In orchestra piano represents its own class. It came along in 1700s. It is a keyboard instrument and is not included in strings. String instruments are first and second violins, violas, cellos and double basses. Percussions neither. In orchestra there is only one piano. Same as only one harp. That one came along in 1800s. Perhaps you know that before piano there were clavichord in 1500s and cembalo in 1600s.