Fire resistance of concrete filled circular hollow columns with restrained thermal elongation

The filling of circular hollow sections (CHS) with concrete is a good solution for strengthening columns since such procedure increase their load bearing capacity at room and high temperatures. However, in the event of a fire, restraining to thermal elongation may change their mechanical behavior. This paper presents the results of a large series of fire resistance tests on CHS columns with restrained thermal elongation. Parameters such as the slenderness of the column, its load level, the stiffness of the surrounding structure, the percentage of steel reinforcement and the degree of concrete filling inside the column, were tested. The results obtained show that the critical time of the columns was less than 46 min. The use of a concrete ring around the internal surface of the column's wall is of no advantage in terms of its behavior under fire conditions because this concrete ring suffers extensive spalling and cracking due to overheating of the steel tube. The main failure mode of the columns was global buckling. However in several cases local buckling also occurred.

► Experimental study on the fire behaviour of CHS columns with thermal restraining.
► Critical times of CHS columns tested were smaller than those found in literature.
► Stiffness of surrounding structure did not lead to major changes in critical times.
► Reducing load level and slenderness of columns enhances their performance in fire.
► Local buckling occurred in columns for a diameter-thickness ratio smaller than 59.