Even with just a quick overview, it was clear to me that these buildings differ from one another, and I wanted to understand why. Because I was unable to find a comprehensive comparative study, I decided that the goal of my PhD research should be to categorize these buildings, analyze them from a design and contextual point of view, and finally present their evolution in the last 20 years of practice. I wanted to show how (and if) they are evolving, in what they differ, and which drivers cause their differences.
I decided to analyze all projects which have most of their load bearing structure made of Engineered Wood Products (EWPs) or EWPs-based. EWPs are products like CLT, NLT, DLT, Glulam, and others. For reasons of feasibility (there were too many 4-storeys projects), I considered all projects taller than 5 storeys, from the period between 2004 (the first case-study) to 2019. The application of these criteria to a wide range of sources about MTBs, resulted in the identification and description of 197 case-studies, covering more than 200 built buildings.
If we look at the location of the case-studies, we can see from figure below, that the highest concentration is in Europe. The UK, Switzerland, Germany, and France are the countries with the highest number of case-studies. Alternatively to Europe, several case-studies were found in Canada, USA, and Australia.
After having selected and located the case-studies, my PhD research focused on their design properties. Particular attention was given to the subdivision of structural categorizations, along with the way in which each system differs. Using the existing literature as the basis, thirty-three structural categories were defined. This categorization proves that there are many ways to design a tall timber building, and there are many hybrid combinations where steel and concrete elements are used as well.
By analyzing the structure (materiality of the core, floor slabs, etc.), and other design properties of each case-study (like visibility of the structural elements, material of the façade, program, etc.), it emerged that the process of building an MTB is not the same everywhere. For example, some countries have a higher number of post-and-beam structures, other have only panalized structures, still others have mainly 3D modular structures (see Figure below). In any case, most of the built case-studies have hybrid structures, with many projects having a concrete podium, and/or a concrete core. This is the reason why I refer to these buildings as “timber-based”.
The design analysis uncovered two other characteristics of MTBs. First, by analyzing and categorizing so many projects for the first time, it was possible to highlight some international trends in the construction of MTBs. Some of these are the common use of CLT as material for floor slabs, the predominance of CLT or concrete cores, or the partial visibility of the structural elements inside the projects. Keep in mind that these trends are not always valid, and some countries adopt specific solutions. Secondly, the analysis also proved the existence of a cluster of solutions which are unique to some countries or professional involved. The reasons were explained in the final part of my research.
The research concluded with the analysis of the contextual environment and influences in each case-study. What resulted is the very complex and heterogeneous forces of influences which drove the realization and the construction of the case-studies. It appeared clear that the legal framework sets the limit of height or the possibility of timber exposure, but there are several examples where rules were modified by individual case-studies, showing that with the support of testing, the legal framework is not always fundamental. The analysis of the stakeholders allowed me to observe that some countries are dominated by very few and common professionals (like architects or timber engineers), while others tend to have more heterogeneous professionals. The private or public influence on the case-studies also changes according to the country. Private and public initiatives in favor of building sustainably, emerged as one of the main drivers behind the decision to realize these buildings. But these are not the only reasons and other relevant findings have been reported.
Overall, MTBs appear to be heterogeneous phenomenon, which will likely evolve as new techniques are established, with EWPs will become more available and cost competitive. I personally found the national (and sometimes even regional) differences in the realization of these projects very interesting. Building with timber today, mostly depends on the location of the project, the availability of the EWPs used, and the local know-how. I also observed that the industry is already following two paths: the first one is focusing on the cutting edge, highly precise, and tailored made structural possibilities of timber. Mjøstårnet and TREET are two examples of this direction. Another path is focusing on the standardization of the structural elements like the projects Life Cycle Tower, and Suurstoffi S22. Both these paths will likely lead the timber industry to stand out in the construction practice and will contribute to the renaissance of timber in tall buildings construction.
You can download the doctoral dissertation at this link:
Business Development Manager at CREE Buildings – PhD Timber-Hybrid Buildings
I obtained my PhD at the Institute of Architectural Sciences, Department of Structural Design and Timber Engineering of the Vienna University of Technology, and I am working as Business Developer Manager at CREE Buildings. My research interests are focused on mid- and high-rise timber-based buildings from a structural and contextual perspective.