The construction of tall buildings has had a marked shift away from a North American dominance that has lasted from the first steel skyscrapers in New York and Chicago in the early 1900s until the emergence of a new globalized field that is seeing a clear ascendancy in China and the Middle East in terms of quantity and height.
This research study is about the invention that has occurred in structural design to support the desire to build taller, and more specifically to the growing shift towards the use of composite construction and away from more traditional all-steel buildings.
According to CTBUH “a composite tall building utilizes a combination of both steel and concrete acting compositely in the main structural elements, thus including a steel building with a concrete core.” The term composite was initially established through the work of Lawrence G. Griffis, P.E. through his 1994 T.R. Higgins Lecture titled “Composite Frame Construction” (Modern Steel Construction. October 1994. p. 36–47). At the time, Griffis included within his study concrete filled tube (CFT) systems, reinforced concrete (RC)/steel reinforced (SRC) systems, and reinforced concrete (RC)/steel reinforced concrete (SRC) wall systems. It is of interest to note that prior to 1994 when Griffis made his speech, there were only 53 composite structures of any height recorded in the CTBUH database, and all were in the United States.
The nature of this material shift must be addressed alongside the recognition of global preferences in construction methodology. Global trends must necessarily be aligned with access to materials and either the strengths or limitations of local expertise in the labor force.
CTBUH: Tall Buildings in Numbers
First Study on Materials and Systems
Shift from Steel to Composite
One of the issues with the term “composite construction” is that it encompasses a variety of ways to structurally combine steel and concrete. Whereas the term may have initially been primarily associated with steel framed buildings having a reinforced concrete core in a North American-dominated tall building scenario, as mentioned in the CTBUH definition, it has grown to include structural variations that, while maybe uncommon in North America, have come to dominate tall building construction in Asia and the Middle East. These systems include concrete filled steel tubes (CFT) and concrete encased steel frames. So the term composite, which five decades ago served as a “catch all” for buildings that did not belong in the major categories of all-steel or reinforced concrete, is now assuming a significant proportion of buildings on an annual basis. This has resulted in a dilution of accurate information in the database as the information about buildings has not been collected with any more specificity than whether it is “composite construction” of some sort. The ever increasing number of tall buildings that is being constructed annually is exacerbating this problem.
The Composite Project
A preliminary study of the data available from the CTBUH Skyscraper Center online database suggested that this incredibly recent and rapid shift towards composite construction could render the database diminished in terms of accuracy and depth if the definition and parameters of composite construction were not more clearly defined and applied as a categorized system in the near future (see Figure 3). The initial basis of the composite type that reflected the North American deviation from all-steel, which is a concrete core with steel framing, may no longer be accurate. It was speculated as the starting point for the research that the predominant globally resultant categories would also include the use of concrete filled steel tubes as well as steel encased concrete. The study would also collect finer data on incidents of the use of diagrids, braced trusses, outriggers and megaframe systems.
The method was simply to search for construction images of the 582 composite skyscrapers (completed or under construction at the time of the study) whose height exceeded 150m in order to categorize them into the initially assumed three composite sub-types: concrete filled steel tubes (CFT), steel frame with reinforced concrete core, and concrete encased steel frame. The use of construction photos was adopted for the primary means of investigation for the following reasons: these could most easily reveal the construction method; are easily available online in several “fan-run” skyscraper forums; and, obtaining drawings and detailed information from architects, engineers and contractors would require more effort and was likely to be less time effective. As it was not the intention of the study to publish the images, copyright was not considered to be an issue. The initial research was conducted by Jasdeep Multani and Jeff So of the University of Waterloo, School of Architecture.
Accuracy and Completeness of the Data
Of the 582 projects, there were 79 for which construction images could not be found, providing us with an 86.4% success rate. The difficulty with finding information for the 79 projects seemed to be divided into buildings that were constructed “pre-internet” and those located in more remote cities where visitor access is less common, in particular, 3 & 4 tier Chinese cities. The largest issues with insufficient data are as follows:
- China: 30 buildings
- Japan: 13 buildings
- United States: 12 buildings
- South Korea: 12 buildings
- Australia: 5 buildings
- Other countries: 7 buildings
The 503 projects with images and data were validated, and the results confirm much of the speculation about the state and evolution of composite construction as well as global realities concerning its spreading use.
It should be clarified that the major aspect of analysis, given that many projects may use complex variations of the systems, was to look primarily at the major structural system for the perimeter of the tower as the basis of the category decision. Buildings that use concrete filled steel tubes (CFT), for instance, will often also have a reinforced concrete core. The focus was then on the nature of the column and beam system and not the specific structural relationship between the concrete and steel reinforcing in the core.
Five Primary Systems
There were also a number of projects that were truly using steel and concrete in unusual and highly complex ways that clearly did not fall in any of the 5 categories. These will remain under the general composite category, as will all buildings with insufficient technical information to define a more particular system.
Figure 5 represents a snapshot of the current state of composite use that includes all 150m+ buildings ever constructed without indicating changes over time. There is a clear dominance of concrete encased steel over the other types with concrete filled steel tubes representing a larger proportion than the concrete core with steel framing, which had been the traditional dominant application of this type.
Relationship between Composite and Building Height
Figure 6 illustrates the main findings of the analysis in terms of the relationship between building height and the choice of composite system. There is clearly a preference for Concrete Encased Steel overall, but with a dominance in the 150m to 299m range. The quantity of composite buildings that exceed 300m is substantially lower, providing a more limited data set for study.
If we compare Figures 7 and 8, also including the other structural systems (with composite as an undivided category), we can see that the use of all-steel for tall buildings has dropped, and that reinforced concrete has taken a dominant role when looking at buildings over 150m in general. This is due in part to the dominance of non-Western countries as well as to a surge in the construction of residential and hotel towers that tend to use reinforced concrete for its suitability to these uses over office occupancies. Data from CTBUH shows this preference. However when we look at Supertall towers (300m+), there seems to be a trend towards the use of composite systems.
For Supertall towers there is an increasing trend towards composite construction over all other methods if the statistics from 2014 and 2015 indeed represent a trend. Information yet to be analyzed from the project will look to discern how other methods of assisting with height have figured into this scenario, including the use of outrigger systems, megacolumns and megaframes.
Composite Type by Geographic Region
There was a sense going into this study, based purely on anecdotal evidence, that it would show a clear preference for the use of composite construction in China and other Asian countries. This was proved true by the data with concrete encased steel framing clearly dominating the choice of type in Asia. The incidents of CFT and concrete encased steel frame systems are absent or low in the North American and European markets. Some markets show very little use of composite systems altogether. There were no composite buildings in the 150m+ range in South or Central America where reinforced concrete construction remains dominant.
Figure 9 shows the clear preference for composite construction in Asia, as well as its domination over the construction of tall buildings in general. Figure 10 shows the preferences within composite types by region. As a percentage of the whole, Asia has the least incidence of concrete core and steel frame, with North American and Europe revealing the opposite.
This information needs to be integrated with the balance of data pertaining to the uses of all-steel and reinforce concrete systems to result in a clearer understanding of the impact that the various composite systems are having on the local situation.
Of note, the use of a precast column system in combination with steel framing is a distinctly Japanese invention. This method of construction is being adopted for residential towers and also includes some significant seismic resistance elements.
Differentiated Skills and Preferences
There are varying reasons for global trends in the construction industry. When it comes to steel construction, there are different factors that will impact the choices. This accounts for the “business as usual” model that seems to maintain current practices in North America and Europe as these areas are working within traditional trades, unions, practices and expertise. There has long existed expertise in the construction of all-steel, steel frame with concrete core and reinforced concrete buildings.
The North American and European steel industries have a preference for the use of hot rolled sections for concealed structural work which are widely available. Tubular steel (HSS) is also widely manufactured but seldom used in the construction of tall buildings. In addition there is a segregation of work that sees welding operations taking place in the shop and bolting operations taking place on site. This also holds for the Australian market. This is due in part to labor rates as well as concerns for safety. The risks of working at height are of significant concern and often precludes extensive welding at heights. Although China encompasses many climate zones, the predominant areas of development experience little to no snow, making site welding less difficult. Climate therefore impacts Western preferences towards on-site bolting as the construction of climate control enclosures for welding operations at height adds significantly to the erection costs of the project and will result in delays.
The most interesting case, and one that is beginning to show a reverse global effect, is the singular project in the United States that was found to be using CFT. The Wilshire Grand Center in Los Angeles, CA, is (not coincidentally) being constructed by a South Korean owner who is working with a structural engineering firm, that (although North American), has considerable experience in designing composite towers for the Asian market.
The important takeaways from the preliminary findings from this study have much to do with creating a clearer picture of tall building structural systems as they are, as well as what this means for future design. As this industry is increasingly global, as indicated by the number of architecture and engineering practices with internationally scattered offices, we are seeing an unprecedented need to understand local construction techniques and preferences. Many firms that are based in Europe or North America, where construction practices still follow more traditional methods such as all-steel and steel frame with a reinforced concrete core, must come to terms when making proposals in Asia and the Middle East where there is a clear preference for the use of steel in composite systems such as concrete encased steel frames and concrete filled steel tubes. This fact must necessarily influence the overall design of the building and will be reflected in the detailing, cladding attachment system and the ability to adapt to desired forms. Education may need to be examined towards including more globally critical practices, as these are not widely included in university level curricula in many Western countries.
Where to this point in time, at least from the exterior, there has existed a visual consistency in the outward appearance of skyscrapers that has tended to acknowledge their Western roots, this remarkable shift in structural systems is pushing the design of skyscrapers into new possible directions, from the inside.
I would like to thank several members of the Council on Tall Buildings and Urban Habitat for their assistance with this project: Antony Wood, Steven Henry, Aric Austermann and Marshall Gerometta. Their continued support will be essential for bringing the revisions in the data collection to the public.
Thanks to Zhi Gao of Architects Crang and Boake Inc., Beijing Office, for the translation of the paper.