Assuming the tank is used for the same kind of chemical throughout its life I would consider it as dead load. Since the weight of the chemical isn't variable, you wouldn't need to consider variability like there is with live load. I would also do two analyses of the tank, one empty (low gravity...
AISC Code of Standard practice section 6.4.1 limits variation in length 1/16 in for members less than 30 ft long and 1/8 in for more than 30 ft long. This agrees with what Canwesteng said about not getting a substantial gap in AISC approved shops.