Composite steel-concrete construction involves linking the beam to the concrete slab mechanically, to optimise materials. Furthermore, the reinforced concrete slab, which already acts as the horizontal loadbearing surface, is used as the compression component of the composite section. It thus increases both the resistance and the rigidity of the beam, which forms the tension element under bending.
Likewise, concrete and steel can be combined in composite columns of various sections.
In the case of multi-storey buildings, composite columns (sections covered with reinforced concrete) contribute not only to fire protection, but also to the transfer of vertical loads, thereby ensuring the stability of the structure and considerably reducing the crosssection and number of columns needed.
The hollow shape of H-sections means that they can be filled level with concrete between the flanges before the parts are assembled, thus eliminating formwork costs. This technique allows part of the metal surfaces to be left visible for architectural purposes.
Composite structures offer unrivalled properties when exposed to seismic loads.
Mega columns with 4 encased steel profiles
Composite mega columns of tall buildings are currently designed with continuous built-up sections, welded in the fabrication shop and spliced on the job site without any prequalified welding procedure. This leads to highly restrained welds and splices which, under severe dynamic loadings, will likely crack before exhibiting any ductile behavior. These tall buildings have not been submitted to severe earthquakes, but it will happen. The 1994 earthquake in Northridge, California, taught us that welding procedures, beam-to-column connections and column splices have to be as simple as possible to properly and reliably work as anticipated.
Using multiple rolled sections encased into concrete is the solution for increasing the safety of tall buildings. It leads to less welding, less fabrication works and reliable simple splices which have been used for decades in high-rise projects.
AISC allows engineers to design composite sections built-up from two or more encased steel. But, it doesn’t explain how to perform and check the design. Those papers offers a method to do it. The method is explained by means of design examples covering combined axial compression, bending and shear.