Table schema:
CREATE TABLE table_name_24 (height__ft_ INTEGER, floors VARCHAR, city VARCHAR, height__m_ VARCHAR)

Question: What is the smallest height (ft) of a building in Frankfurt with a height (m) of 257 and less than 55 floors?
SQL: SELECT MIN(height__ft_) FROM table_name_24 WHERE city = "frankfurt" AND height__m_ = 257 AND floors < 55