Analyzing the key architectural and structural factors in the formation of tall timber projects in Europe

Abstract

As tall wooden structures emerge as a burgeoning and promising sector, offering considerable ecological and financial benefits across their life cycles, their prevalence is on the rise, particularly in Europe. However, the current corpus of literature fails to offer a detailed assessment of the fundamental architectural and structural planning parameters for European tall timber towers (≥9 stories). To span this gap and improve the comprehension of the developing European tendencies, this study meticulously examined information from 33 tall timber towers in Europe. The principal findings uncovered the following: (i) tall timber buildings predominantly favored residential applications as their primary function, (ii) the prevailing preference for the central core arrangement was evident, (iii) the most prevalent design preference for tall timber buildings was prismatic forms, (iv) widespread adoption of composite materials was evident, with combinations of timber and concrete being particularly prominent, and (v) the structural systems were primarily characterized by shear-frame configurations. By revealing these present-day attributes of tall wooden towers in Europe, this article is anticipated to offer valuable insights to architectural designers, assisting and directing them in the formulation and implementation of forthcoming developments in this domain.

Keywords:

• Timber/wood
• tall timber projects
• Europe
• architectural design factors
• structural design factors

Reviewing Editor:

• Dr Montemurro Marco, Arts et Métiers Sciences et Technologies, France

Subjects:

• Design
• Architecture
• Building and Construction

1. Introduction

In recent decades, the worldwide construction industry faces the challenge of balancing urbanization with the need for environmental sustainability amid climate change concerns (Rahko & Alola, 2022). As cities grow, there’s a heightened demand for innovative and eco-friendly building practices that go beyond mere structural functionality, incorporating design and long-term environmental impact considerations (Meena et al., 2022).

Tall timber buildings have emerged as a forward-thinking solution, addressing urban development challenges while demonstrating a commitment to ecological responsibility (Safarik et al., 2022). Timber, being renewable and low-carbon, contrasts with traditional materials like steel, contributing to lowering the building sector’s carbon footprint (Morales-Beltran et al., 2023). Beyond their environmental advantages, tall timber buildings bring a unique aesthetic warmth, countering the monolithic facades of conventional structures (Laboy, 2022). This integration of sustainability and visual appeal contributes to creating urban spaces that prioritize well-being and a connection to nature.

The erection of tall wooden structures aligns with the growing trend of green urbanism, promoting green spaces and sustainable design (Ahn et al., 2022). As these eco-friendly structures gain traction, they represent a paradigm change in the building sector, harmonizing the practical needs of urban living with a conscientious effort to coexist with the environment. In Europe, especially in Scandinavia, a deep-rooted connection to timber construction reflects a rich cultural identity and history, emphasizing craftsmanship, sustainability, and a sense of place (Zahiri, 2023). The resurgence of timber as a main building material in Europe signifies a pivotal shift towards more sustainable alternatives, acknowledging the ecological effect of conventional materials.

Amidst growing attention to timber load-bearing systems and notable advancements in the building sector, extensive study has delved into the technical, environmental, societal, and financial aspects of engineered timber products across various building applications (e.g. Millaniyage et al., 2024; Sciomenta et al., 2024). However, a noticeable gap remains in the research landscape, particularly regarding tendencies and classifications about architectural and structural planning parameters in tall wooden projects within the European context.

Fink et al. (2023) emphasized interdisciplinary evaluation in designing tall timber structures. Tuure and Ilgın (2023) analyzed spatial efficiency in mid-rise wooden apartments, highlighting square layouts and prismatic forms. Zahiri (2023) outlined trends in tall wooden constructions in the Nordic zone, emphasizing prefabrication and sustainability. Ilgın et al. (2023) revealed preferences for central core layouts and pure timber construction. González-Retamal et al. (2022) categorized advancements and challenges in multi-story timber towers, focusing on engineering aspects. Santana-Sosa and Kovacic (2022) studied methodologies for wooden constructions in Austria, considering planning and manufacturing challenges. Svatoš-Ražnjević et al. (2022) explored architectural diversity and spatial capacities in high-rise wooden constructions. Žegarac Leskovar and Premrov (2021) investigated methodologies in architectural and structural planning in tall wooden constructions, noting significant alterations in architectural design. Salvadori (2021) examined geographical distinctions and structural categorization of multi-story timber structures. Tupėnaitė et al. (2019) assessed financial and environmental efficiency of tall timber structures, revealing improved effectiveness in both dimensions. Kuzmanovska et al. (2018) identified evolving tendencies in high-rise wooden projects, including a growing preference for rigid frame structures. Smith et al. (2015) recognized advantages and drawbacks of off-site solid timber manufacture, while Perkins + Will (2014) surveyed timber towers exceeding five stories, emphasizing the feasibility of wood in tall buildings to mitigate ecological effects. Holt and Wardle’s investigation (Holt & Wardle 2014) justified integrating timber in tall building projects.

However, despite the increasing popularity of tall timber projects, there is a lack of thorough academic investigation into the unique architectural and structural planning parameters specific to Europe. Our paper aims to fill this gap by analyzing data from 33 tall timber building projects across this region, focusing on fundamental details, architectural and structural design features. The goal is to provide architects with valuable insights to enhance the planning and execution of future tall timber structures, serving as a foundational resource for innovation, sustainability, and excellence in European tall timber projects.

In Europe, tall wooden buildings are increasingly being developed for residential purposes, reflecting a growing trend towards sustainable construction practices. These structures typically incorporate a combination of innovative design elements and advanced materials to ensure both structural integrity and environmental efficiency. Prismatic forms, characterized by their geometric simplicity and uniformity, are favored for their ability to optimize space utilization while maintaining structural stability. Central service cores play a crucial role in supporting vertical loads and facilitating efficient circulation within the building. Shear walled frames are commonly employed to enhance lateral stability, ensuring the safety and resilience of the structure. Additionally, the integration of composite materials, such as engineered wood products and reinforced concrete, offers enhanced strength-to-weight ratios and durability, further contributing to the overall performance and longevity of tall wooden buildings in the European context. Through the adoption of these design principles and material innovations, Europe continues to push the boundaries of timber construction, establishing itself as a leader in sustainable urban development.

2. Materials and methods

The utilization of case studies was employed to collect, organize, and integrate information regarding European tall timber buildings, accelerating a systematic examination of both their architectural and structural features. The application of the case study methodology is an established practice in evaluations associated with built environment (e.g. Gao et al., 2024; Wang et al., 2024). Figure 1 illustrates the methodical approach employed in the selection of the cases examined in this paper.

Figure 1. Flowchart of the methodology and process (Created by the authors).

This research covered a comprehensive set of 33 tall timber structures that were either erected or in the process of construction. The inclusion criteria comprised all timber buildings with nine stories or more, as identified by the Council on Tall Buildings and Urban Habitat (CTBUH, 2024), and as seen in Figure 2, the selected structures were situated in various locations across Europe (seven from Norway, six from Sweden, four from France, three from Netherlands, two from Finland, two from Germany, two from Switzerland, one from Austria, one from Italy, and finally, five from UK) (Appendix A).

Figure 2. Tall timber buildings located throughout Europe (Created by the authors).

Architectural and structural planning features of tall timber towers (Appendix B) are summarized as follows (Ilgın et al., 2021):

In the context of architectural attributes, the subsequent features play a substantial role:

• Allocated function of the tower.

• Designing the core, with potential implications for the arrangement of vertical circulation and, in specific conditions, the layout of shaft distribution.

• Forms, capable of impacting the dimension and shape of floor slabs.

Concerning structural features:

• Preferences for structural materials can affect the size of the load-bearing components.

• The chosen load-bearing system can influence the arrangement and size of structural elements.

Characteristically, when classifying tall structures according to their anticipated use, they are grouped into those exclusively designed for a singular purpose or for multiple purposes. This article grounds its assessment of functionality on the following formations: (1) hotel, (2) residential, (3) office, and (4) mixed-use.

Additionally, core arrangement proposed by Ilgın et al. (2021) is employed for its com-prehensive framework, including the following classifications: (i) central, (ii) atrium, (iii) external, (iv) peripheral.

This paper establishes the classification of forms based on the following patterns, as illustrated in Figure 3 (Ilgın et al., 2021):

1. prismatic forms pertain to buildings distinguished by similarities, uniformity, and parallel geometric forms at both ends, showcasing identical sides and vertical axes, especially perpendicular to the ground, as in the case of Mjøstårnet in Norway (Figure 4).

2. leaning structures pertain to buildings characterized by an architectural design that incorporates an inclined form.

3. tapered structures are recognized for demonstrating a narrowing phenomenon as they rise, achieved through the reduction of floor plans and surface areas.

4. setback configurations are identified in structures that showcase horizontally indented segments along their vertical height.

5. twisted forms signify structures in which the floors or outer surfaces undergo a steady rotation as they ascend along a central axis, involving a twisting angle.

6. free structures are associated with structures that deviate from the previously mentioned formations, as in the case of HAUT in Netherlands (Figure 5).

Figure 3. Tall timber building forms (Created by the authors).

Figure 4. Mjøstårnet (photo by authors).

Figure 5. HAUT (photo courtesy of Jannes Linders).

Structural materials can be grouped into 2 major classes: (a) ‘timber’ or ‘all-timber’ and (b) ‘composite’ or ‘hybrid’ materials, involving arrangements like timber with concrete, timber with steel, or amalgamations of timber, concrete, and steel. Our study particularly focusses on main structural members, encompassing such as columns, beams with the elimination of considering floor slabs in this sense. It is also critical to emphasize that modifications in the structural material combination on the ground floor do not impact of the overall load-bearing system classification.

In line with this categorization of structural materials, to elaborate further, a structure qualifies as ‘timber’ only when both its main vertical and horizontal load-bearing elements are constructed completely from timber (CTBUH, 2024). It is notable that a structure classified as ’timber’ may include non-wood fasteners in certain regions linking wooden members. Even in instances where a structure is chiefly built using timber but incorporates a floor system comprising a concrete slab positioned atop timber beams, it retains its categorization as a ‘timber’ structure since concrete members do not function as the main structural element, as in the case of Mjøstårnet.

Alternatively, in the composite/hybrid categorization that incorporates timber, a significant percentage of the structure is composed of materials distinct from timber, namely steel, concrete, or both. As an illustration, in constructions that integrate timber and concrete, it is typical to observe a concrete core supporting a wooden frame, as in the case of HoHo in Vienna, Austria (Figure 6). It stands at a height of 84 meters across 24 stories. On the flip side, in constructions that amalgamate timber and steel, a significant percentage of the vertical or lateral structural system depends on steel. This frequently involves components like steel-trussed cores, braces, exterior frames, as in the case of Sara Kulturhus in Sweden (Figure 7).

Figure 6. HoHo (photo courtesy of DERFRITZ).

Figure 7. Sara Kulturhus (photo by authors).

Likewise, hybrid buildings that combine timber, concrete, and steel utilize a blend of these 3 materials to withstand main loads. A common layout includes a concrete core collaborating with steel beams and pillars, whereas timber is employed for floors and separation walls, as in the case of De Karel Doorman in Rotterdam, Netherlands.

Concerning the furnishing of lateral load-bearing mechanism of tall buildings, especially in mitigating forces like wind and earthquake, different load-bearing systems and categories have been employed in practical implementations. This aspect has been a central topic of discussion in the current literature (Kuzmanovska et al., 2018). In our study, we chose to use the load-bearing system categorization defined in Ilgın et al., 2021) due to its complete nature, as seen in Figure 8.

Figure 8. Tall timber building structural systems (Created by the authors).

It is noteworthy that supertall buildings surpassing 300 meters in height commonly utilize outriggered frames, diverse tube, and buttressed core systems. These are favored for their effectiveness and cost-efficiency. Therefore, these systems were excluded from our study, given its focus on tall structures. However, examples such as Mjøstarnet (Norway) and Treet (Norway) exemplify rare occurrences of tall timber buildings that incorporate elements akin to tubular systems.

In Mjøstarnet, the primary structural support comprises extensive glulam trusses positioned along the building’s exteriors, alongside internal columns and beams (Ascione et al., 2024). These trusses effectively manage both horizontal and vertical forces, providing essential rigidity to the structure. Additionally, cross-laminated timber (CLT) walls are employed for secondary support, specifically for three elevators and two staircases. However, it’s important to note that the CLT walls do not play a role in enhancing the building’s horizontal stability. Similarly, in Treet, the structural integrity of the building is provided by glulam trusses positioned along the exterior walls. CLT walls function separately from the primary load-bearing system and do not enhance the building’s lateral stability. The primary structure of the building consists of prefabricated modules, which can be stacked up to four stories high.

The precise definition of tall constructions faces a lack of globally accepted criteria concerning their height, number of floors, and the conceptualization of ‘tall’ within the domain of wooden edifices, sparking continuous discussions. In the study by Smith and Frangi (2008) that tall wooden structures were termed as timber frameworks spanning from about 10 to a maximum of 20 floors. Alternatively, Wood Solutions (Wood 2024) delineates mass timber tall as edifices with an altitude exceeding 25 meters above ground floor or, in the absence of specific measurements, buildings surpassing 8-story.

In the scope of our study, a ‘tall timber building’ refers to a construction exceeding 8 stories (Ilgın et al., 2023). It’s worth noting that this height specification corresponds to the prevailing maximum permissible height for the ‘P2 class’ wooden construction solution in specific European nations, like Finland. Throughout history, ‘fire’ has been a crucial factor in establishing the technical parameters of ‘building height’, acting as a foundational limitation on height (Calder et al., 2014).

3. Results

3.1. Examination of aspects pertaining to architectural design

This part offers a detailed investigation of architectural design aspects for 33 tall wooden towers, either in the erected or presently undergoing construction. These parameters include: (i) function, (ii) core type, and (iii) form.

3.1.1. Function

Figure 9 highlights a significant pattern observed in a dataset containing 33 European tall timber buildings. The data indicates that residential purposes are the most prevalent, accounting for more than 70% of the total.

Figure 9. Functional distribution of selected buildings (Created by the authors).

Residential function in tall timber buildings across Europe is driven by various factors (Li et al., 2023; Ojala et al., 2023; Tupenaite et al., 2023), including ecological advantages. Timber, a sustainable material, aligns with efforts to reduce carbon emissions. The choice of residential applications maximizes positive environmental impact. Additionally, psychological and health benefits associated with timber constructions contribute to the appeal. Proximity to timber and organic elements in domestic environments positively affects mental and physical well-being. Tall timber towers also align strategically with market demand and economic viability, offering a solution to the enduring need for housing units in densely populated urban regions. This convergence of environmental consciousness, well-being considerations, and economic viability establishes a strong foundation for the prevalent preference for residential use in tall timber towers in Europe.

Several factors contribute to the limited prevalence of mixed-use, office, and hotel functions in tall timber buildings in European construction compared to residential use (Abdullah et al., 2023; Nguyen et al., 2023; Saleh et al., 2023). Regulatory and safety concerns present challenges, as compliance with stringent safety and fire codes is more difficult in timber construction for structures with these functions. Timber may struggle to meet distinct structural requirements for commercial and office spaces, particularly in large open areas. Market demand and developer familiarity with traditional materials like concrete and steel may overshadow timber use in commercial spaces. Skepticism or resistance may arise due to perceptions of safety and durability issues, especially in markets where such concerns prevail. Cost considerations, including the perception of higher upfront costs for tall timber buildings with commercial functions, may discourage developers focused on cost efficiency. The construction industry’s alignment with traditional materials makes transitioning to tall timber construction for commercial spaces challenging. Limited availability of advanced technologies and construction methods for tall timber buildings in mixed-use, office, and hotel applications, compared to residential use, further contributes to hesitation in adopting timber in these sectors.

3.1.2. Core planning

Examining Figure 10 reveals that the central core configuration, comprising more than 65% of instances, is the prevailing choice for core arrangement. Following is the peripheral core layout, accounting for one-third of the case study samples.

Figure 10. Core planning distribution of selected buildings (Created by the authors).

The benefits linked to a central core arrangement are diverse and use a substantial impact on its extensive acceptance (Oldfield & Doherty, 2019) including enhanced structural strength, space-efficient design optimization, creation of open spaces, enhanced fire safety measures.

Within the examined sample group, a noteworthy observation pertains to the prevalence of rectangular floor plans in several structures. These architectural characteristic gains significance in scenarios where a building faces constraints in its dimensions, particularly when it assumes a form characterized by narrowness and rectangularity.

In such constrained settings, architects often adopt a strategic approach that involves siting the core of the building near its outer edge. This architectural strategy emerges as a prominent practice due to its potential to optimize the effectiveness of the floor plan. The choice of positioning the core at the periphery is justified by its notable standing as the second most favored configuration within this certain framework.

The primary objective behind this deliberate placement of the core is to ensure the maximal utilization of available interior space. By strategically situating the core near the outer edge, architects aim to enhance the flexibility of space use. This tactical maneuver proves to be markedly advantageous in structures characterized by restricted dimensions, where every inch of space becomes a valuable resource.

In essence, this approach not only addresses the challenges posed by constrained dimensions but also contributes to the creation of a spatially efficient and flexible environment. The emphasis on optimizing interior space underscores the significance of peripheral core configurations as a pragmatic choice in architectural design, particularly when confronted with the constraints imposed by narrow and rectangular building forms.

3.1.3. Form

The predominant design preference is the prismatic form, accounting for more than 70% of instances. Meanwhile, free forms represent 27% of one-third of the overall cases (Figure 11).

Figure 11. Building form distribution of selected buildings (Created by the authors).

Preference for prismatic forms, particularly in tall timber buildings (Tuure & Ilgın, 2023) can be attributed to their advantageous properties that simplify the construction process. The geometric simplicity of these forms streamlines structural design, material handling, and assembly, contributing to an uncomplicated construction process. Practicality is another key factor, as prismatic forms align seamlessly with conventional building practices, leading to lowered labor and material costs. Additionally, their functional nature makes them adaptable for various purposes. Optimal space utilization is achieved through prismatic configurations, especially when combined with rectangular floor layouts, enhancing internal space use, and facilitating effective distribution of spaces, passages, and services. This effectiveness is specifically beneficial in housing and office environments. Furthermore, the economic efficiency of prismatic forms stems from their simplicity with standardized building methods, minimizing complexities, reducing the risk of faults, and enhancing cost-effectiveness for developers and builders.

The increasing embrace of free forms in the design of tall wooden buildings in Europe can be attributed to architectural designers’ fervent pursuit of innovative and distinctive projects. Motivated by a deep-seated aspiration for original expression and the development of structures that are iconic and visually impactful, architects are progressively inclined toward exploring free forms (Shahbazi et al., 2023). These architectural expressions signify a departure from the traditional constraints of rectilinear or prismatic geometries, offering architects an expansive canvas to delve into imaginative and pioneering concepts.

This trend reflects a departure from the constraints of more conventional design principles, allowing architects to push the boundaries of creativity and redefine the aesthetic possibilities in tall timber constructions. The adoption of free forms not only serves as a testament to the evolving nature of architectural expression but also opens opportunities for groundbreaking and innovative solutions in the construction of tall wooden structures. As architects embrace the freedom afforded by these non-traditional geometries, the resulting structures become not only functional but also captivating works of art, contributing to the evolving tapestry of modern architectural landscapes.

3.2. Examination of aspects pertaining to structural design

This part delves into an assessment of structural design concerns for 33 tall timber towers in Europe. The examination encompasses two key factors crucial to the overall structural integrity and performance: (i) structural material, (ii) structural system.

3.2.1. Structural material

Figure 12 underlines a notable predominance of the hybrid/composite use, accounting for 55% of the instances, while timber follows closely, constituting 45% of the dataset, which comprises 33 tall timber structures.

Figure 12. Structural material distribution of selected buildings (Created by the authors).

Hybrid materials offer several benefits that actively drive the selection of tall timber buildings in Europe. First, timber’s lightweight nature with an excellent strength-to-weight ratio is suitable for tall structures, and additional load-bearing capacity can be achieved by strategically integrating steel or concrete components. Second, while timber possesses unique structural properties, combining it with materials like steel or concrete enhances overall stability, reducing sway and ensuring structural integrity during environmental events. Third, fire resistance is addressed by incorporating non-combustible materials in key structural elements, meeting building code requirements. Fourth, hybrid structures provide high design flexibility, allowing different materials to be employed based on their specific properties, enhancing both aesthetics and structural requirements. Fifth, the environmental sustainability of wood aligns with the increasing focus on sustainable construction practices in Europe, reducing the overall ecological impact of structures. Sixth, the natural insulating properties of wood contribute to energy efficiency and combining it with other materials optimizes insulation and thermal performance. Seventhly, the prefabrication of components expedites construction and enhances cost efficiency, as hybrid structures allow the utilization of materials with diverse manufacturing processes. Lastly, hybrid solutions enable compliance with building codes and regulations, leveraging the strengths of different materials to meet specific performance criteria for tall structures.

In Figure 13, composite structures are displayed, classified by the grouping of structural materials. Remarkable is the predominance of timber combined with concrete, constituting more than 75% of the cases. Following is the amalgamation of timber with both concrete and steel with 17%.

Figure 13. Composite tall timber buildings in Europe by structural material combination (Created by the authors).

The decision to incorporate a concrete core in composite structures is informed by a multifaceted set of considerations. Firstly, this choice significantly contributes to the horizontal stability of the tower. The inclusion of concrete, a material known for its robustness and rigidity, enhances the structural integrity, particularly in scenarios where lateral forces come into play, such as during seismic events or strong winds. Secondly, the use of concrete in the core brings forth an inherent advantage in terms of fire resistance. Concrete possesses properties that make it resistant to high temperatures, thus serving as a protective element against the potential spread and impact of fire. This is a critical aspect in ensuring the safety and resilience of tall buildings, aligning with stringent safety standards and regulations. Thirdly, the decision to integrate a concrete core is motivated by the superior ability of concrete to dampen building sway, a frequent challenge faced in high-rise structures. Wind forces can induce lateral movement in buildings, and concrete’s mass and damping properties help mitigate this swaying effect, contributing to the overall stability and occupant comfort in the structure.

As illustrated in specific cases of timber-concrete hybrid construction, as seen in the project like HoHo, the concrete core assumes a pivotal position. In these notable cases, the concrete core acts as a key element in enhancing the lateral stiffness of the structure, ensuring it meets or exceeds the stringent performance criteria demanded by the dynamic conditions of tall buildings. The interaction between timber and concrete in these composite structures exemplifies a thoughtful integration of materials to optimize structural performance, safety, and resilience in tall building construction.

In other words, timber-concrete hybrid tall building construction, commonly characterized by the integration of a cast-in-place concrete core and a timber framework, represent a promising advancement in structural engineering. This hybridization strategy effectively distributes the structural responsibilities, with the concrete core adeptly managing lateral loads while the timber structure efficiently bears the gravitational and diaphragm loads. Extensive research has demonstrated the significant advantages of timber concrete hybrid structures over their purely wooden counterparts (Larsson et al., 2022; Zhang et al., 2022). These benefits include heightened load-bearing capacity, improved resistance to fire hazards, and superior performance under seismic stresses. By leveraging the inherent properties of both wood and concrete, such hybrid structures exhibit a synergistic effect, achieving a level of structural robustness and resilience that surpasses conventional wooden structures. Furthermore, the combination of these materials not only enhances the structural performance but also contributes to sustainability objectives, as timber is a renewable resource and concrete’s environmental impact can be mitigated through optimized design and construction practices.

Emphasizing the critical importance of addressing building sway management becomes paramount, particularly in the context of taller structures, where it presents a formidable challenge impacting structural safety and the building’s overall usability (Zhou & Li, 2022). This challenge transcends variations in building materials, necessitating a universal focus on effective sway control strategies. Designers are tasked with the responsibility of mastering the art of controlling building sway, a task that takes on heightened significance during turbulent weather conditions such as windstorms. Beyond merely safeguarding structural integrity, the overarching goal is to guarantee the comfort and well-being of building inhabitants. This is specifically pertinent for those situated on the uppermost floors, where the effects of building sway are most pronounced.

Moreover, the evolving landscape of modern tall building design, including the innovative realm of tall timber towers, introduces a distinctive characteristic—reduced mass in comparison to their predecessors (Foster & Ramage, 2020). This alteration in mass distribution brings forth a noteworthy challenge in the form of heightened susceptibility to lateral drift, primarily attributable to lower damping characteristics. These dynamic underscores the pressing concern of effectively managing wind-induced building sway during the design phase. In response to this challenge, the incorporation of concrete materials emerges as a strategic advantage. Concrete, renowned for its density and mass, becomes a pivotal element in mitigating the potential consequences of lateral drift in tall timber towers. A compelling example of this integration can be found in Mjøstårnet, where the uppermost six floors are fortified with 300-mm-thick reinforced concrete slabs. This deliberate inclusion serves the dual function of augmenting the overall mass of the structure and, consequently, counteracting wind loads effectively (Tulebekova et al., 2022).

3.2.2. Structural systems

As depicted in Figure 14, shear-frame systems, specifically the subcategories ‘shear walled frame’ and ‘shear trussed frame’, are the prevalent preference, composing more than 65% of the preferences. Following is shear wall system, making up 27%. Notably, within shear-frame systems, the prevalent selection in the examined cases is the shear walled frame systems, representing 95% of the samples. In contrast, tube system demonstrates a lower adoption rate, with only two instances among the load-bearing systems for tall timber towers in Europe.

Figure 14. Structural system distribution of selected buildings (Created by the authors).

In the realm of shear-frame systems, which include both shear trussed frame and shear walled frame configurations, a nuanced and comprehensive understanding emerges when evaluating the relative drawbacks and limitations of each constituent system. Research suggests (Wang, 2020) that the inherent weaknesses of a rigid frame, when juxtaposed with the advantages of shear truss or wall systems, and vice versa, can be effectively mitigated through the strategic integration of these elements.

The synergy between shear trussed frame and shear walled frame systems becomes apparent in such integrated applications. It becomes a symbiotic relationship where the drawbacks of one system are compensated by the strengths of the other. This intricate balance is particularly evident in scenarios where the frame plays a reinforcing role in upper levels, augmenting the capacity of the shear truss or wall, while reciprocally, the shear truss or wall improves the structural integrity of the system in lower stories, as in the case of HoHo (Safarik et al., 2022).

The distinctive cantilever behavior inherent in shear wall systems manifests as a substantial architectural challenge, particularly notable in the tall timber towers. This behavior leads to a noteworthy escalation in inter-story drift, representing the lateral displacement between successive floors within a building. Importantly, this phenomenon is consistently more pronounced in the upper levels of structures employing shear wall systems when compared to their lower counterparts (Aloisio et al., 2021). The consequential increase in inter-story drift, especially in the upper reaches of the building, presents a noteworthy challenge for engineers and architects involved in tall timber tower projects. The heightened horizontal displacement in these upper levels can potentially compromise the structural integrity and overall stability of the building, necessitating careful consideration in the selection of structural systems.

The concurrent achievement of maximizing usable space and minimizing core structural dimensions presents a compelling rationale for the preference for tube systems in tall wooden structures (Wang et al., 2020). The outer tubular frame not only ensures structural stability and efficiency but also contributes significantly to the overall spatial design and utilization of the building. This multi-faceted advantage aligns with the evolving architectural and engineering considerations in the construction of tall timber structures, making tube systems an appealing choice for those seeking an optimal balance between structural performance and spatial efficiency. Moreover, the integration of external braces into the design of a framed-tube system represents a strategic enhancement aimed at achieving a more pronounced tubular cantilever behavior. This modification contributes to an elevation in structural stiffness and overall effectiveness. Simultaneously, it addresses the challenge of ‘shear lag’, a phenomenon resulting from the flexibility of spandrel beams, by minimizing its adverse impact (Scaramozzino et al., 2020).

4. Discussion

We aim to collect and amalgamate extensive data concerning 33 contemporary tall wooden structures in Europe. The emphasis of this study revolves around scrutinizing the architectural and structural facets of these structures. The results unveil similarities and differences when juxtaposed with earlier inquiries, including those carried out by Ilgın et al. (2021). The major discoveries stemming from this investigation can be concisely encapsulated as follows:

1. When considering functionality, residential use emerges as the predominant choice, underscoring the inclination towards incorporating timber in structures intended for living spaces.

2. In terms of core configuration, central core design stands out as the prevailing choice, showcasing a preference for structural layouts that emphasize centralized strength and stability.

3. Prismatic form takes precedence as the dominant design preference, suggesting a predilection for sleek and streamlined architectural profiles.

4. A noteworthy trend is the prevalent use of hybrid materials, with the combination of timber and concrete as the primary preference in composite use.

5. Structural systems employed exhibit a pronounced preference for shear-frame configurations, specifically emphasizing shear-walled frames, highlighting a commitment to robust structural solutions within the realm of European tall timber towers.

In the domain of tall wooden towers in Europe, a conspicuous choice for central cores emerged as the predominant architectural choice. This inclination towards central cores was not isolated to wooden structures alone but extended to other construction materials as well. A parallel observation was noted in research focused on spatial efficiency within mid-rise wooden apartment buildings in the Finnish context. In this sense, floor layouts characterized by square shapes exhibited a consistent tendency to favor the utilization of central core (Tuure & Ilgın, 2023). Furthermore, investigations into skyscrapers constructed from non-wooden materials consistently highlighted a prevalent trend of central core domination (Ilgın, 2021a, 2021b; 2023a, 2023b). This suggests that the strategic incorporation of central cores is a recurrent and effective approach across various materials and building types when aiming for height and structural efficiency.

Oldfield and Doherty’s study (Oldfield & Doherty, 2019) delving into a sample of 500 tall buildings constructed from non-timber materials substantiates this trend. Their findings revealed that a remarkable 85% of the examined tall buildings featured central core configurations. This further underscore the significance and widespread adoption of central core designs in tall building construction, irrespective of the underlying materials.

Tall timber buildings consistently embraced the utilization of prismatic profiles, depicted by their rectilinear arrangements and uniform extrusions. This tendency was substantiated by the research conducted by Tuure and Ilgın (2023), where an examination of 55 mid-rise timber apartments showed a predominant inclination towards these uncomplicated configurations. This architectural trend was further validated by the findings of Kuzmanovska et al. (2018), affirming the dominance of prismatic shapes in the architectural profiles of multi-story structures.

In a parallel context, the study conducted by Ilgın et al. (2021) provided additional support to the prevalence of prismatic forms, exceeding 40%, among 18 supertall non-wood residential structures within their comprehensive study of 93 buildings. This widespread adoption of prismatic shapes appears to transcend the material composition.

It is notable that modern supertall residential towers, primarily erected using concrete, distinctly lean towards prismatic shapes, as underscored by the insights gleaned from (Ilgın, 2021b). This architectural preference for prismatic shapes evidently extends across different construction materials, emphasizing the significance of this design approach in the construction of modern tall structures.

There was a noteworthy frequency of composite materials, and among the various options available, the amalgamation of timber and concrete stood out as the most leading preference in the realm of composite construction. This strategic integration of materials appears to be a favored choice, possibly due to the complementary qualities and synergies that timber and concrete bring to the construction process. The combination of these materials not only offers structural strength but also addresses considerations such as durability and resilience. Additionally, this trend was not confined to specific scales of construction; it extended to supertall tower projects as well. The prevalent utilization of composite construction in the context of supertall structures suggests that the benefits associated with this approach, such as enhanced structural performance and versatility, are particularly advantageous in the ambitious and complex designs often associated with supertall buildings (Ilgın et al., 2021). These underscores the recognition of composite construction as a viable and effective solution in advancing the structural and functional aspects of tall structures.

Within the realm of load-bearing systems tailored for timber structures, a clear hierarchy has been observed, organized according to the height of the structures in question. In the tall timber tower projects, a notable and preferred choice has been the shear-walled frame system, establishing itself as the primary structural solution for such structures. This selection is indicative of the emphasis on optimizing vertical stability and load distribution, aligning with the specific challenges associated with taller timber constructions.

In mid-rise wooden buildings, a prevailing preference was identified for shear wall systems, distinguishing them from their taller counterparts. This preference suggests a nuanced approach, where the focus may be on balancing structural integrity and design efficiency in mid-rise timber constructions, where considerations of scale and load distribution differ from those in taller structures.

Conversely, the construction of supertall buildings introduces a distinctive trend in structural choices. It has become commonplace to employ outriggered frame systems in such exceptionally tall structures. This choice reflects a strategic response to the unique challenges posed by the extreme height of supertall buildings, emphasizing the need for lateral stability and load distribution through innovative structural configurations (Salman et al., 2020). Therefore, the hierarchy of structural systems in timber buildings appears to be intricately linked to the specific height categories, reflecting a nuanced understanding of the structural demands at different scales of construction.

The empirical data available for our study is specifically confined to buildings that have reached completion or are currently under construction, emphasizing structures with a height exceeding 8 stories. The rationale behind this limitation is grounded in the global scarcity of tall timber towers, rendering further subdivision with an in-depth analysis of 33 specific tall timber structures unfeasible due to the risk of introducing bias into the results. Nonetheless, it is critical to state that the total number of structures meeting the predefined parameters of the study has witnessed a substantial increase in recent decades. As the count of tall timber buildings continues to rise, there is a prospect of a more significant pool of structures for potential sub-categorization in future analyses. This potential expansion in the dataset holds promise for gaining deeper insights into specific subsets of tall timber constructions, fostering a more nuanced understanding of the diverse characteristics within this category.

Future research on tall timber buildings in Europe should focus on environmental consequences through life cycle assessments, carbon footprint analyses, and comparisons with traditional materials. Technological advancements and innovation, including new materials, advanced timber technologies, and smart building systems, are essential. Urban planning and policy considerations, such as zoning regulations and incentives, need thorough investigation. Longitudinal studies on existing tall timber buildings can provide insights into structural integrity, fire resistance, and maintenance. Research into social and economic implications, including local impact and economic feasibility, is crucial. Exploring architectural design variations can offer insights into regional preferences and cultural influences. Comparative studies between European and global practices can identify lessons and best practices for global application.

By exploring these avenues, future studies have the potential to further enrich our understanding of tall timber buildings in Europe, contributing to sustainable architectural practices and fostering innovation in the building sector.

5. Conclusions

In Europe’s current architectural scene, tall timber structures, notably residential complexes, stand out for their central core layout and shear-walled frame systems built with composite materials. Architects face the intricate challenge of balancing aesthetics, functionality, and environmental sustainability to create remarkable timber skyscrapers. Achieving this delicate equilibrium is crucial for realizing these extraordinary structures, as architects harmonize visual appeal, design finesse, spatial optimization, and seamless integration of functionalities while prioritizing environmental responsibility. Navigating the regulatory landscape and expectations for tall timber buildings involves inherent uncertainties in this relatively new architectural category. The design process is dynamic, evolving with technological progress and innovative wood construction methods, influenced by regulations, market needs, context, and environmental conditions. Our article contributes to the ongoing discourse by offering the latest assessment of tall timber construction in Europe, offering insights into the challenges, advancements, and considerations shaping its present and future.

Author contributions statement

Conceptualization, H.E.I.; methodology, H.E.I. and Ö.N.A.; software, Ö.N.A.; formal analysis, H.E.I. and Ö.N.A.; investigation, H.E.I. and Ö.N.A.; writing—original draft preparation, H.E.I.; writing—review and editing, Ö.N.A.

Data availability statement

Authors agree to share the data upon reasonable request.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Notes on contributors

Hüseyin Emre Ilgın

Hüseyin Emre Ilgın received his Ph.D. (2018) in Building Sciences about tall building design in Architecture from METU in Ankara. Since December 2019, he has been conducting post-doctoral research on wood construction at Tampere University. Dr. Ilgın worked as a Marie Skłodowska-Curie postdoctoral research fellow on dovetailed massive wood board elements for 2 years between 2021–2023.

Özlem Nur Aslantamer

Özlem Nur Aslantamer completed her Ph.D. in Landscape Architecture at the School of Natural and Applied Sciences, Ankara University, in 2021. With 26 years of combined experience in academia and the professional field of interior architecture, she has established a robust foundation in her field. Since 2021, she has been serving as a full-time instructor at Atılım University, dedicating her expertise to both teaching and research endeavors.

Appendix

 

Appendix A. European tall timber buildings.

#	Building name	Country	City	Height (m)	# of stories	Completion date	Function
1	Mjøstårnet	Norway	Brumunddal	85	18	2019	M
2	HoHo	Austria	Vienna	84	24	2020	M
3	HAUT	Netherlands	Amsterdam	73	22	2022	R
4	Sara Kulturhus	Sweden	Skellefteå	73	19	2021	H
5	De Karel Doorman	Netherlands	Rotterdam	71	22	2012	R
6	Roots Tower	Germany	Hamburg	65	19	UC	R
7	Abro	Switzerland	Risch-Rotkreuz	60	15	2019	O
8	Kaj16	Sweden	Göteborg	60	15	UC	M
9	Hyperion	France	Bordeaux	55	16	2021	R
10	Rundeskogen Hus B	Norway	Sandnes	55	16	2013	R
11	Albizzia	France	Lyon	53	17	UC	M
12	Treet	Norway	Bergen	49	14	2015	R
13	Lighthouse Joensuu	Finland	Joensuu	48	14	2019	R
14	Cederhusen	Sweden	Stockholm	44	13	UC	R
15	Hoas Tuuliniitty	Finland	Espoo	44	13	2021	R
16	Palazzo Nice Meridia	France	Nice	44	10	2019	O
17	Sensations	France	Strasbourg	38	11	2019	R
18	Rundeskogen Hus C	Norway	Sandnes	38	11	2013	R
19	Trafalgar Place	UK	London	36	10	2015	R
20	Suurstoffi 22	Switzerland	Risch-Rotkreuz	36	10	2018	O
21	Kringsja Studentby	Norway	Oslo	34	10	2018	R
22	Hotel Jakarta	Netherlands	Amsterdam	34	9	2018	H
23	Rundeskogen Hus A	Norway	Sandnes	34	10	2012	R
24	SKAIO	Germany	Heilbronn	34	10	2019	R
25	Dalston Works	UK	London	34	10	2017	R
26	The Cube Building	UK	London	33	10	2015	R
27	Botanikern	Sweden	Uppsala	31	9	2019	R
28	Cenni di Cambiamento	Italy	Milan	31	9	2013	R
29	Kajstaden	Sweden	Vasteras	31	9	2019	R
30	Press House	UK	London	31	9	2017	R
31	Vallen	Sweden	Vaxjo	31	9	2015	R
32	Stadthaus	UK	London	29	9	2009	R
33	Moholt 50/50	Norway	Trondheim	28	9	2016	R

Note on abbreviations: ‘M’ indicates mixed-use; ‘H’ indicates hotel use; ‘R’ indicates residential use; ‘O’ indicates office use; ‘UK’ indicates United Kingdom; ‘UC’ indicates Under construction.

Appendix B. European tall timber buildings by core type, building form, structural system, and structural material.

#	Building name	Building form	Core type	Structural system	Structural material
1	Mjøstårnet	Prismatic	Peripheral	Trussed-tube	Timber
2	HoHo	Prismatic	Central	Shear walled frame	Composite (T + C)
3	HAUT	Free	Peripheral	Shear walled frame	Composite (T + C)
4	Sara Kulturhus	Prismatic	Peripheral	Shear walled frame	Composite (T + S)
5	De Karel Doorman	Prismatic	Peripheral	Shear walled frame	Composite (T + C + S)
6	Roots Tower	Prismatic	Central	Shear walled frame	Composite (T + C)
7	Abro	Prismatic	Central	Shear walled frame	Composite (T + C)
8	Kaj16	Free	Central	Shear walled frame	Composite (T + C)
9	Hyperion	Free	Central	Shear walled frame	Composite (T + C + S)
10	Rundeskogen Hus B	Free	Central	Shear walled frame	Composite (T + C)
11	Albizzia	Prismatic	Central	Shear walled frame	Composite (T + C)
12	Treet	Prismatic	Peripheral	Trussed-tube	Timber
13	Lighthouse Joensuu	Prismatic	Central	Shear wall	Timber
14	Cederhusen	Prismatic	Central	Shear wall	Timber
15	Hoas Tuuliniitty	Prismatic	Peripheral	Shear wall	Timber
16	Palazzo Nice Meridia	Prismatic	Peripheral	Shear walled frame	Composite (T + C)
17	Sensations	Free	Central	Shear walled frame	Timber
18	Rundeskogen Hus C	Free	Central	Shear walled frame	Composite (T + C)
19	Trafalgar Place	Prismatic	Peripheral	Shear wall	Timber
20	Suurstoffi 22	Prismatic	Central	Shear walled frame	Composite (T + C)
21	Kringsja Studentby	Prismatic	Central	Shear walled frame	Timber
22	Hotel Jakarta	Prismatic	Peripheral	Shear walled frame	Composite (T + C)
23	Rundeskogen Hus A	Free	Central	Shear walled frame	Composite (T + C)
24	SKAIO	Prismatic	Central	Shear walled frame	Composite (T + C)
25	Dalston Works	Prismatic	Central	Shear wall	Timber
26	The Cube Building	Free	Central	Shear walled frame	Composite (T + C + S)
27	Botanikern	Prismatic	Peripheral	Shear trussed frame	Timber
28	Cenni di Cambiamento	Free	Central	Shear wall	Timber
29	Kajstaden	Prismatic	Peripheral	Shear wall	Timber
30	Press House	Prismatic	Central	Shear walled frame	Timber
31	Vallen	Prismatic	Central	Shear walled frame	Composite (T + C)
32	Stadthaus	Prismatic	Central	Shear wall	Timber
33	Moholt 50/50	Prismatic	Central	Shear wall	Timber

Note on abbreviations: ‘(T + C + S)’ indicates composite/hybrid structures combining timber and concrete and steel; ‘(T + C)’ indicates composite/hybrid structures combining timber and concrete; ‘(T + S)’ indicates composite/hybrid structures combining timber and steel.