You might find something on standards API 650 (atmospheric petroleum products storage tanks) or API 12D (low capacity prefabricated storage tanks). Try also the American Institute of Steel Construction: www.aisc.org Giovanni S. Crisi Sao Paulo - Brazil
It would depend on which code the tank is being built to. Are you talking about the bolted stairways on 12F or a vertical ladder going up the side of the tank? BABRT's