A piano is a sophisticated and complex creation. With over 10,000 moving parts and 230 strings beneath its unique and flawless encasing. The interior of a piano is so intricately constructed, each string holding approximately 165 pounds of tension. Totaling more than 18 tons of combined tension weight.
"Piano makers gradually learned that pianos could be made louder by increasing the weight and tension of the strings with the result that wooden frames soon became inadequate to support the increased stresses. Until around the beginning of the 19th century the load-bearing structure of pianos was made entirely of wood. By the end of that same century almost all pianos had cast-iron string plates. The metal plate brought improved tuning stability and, at least to most modern ears, better tone. " (https://www.speech.kth.se/music/5_lectures/conklin/thecastiron.html)