STUDIES IN STEEL ENCASED RC COLUMNS

Abstract

Tubular steel sections are increasingly being used as columns to carry axial load along with moments. These columns are presently being designed just as hollow sections even when they are filled with concrete. It is pertinent, therefore, to study the additional load carrying capacity on account of concrete infill and also due to appropriate provision of reinforcing steel bars. Although, capacity enhancement is expected due to confinement of concrete, only carefully carried out experiments would bring out the quantum of enhancement with varying strength of concretes. The present study is an attempt to clearly bring out the significant contributing factors in terms of capacity enhancement. Experiments have been carefully designed using mild steel tubes available in 140, 166 and 194 mm outer diameter. Wall thickness of the tubes is uniformly 4 mm. These tubes have been obtained out of the long pole used for electrical power transmission. Tube lengths in 300mm and 500mm have been cut and ends processed to result in true transverse sections. Concrete of compressive strength 38, 44, 52 and 57MPa have been used as infill material in the steel tubes mentioned above. Plain concrete as well as reinforced concrete have been tested for their individual load carrying capacity and later on tested with steel tube with concrete infills. As expected, the load carrying capacity of these columns is in excess of the sum total of capacities of tube and concrete taken independently. This is the primary aim of the study wherein the advantage of steel column with concrete infill gets highlighted against hollow steel column. Tube length of 300mm primarily represents the basic characteristics of the composite action of steel and the infill concrete. The structural characteristics would come out when length-to-diameter ratio is increased meaningfully so as to make the column fail not just due to exhaustion of materials strengths but the structural element becoming unstable. This aspect needs large dimension loading machine along with high load carrying capacity. The existing testing facility permitted just doubling of the length to 500mm, which does show local failures through wrinkle formation.