Qhov hluav taws kub tsis kam ntawm steel qhov rooj hluav taws sib txawv, thiab lawv cov hluav taws kub tuaj yeem muab faib ua peb qib: A, B, thiab C. Hlau qhov rooj hluav taws ntawm qib sib txawv tuaj yeem siv rau ntau qhov chaw. Ntau qhov zoo ntawm cov qhov rooj hluav taws steel ua rau lawv nyob hauv kev lag luam loj hauv cov tsev pej xeem.