A standard indoor temperature rising curve is simulated by using a 10 kW electrical furnace. DN20 seamless steel tubes are horizontally placed on top of the electrical furnace. The distances between the bottom of the steel tubes and the mouth of electrical furnace are 0.5 cm, 5 cm, 10 cm and 15 cm, respectively. Experiments regarding the thermal response of an unshielded steel tube, a steel tube with an external fire retardant coating, and a concretefilled steel tube are carried out. When the temperature of lowest point of the bottom of the unshielded steel tube is 400 ℃, the temperature of corresponding points of the steel tube with an external fire retardant coating and the concretefilled steel tube are 215 ℃ and 200 ℃, respectively. It is concluded that the steel tube fireresistance limit exceeds 2 hours when the exterior of a steel tube is coated with fire retardant or if steel tubes are filled with concrete because these modifications improve their thermal insulation and decalescence.