The rise of wood buildings in the skylines of cities forces structural dynamic and timber experts to team up to solve one of the new civil-engineering challenges, namely comfort at the higher levels, in light weight buildings, with respect to wind-induced vibrations. Large laminated timber structures with mechanical joints are exposed to turbulent horizontal excitation with most of the wind energy blowing around the lowest resonance frequencies of 50 to 150 m tall buildings. Good knowledge of the spatial distribution of mass, stiffness and damping is needed to predict and mitigate the sway in lighter, flexible buildings. This paper presents vibration tests and reductions of a detailed FE-model of a truss with dowel-type connections leading to models that will be useful for structural engineers. The models also enable further investigations about the parameters of the slotted-in steel plates and dowels connections governing the dynamical response of timber trusses.
In this paper, an adaptable and architecturally flexible lateral stiffening system for tall timber buildings between 50 and 147 m is developed and investigated. The system is based on a tube-in-tube concept. The internal tube consists of a braced timber core, and the external tube consists of a frame structure with semi-rigid beam-column joints in the façade. Based on a finite element framework, more than 500 000 simulations with different configurations are carried out to assess the performance of the lateral stiffening system subjected to wind loading. The resulting data is used to assess the feasibility of the tube-in-tube system and stiffness requirements for the beam-column joints.
Cross-laminated timber (CLT) is an innovative engineered timber product and has been widely used for constructing tall timber buildings due to its excellent structural performance and good strength with its multi-layers of boards in both perpendicular directions. However, the global serviceability performance of tall timber buildings constructed from CLT products for the lift core, walls, and floors under wind load is not well known yet, even though it is crucial in a design. In this study, the finite element software SAP2000 is used to numerically simulate the global static and dynamic serviceability behaviours of a 30-storey tall CLT building assumed in Glasgow, Scotland, UK. The maximum horizontal storey displacement due to wind is only 16.6% of the design limit and the maximum global horizontal displacement is only 13.8% of the limit set to the Eurocodes. The first three lowest vibrational frequencies, modes and shapes were obtained, with the fundamental frequency being 19.9% larger than the code-recommended value. Accordingly, the peak acceleration of the building due to wind was determined as per the Eurocodes and ISO standard. The results show that the global serviceability behaviours of the building satisfy the requirements of the Eurocodes and other design standards. Parametric studies on the peak accelerations of the tall CLT building were also conducted by varying the timber material properties and building masses. By increasing the timber grade for CLT members, the generalised building mass and the generalised building stiffness can all be adopted to lower the peak accelerations at the top level of the building, so as to reduce human perceptions of the wind-induced vibrations with respect to the peak acceleration.
The dynamic response of semi-rigid timber frames subjected to wind loads is investigated numerically in this paper. The dynamic response of more than one million unique frames with different parameters was assessed with the frequency-domain gust factor approach, which is currently adopted by Eurocode 1, and the time-domain generalized wind load method. In the generalized wind load method, the frames were simulated for three different wind velocities with five simulations per unique combination of parameters, resulting in more than twelve million simulations in total. Qualitative and quantitative observations of the dataset were made. Empirical expressions for the accelerations, displacements, and fundamental eigenfrequency were proposed by the use of nonlinear regression applied to the obtained numerical results and a frequency reduction factor was developed. The wind-induced accelerations obtained by the two methods were compared to the corresponding serviceability criteria according to ISO10137, providing insight about the feasibility of moment-resisting frames as a lateral load-carrying system for mid-rise timber buildings. Comparison between the theoretical gust factor approach and the generalized wind load method showed that the gust factor approach was nonconservative in most cases. Finally, the effect of uniform and non-uniform mass distributions was investigated, with a theoretical reduction in top-floor accelerations of 50% and 25% respectively.
Despite all being less than 100 metres tall, the world's tallest timber buildings all utilise concrete to increase their mass such that they do not vibrate excessively under wind loading. Wind-induced vibrations must be minimised to ensure that the building's occupants remain comfortable and do not regularly experience motion sickness during high winds. Despite the difficulties with wind dynamics for the current generation of timber towers, numerous concept designs have been announced that propose to build much taller with timber. However, at present, there has been little consideration of how the architecture of timber towers can be suitably designed to help combat the problem. This thesis investigates the effects of different structural typologies on the dynamic performance of timber buildings by studying four iconic skyscrapers; the Gherkin, the Shard, the John Hancock Center and 432 Park Avenue and examining how they would perform if built from timber. First, they are assessed at their existing heights and across a range of shorter heights, with their steel or concrete frames but examining the effect of replacing their concrete floors with CLT. Secondly (and again across a range of heights), the buildings are redesigned with a timber frame to test how their dynamics would change if their steel or concrete beams, columns and walls were replaced with glulam sections and CLT panels. The Shard and 432 Park Avenue, which have concrete cores, have also been examined to see how they perform if they kept their concrete cores, but if the remainder of their structures were built from timber.
In total, 144 combinations of building, floor material, height, and frame material are assessed. Retaining their existing steel or concrete frames but replacing their concrete floors with CLT resulted in the buildings' natural frequencies increasing by an average of 30% and the peak accelerations by 47%. These changes are due to the CLT floors being considerably lighter than the original concrete floors. By comparison, the change from a steel or concrete-framed structure to a timber-framed structure (with no change in floor type) made little difference to the peak accelerations, but caused natural frequencies to increase by 11%.
If their existing structures were retained, but CLT panels with a thin layer of concrete screed were used for their floors (instead of deep concrete slabs), then the Gherkin at 182 m, the Shard at 200 m, the John Hancock Center at 196 m and 432 Park Avenue at 137 m would have acceptable vibrations (for residential occupancy) if located in a low wind speed environment like London. Across the four buildings, this change in floor type would save an average of 24 kgCO2 per m2 of floorspace if sequestered carbon is excluded, and 170 kgCO2/m2 if sequestered carbon is included. When sequestered carbon is included in the calculation, the net carbon stored in CLT is enough to offset the embodied carbon of the steel and concrete of the Shard (at 200 m) and 432 Park Avenue (at 137 m). When sizing the columns and diagonals of the Gherkin and the John Hancock Center, the strength criteria was the limiting factor (rather than stiffness). This is because both towers have well-braced tubular designs that are inherently stiff, thanks to the majority of their columns and diagonals being located on their perimeters. With strength as the governing criterion, the size of the structural members could be reduced when lightweight CLT floors were used instead of concrete. For example, the columns of the Gherkin would have required 32% less steel if CLT floors had been used instead of concrete decks. Such savings would not be possible for the Shard or 432 Park Avenue, where the stiffness criterion limits the sizes of the sections.
If the four skyscraper designs were built with a timber frame, the Gherkin would comfortably be the best performing structure thanks to its inherently stiff diagrid shell and its circular cross-section. It could easily satisfy the ISO 10137 human comfort criterion for residential occupancy in most locations at its full height of 182 metres. Taller versions of the structure are also likely to be viable. If built in London, a fully-timber Shard at 134 m (or 200 m with a concrete core and glulam frame), a timber John Hancock Center at 196 m, and a fully timber 432 Park Avenue at 80 m (or a hybrid at 137 m) could also satisfy the same criterion (all with CLT and screed floors). Across a set of the 135 m versions of the four skyscrapers, the change from a steel or concrete frame to a glulam and CLT structure would result in a saving of 130 kgCO2/m2 (including sequestered carbon) or a saving of 92 kgCO2/m2 for a hybrid (timber beams and columns, but retaining a concrete core).
Overall, when different typologies were compared on a like-for-like basis, braced tubular forms like the Gherkin and the John Hancock Center worked the best in timber, producing lower wind-induced vibrations than 432 Park Avenue and the Shard. Furthermore, their tubular structures required smaller column sizes (which occupy a lower percentage of their floor space), have lower material costs per m2 of floor space and would result in less embodied carbon per m2 (if sequestered carbon is ignored) than those which rely on an internal core for lateral stability.
The next generation of tall timber buildings looks unlikely to reach some of the super tall heights proposed without significant additional damping, added mass or suitable aerodynamic cross-sections that can minimise wind-induced vibrations. However, this thesis has shown that timber does have the potential to be used in suitably designed tall buildings up to at least 200 m tall, without additional damping or mass, and as the primary structural material in the frame or as an alternative to concrete floors.
This paper reflects on the structural design of Haut; a 21-storey high-end residential development in Amsterdam, the Netherlands. Construction started in 2019 and is in progress at the time of writing. Upon completion in 2021, Haut will be the first residential building in the Netherlands to achieve a 'BREEAM-outstanding' classification. The building will reach a height of 73 m, making it the highest timber structure in the Netherlands. It contains some 14.500 of predominantly residential functions. It features a hybrid concrete-timber stability system and concrete-timber floor panels. This paper describes the concepts behind the structural design for Haut and will touch upon the main challenges that have arisen from the specific combination of characteristics of the project. The paper describes the design of the stability system and -floor system, the analysis of differential movements between concrete and timber structures and wind vibrations. The paper aims to show how the design team has met these specific challenges by implementing a holistic design approach and integrating market knowledge at an early stage of the design.
Wind-induced dynamic excitation is becoming a governing design action determining size and shape of modern Tall Timber Buildings (TTBs). The wind actions generate dynamic loading, causing discomfort or annoyance for occupants due to the perceived horizontal sway – i.e. vibration serviceability failure. Although some TTBs have been instrumented and measured to estimate their key dynamic properties (natural frequencies and damping), no systematic evaluation of dynamic performance pertinent to wind loading has been performed for the new and evolving construction technology used in TTBs. The DynaTTB project, funded by the Forest Value research program, mixes on site measurements on existing buildings excited by heavy shakers, for identification of the structural system, with laboratory identification of building elements mechanical features coupled with numerical modelling of timber structures. The goal is to identify and quantify the causes of vibration energy dissipation in modern TTBs and provide key elements to FE modelers. The first building, from a list of 8, was modelled and tested at full scale in December 2019. Some results are presented in this paper. Four other buildings will be modelled and tested in spring 2021.
As the interest in timber buildings is increasing, more attention is pointed towards highrise timber buildings. Partly because it is one of the main areas pushing the development within the field of timber structures. As the current tallest timber building, Mjöstornet in Brumunddal is approximately 10 times shorter than the world’s tallest building, Burj Khalifa, the intuition says that there is room for major improvements regarding tall timber structures. The aim of this thesis is therefore to investigate the possibilities to build a 200 m tall timber tower while still fulfilling the requirements for strength, stability and dynamics. In order to anchor the project in reality, the assumed building location is Gothenburg with the ground conditions of solid rock.
Early in the study it was concluded that in order to push the height limits, the building design had to be improved compared to the existing timber buildings. The main geometries of interest turned out to be the circular shape thanks to its aerodynamical benefits. This base shape was applied in various ways, generating five different concepts ready for evaluation.
Each of the five concepts were modelled and preliminary sized using Grasshopper and Karamba 3D, whereafter they were evaluated based on their dynamic performance, global stiffness, and a few other evaluation criteria. The evaluation was primary made with structural performance in mind and secondary with regard to comfort, quality and economical aspects.
The results show that one of the concepts have great potential of reaching 200 m despite the uncertainties regarding joint stiffness and structural damping. Also, a few of the other concepts might be able to reach 200 m if subject to some structural and dynamical improvements.
Advancement in engineered wood products altered the existing building height limitations and enhanced wooden structural members that are available on the market. These coupled with the need for a sustainable and green solution to address the ever-growing urbanization demand, avails wood as possible candidate for primary structural material in the construction industry. To this end, several researches carried out in the past decade to come up with sound structural solutions using a timber based structural system. Green and Karsh (2012) introduced the FFTT system; Tesfamariam et al. (2015) developed force-based design guideline for steel infilled with CLT shear walls, and SOM (2013) introduced the concrete jointed mass timber hybrid structural concepts. In this research, the basic structural concepts proposed by SOM (2013) is adopted. The objective of this research is to develop a wind and earthquake design guideline for concrete jointed tall mass timber buildings in scope from 10- to 40-storey office or residential buildings. The specific objective of this research is as follow:
1. Wind serviceability design guideline for hybrid mass-timber structures.
2. Calibration of design wind load factors for the serviceability wind design of hybrid tall mass timber structures.
3. Guidelines to perform probabilistic modeling, reliability assessment, and wind load factor calibration.
4. Overstrength related modification factor Ro and ductility related modification factor Rd for future implementation in the NBCC.
5. Force-based design guideline following the capacity based design principles.
The interest in building taller structures in timber is increasing in the building sector. However, the high strength-to-weight ratio of timber leads to a relatively light structure which is often associated with vibrations. The dynamic properties are essential in the design of tall timber structures, where wind-induced vibrations of the building in service state is addressed. The dynamic response is influenced by mass, stiffness and damping. These parameters influence the acceleration of the building which can be perceived as a discomfort for human occupancy. The aim is to find a structural concept that makes a taller structure than the usual today feasible. The objective is to make a parametric study and investigate how a multi-storey residential building of timber can be optimized with respect to dynamic wind loading. With a combination of numerical and analytical methods, accelerations are calculated and evaluated against the criteria for human comfort according to ISO 10137 and ISO 6897. An analytical calculation sheet is set up according to SS-EN-1991-1-4 and EKS 10 to define wind-induced acceleration. Starting from a beam-column structure with a central core, the effect of adding inner walls and exterior bracing is studied to see what limits the number of storeys for an open plan building. Analysis of the dynamic response due to wind shows the fundamental mode shape in torsion before exterior bracing is added. Results have shown that the structure can reach 5-storeys with inner walls of cross-laminated timber and 4-storeys with no walls. Moreover, it’s found that diagonal bracing in the facades improves the torsional stiffness significantly and the fundamental mode becomes a transversal mode. An outrigger bracing system has been found to be the most efficient, leading to a structure of 12-storeys. The parameters mass and stiffness are modified by adding concrete floors and assigning larger sections to the structure. Results show that the building can achieve 15-storeys with pure timber and 21-storeys when concrete floors are added. Secondary parametric action i.e. adding another outrigger generates a gain of one-storey and modifying the truss-work to steel gives a structure of 23-storeys.
