With tall buildings growing ever higher, it is becoming increasingly critical to optimise structural systems in order to ensure strength and stability. The rise of composite megacolumns could be the most effective answer.
As part of the Council on Tall Buildings and Urban Habitat’s (CTBUH) roadmap for the Future Research Needs of Tall Buildings, developed in conjunction with CIB and UNESCO, composite materials and structural optimisation were identified as priority objectives for the industry.
ArcelorMittal, the world’s largest steel producer, has now given $150,000 to help research the constructability and the engineering properties of this structural solution.
In tall buildings especially, the large dimensions required for concrete columns mean developers are finding it increasingly difficult to make projects stack up financially as structural elements comprise an increasingly significant share of the building’s lettable areas. This is especially the case at a building's the lower levels.
To mitigate this issue, many super tall buildings are now being designed and built using composite materials, with steel and concrete employed together so the strengths of one material can compensate for the limitations of the other.
Composite structures integrate multiple large-scale steel profiles embedded in high-strength concrete columns that can be larger than two metres by two metres. Via the use of shear connectors, the steel elements and the concrete act as a single material.
The Chinese Institute of Earthquake Engineering is already recommending the use of multiple jumbo H-shapes rather than large continuous caissons. These high performance steels not only fully comply with American and European standards, they can also meet the stringent requirements of the Chinese standards, such as the 20 per cent minimum elongation.
The Shanghai World Financial Centre has used the approach to outstanding effect. The structural designs for the 95-storey project, by Leslie E Robertson Associates (LERA), called for a megastructure consisting of composite perimeter columns, one at each corner of the rectilinear base and six as the floor plan morphs into a six-sided form at higher elevations.
Diagonal perimeter braces zigzag up the faces, connecting mega-columns vertically and connecting steel belt trusses horizontally. The building has a reinforced concrete services core which interacts with the exterior framing through outrigger trusses that span the space between the core and the belt trusses.
In order to take this approach to the next level, however, the new composite structures need to be fully tested to understand their strengths and limits, as well as to provide the design community with the awareness, guidance and tools necessary for integrating the use of composite structures into future super tall building designs.
With this in mind, the research team will work on design specifics derived from a real case study, a 300-plus-metre tower to be built in a major Chinese city. Different design solutions will be tested to identify the optimum system for columns of such an unprecedented size.
ArcelorMittal is also supporting CTBUH's research activity with a grant for research on the Life Cycle Assessment of Tall Building Structures, which was initiated in January 2013 and is due to report in early 2015.