Why do modern pianos have cast-iron frames?

Modern pianos utilize cast-iron frames to withstand the immense tension exerted by their strings, which can reach approximately 18 to 30 tons in modern instruments.12

Early pianos were constructed entirely of wood, which proved insufficient to support the increasing string tensions required for greater volume and projection in larger concert halls. As string tension rose, wooden cases and pinblocks were prone to failure, necessitating the development of metal bracing. The transition to a single, massive cast-iron frame—first applied to square pianos by Alpheus Babcock in 1825 and later to grand pianos by Jonas Chickering in 1843—allowed for significantly higher string tension compared to the lightly strung wooden instruments of the 18th century. This structural reinforcement is essential for the modern piano's increased loudness, dynamic range, and tonal spectrum.12