forces are combined, the total inward pressure exerted on the piano's frame reaches between 18 and 22 tons, or roughly 40,000 to 44,000 pounds. To put this in perspective, a standard American school bus weighs about 20,000 to 24,000 pounds. This means the internal tension of a single piano is equivalent to the weight of two full-sized vehicles. This immense physical load is why the cast iron plate, or 'harp,' was such a revolutionary invention in piano manufacturing during the 19th century. Before the introduction of the one-piece cast iron frame by Alpheus Babcock in 1825, pianos were made primarily of wood. These older instruments could not handle high tension, resulting in a quieter sound and frequent structural failure. The modern iron frame allows for thicker strings and higher tension, which produces the powerful, resonant volume required for large concert halls. The strings themselves are engineered to withstand stress near their breaking point to achieve the best tonal quality. If the cast iron frame were to crack, the sudden release of those 20 tons of pressure would be catastrophic to the instrument. This engineering marvel allows the piano to remain in tune while balancing forces that would easily crush most other wooden objects.