Composite megacolumns : testing multiple, concrete-encased, hot-rolled steel sections

There is an ongoing need to optimize construction materials and reduce the size of elements required within the structural systems of high-rise buildings. Minimizing the size of the vertical structural elements, without compromising the economic feasibility of projects and limiting their significant share on tall buildings’ floor plans, is a consistent challenge. The use of composite structural elements, such as combining concrete and steel, along with higher grade materials within each, is a viable solution. Currently, concrete filled tubes (CFT) or concrete filled continuous caissons built-up by welding heavy plates are the common structural solutions. Their main drawbacks include high costs, the need for skilled labor, complex connections, and requiring welding conditions for heavy plates, such as preheating and repairing. Composite megacolumns considered in this research are defined as vertical structural systems with more than one hot-rolled steel section, longitudinal rebar and ties embedded in concrete, and they are subject to significant vertical loads and secondary bending moments from wind and seismic actions. They are believed to be a convenient solution in terms of structural behavior, cost, and constructability for the design of tall buildings, including towers over 300 meters tall. Although codes and specifications do consider composite structural elements, they do not offer specific provisions on the design of composite sections with two or more encased steel sections (AISC 2010 Specifications for instance). The lack of knowledge on the axial, bending, and shear behavior of composite megacolumns, along with the resulting lack of clarity in the codes, leads to the need for experimental performance tests. These tests, and the resulting findings, suggest a simplified design approach and help develop numerical methods to describe the designs and to validate the results. The laboratory tests took place between February and September 2015 within CABR Laboratories and the Laboratories of Tsinghua University, Beijing. The column specimens’ overall layout and geometry have been based on suggested sections, from MKA and others, of representative full scale composite columns considered for high-rise buildings. Overall dimensions of the representative full scale columns considered for this testing program are 1,800 by 1,800 millimeters, with a height of 9 meters at the Lobby level (base of the tower) and 4.5 meters at the typical floor. The laboratory tests consisted of two sets of tests that attempt to define the axial load and moment (P-M) interaction curves of the representative columns at failure. Static tests were accomplished by applying 0%, 10%, and 15% eccentricity axial loads, on six 1:4 scaled specimens, until failure. Quasi-static tests were accomplished by applying 10% and 15% eccentricity axial loads with horizontal forces on four 1:6 scaled specimens, until failure. Results are used to investigate the specimens’ maximum capacity, displacements, stress distribution, ductility, and stiffness. Experimental results are validated by finite element method (FEM) models developed by CABR and AMBD with Abaqus and Safir software, with the numerical values in accordance with the experimental values. FEM models allow also for a deeper insight on steel-concrete interaction forces and stress distribution.