Cross-laminated Timber Buildings Research Articles

Modular multi-storey construction with cross-laminated timber: Life cycle environmental implications

ABSTRACT In this study, the life cycle environmental implications of modular multi-storey building with cross-laminated timber (CLT) volumetric elements are analysed, considering the product, construction, service life, end-of-life and post-use stages. A bottom-up attributional approach is used to analyse the environmental flows linked to the global warming potential (GWP), acidification potential (AP) and eutrophication potential (EP) impacts of the building for a 50-year reference study period. The result shows that the building’s life cycle impacts can vary considerably, depending on the energy production profile for the operation of the building. The product, construction and end-of-life stages constitute a significant share of the life cycle impacts, and the importance of these stages increase as the energy production profile evolves towards a low-carbon energy mix. For the GWP, the product and construction stages constitute 13% of the total life cycle impact when the operational energy is based on a coal-based marginal electricity. The contribution of this stage increases to 81% when electricity is based on a plausible long-term Swedish average mix. The patterns of the life cycle EP and AP impacts are also closely linked to the energy production profile for the assessment. The analysis shows that a 5% reduction in the GWP impact in the product stage is achievable with emerging solutions for the improved structural design of CLT buildings. This study highlights the need for strategies to improve the life cycle environmental profile of modular CLT buildings.

Cross-Laminated Timber Floor: Analysis of the Acoustic Properties and Radiation Efficiency

Cross-Laminated Timber (CLT) is a building technology that is becoming increasingly popular due to its sustainable and eco-friendly nature, as well as its availability. Nevertheless, CLT presents some challenges, especially in terms of impact noise and airborne sound insulation. For this reason, many studies focus on the vibro-acoustic behavior of CLT building elements, to understand their performance, advantages and limitations. In this paper, a 200 mm CLT floor has been characterized in the laboratory, according to ISO standards, by three noise sources: dodecahedron, standard tapping machine and rubber ball. In order to understand the vibro-acoustic behavior of the CLT floor, measurements through the analysis of sound pressure levels and velocity levels, measured by dedicated sensors, were performed. Analysis was carried out in order to understand what is prescribed by the prediction methods available in the literature and by the simulation software. Then, a specific prediction law for the CLT floor under investigation was derived. Finally, an analysis on sound radiation index is provided to complete the vibro-acoustic study.

Evaluation of Bearing Strength of Self-Tapping Screws according to the Grain Direction of Domestic Pinus densiflora

To evaluate the bearing strength of red pine cross-laminated timber (CLT) with self-tapping screw (STS), which is widely used as a fastener for connection in CLT building, the bearing test was conducted. Accoring to the STS’s diameters (8, 10, 12 mm), the bearing test specimens with half hole were manufactured. Bearing strength was compared and reviewed in consideration of the configuration in STS and the loading direction to the grain of red pine. As a result of the bearing test on the STS’s diameter, the yield bearing load increases as the larger diameter of the STS in all directions of the red pine. The bearing strength of the thread part (thread + tip) was higher than the shank part (shank + shank cutter). In compared with the directions to the grain of red pine, the bearing strength of the cross section parallel to the loading direction was the highest, and the tangent section was the lowest bearing strength. The average bearing strength of the loading direction in parallel to the grain was 23.43 MPa, which was about 45% higher than the average 16.16 MPa in perpendicular to the grain. The predicted bearing strength calculated by Eurocode (EN) and Korean Building Code (KBC)’s equation was lower than the experimental value. It is nessesary to propose the neuations of bearing strength reflected the configuration information of STS.

Experimental and analytical studies on mechanical performance of innovative energy-dissipating hold-down for CLT structures

Connection is the most important part in cross-laminated timber (CLT) buildings as it guarantees necessary strength, stiffness, ductility, and integrality for the whole CLT structure. This paper proposes an innovative energy-dissipating hold-down connection for CLT structures, which combines the advantages of the soft-steel bracket and high-damping rubber for providing great energy-dissipating capacity and high ductility. A series of tests were performed under quasi-static monotonic and reversed cyclic loading to investigate the failure mechanisms and mechanical properties of the novel hold-down connection. Test results demonstrate that the connections can continue to work as a whole even after the occurrence of preliminary failures of steel ribs rupture and weld fracture. Final failure modes, including debonding between the rubber and steel plates, rupture of the front steel plate and breakage of screws, caused the invalidation of the connection. Meanwhile, load–displacement curves of the connections usually exhibit a bi-linear form, and the connection’s yielding is caused by the yielding of steel ribs. All the tested specimens exhibited stable energy-dissipating capacity in the working stage and were all classified as highly ductile. Furthermore, efforts were made to develop a simplified analytical model for estimating the basic mechanical properties of the novel hold-down connections. Comparison with test results shows that the analytical model can provide reasonable estimations of the initial stiffness, post-yield stiffness, yield force and failure force. The test results and analyses presented herein provide useful technical bases for supporting future studies and practical applications of the novel hold-down connection for CLT buildings.

Assessment of effect of climate change on hygrothermal performance of cross-laminated timber building envelope with modular construction

Cross-laminated timber (CLT) modular construction possesses the advantages of wood, such as excellent carbon storage and thermal insulation, and of modular construction, such as considerably reduced construction period and cost as well as high productivity. This study evaluates the hygrothermal performance of CLT walls considering modular construction in future climatic conditions. Firstly, CLT walls with plywood applied to a core layer were manufactured. A mock-up of a CLT building was produced and its construction process was analyzed. Hygrothermal behavior of the CLT walls was simulated using WUFI simulation program, and the predicted results were verified against measurements obtained from the mock-up experiment. Finally, the hygrothermal performance of the CLT wall was evaluated for four types of insulation and future climate in eight cities of USA. The coefficient of variation—root mean square error (CV(RMSE))—of the temperature and relative humidity inside the ply-lam CLT wall from mock-up experiments and simulation evaluation were 6.43% and 7.02%, respectively, which met the validation criteria. Based on the hygrothermal performance, the ply-lam CLT wall with extruded polystyrene insulation was evaluated as safe from moisture problems in all the eight cities considered in this study. However, the risk of mold growth in all regions and insulation types increased under climate change with a rise of average annual temperature.

Comparative life cycle assessment of cross laminated timber building and concrete building with special focus on biogenic carbon

This study conducted a consequential Life Cycle Assessment (LCA) on two similar mid-rise apartment buildings applying either concrete or cross laminated timber (CLT) as the main structural material. The study further investigated inclusion of biogenic carbon and how this affects environmental impacts related to Global warming. Thus, two assessment scenarios were applied: A Base scenario, without accounting for biogenic carbon and a Biogenic carbon scenario that include a GWPbio factor to account for the use of biogenic carbon. The CLT building had the lowest impact score in 11 of 18 impact categories including Global warming. Operational energy use was the main contributor to the total impact with some variation across impact scores, but closely followed by impacts embodied in materials (incl. End-of-Life). An evaluation of the potential forest transformations required for fulfilling future projections for new building construction in 2060 showed that about 3% of current global forest area would be needed. This share was essentially independent of the selected building material as the main driver for forest transformation was found to be energy use during building operation. Thus, focus should primarily be on reducing deforestation related to energy generation rather than deforestation from production of building materials.

Impact on a CLT structure concerning moisture and mould growth using weather protection

Timber buildings, including cross laminated timber (CLT), are gaining market shares globally, mainly due to anticipated environmental benefits, but a new technical solution also raises new questions. Durability is critical to obtain real sustainable constructions built for the future. There are field studies concerning hygrothermal conditions of timber structures, however, there is a lack of documented experiences combining hygrothermal conditions, mould growth potential and weather protection during construction using CLT. The use of full weather protection is being debated in the building industry as well as in the research community, due to lack of knowledge of the combined effects. How does weather protection during the construction affect hygrothermal conditions and risk of mould growth in a CLT structure? A case study using a weather protected six-storey CLT building was performed. The hygrothermal conditions – indoors and outdoors – were monitored during construction and samples from CLT were analysed with respect to mould. The results were analysed together with simulations of mould growth using actual hygrothermal conditions. Theoretical conclusions show the weather protection gives significantly improved conditions resulting in lower potential of mould growth compared to outdoor conditions. The results also show lessons to be learned concerning planning of the construction site.

The effect of the floor-to-wall interaction on the rocking stiffness of segmented CLT shear-walls

The mechanical behaviour of Cross-Laminated Timber (CLT) buildings under lateral loads is mainly governed from the connections and is strongly influenced from the internal structure of the shear-walls, which can be monolithic or segmented. In comparison with monolithic shear-walls, segmented shear-walls have a more flexible behaviour and can dissipate higher amount of energy due to the contribution of the vertical panel-to-panel connections. However, in CLT buildings other connections, such as the connections between the wall heads and the upper floor, can influence the lateral behaviour of the shear-walls. This paper presents a study on the phenomenon of the floor-to-wall interaction and on the influence of this mechanism on the rocking behaviour of segmented CLT shear-walls. An analytical elastic model that describes the floor-to-wall interaction and allows to calculate transversal displacements and internal actions along the floor is presented and validated against numerical models developed in SAP2000. The analytical model is subsequently used for the definition of an equivalent spring, which can be used for easily taking into account the more complex phenomenon of the floor-to-wall interaction. Parametric analyses, in which different geometries and connection stiffnesses were used, showed that the floor-to-wall interaction increases the rocking stiffness of segmented shear-walls and modifies the shear-wall kinematic behaviour. The increase of rocking stiffness was found to be dependent on the floor bending stiffness as well as the withdrawal stiffness of the floor-to-wall connections.

Resilience-based predictive models for the seismic behaviour of mid-rise, base-isolated CLT buildings for social housing applications in Chile

Based on a six-storey cross-laminated timber building equipped with a base isolation system, this article proposes predictive models for the fundamental period of vibration, a parameter that needs to be selected in early stages of a seismically isolated building's design process, in relation to economic losses, such as repair costs and repair time, and environmental losses, such as repair-related carbon footprint and embodied energy. The predictive models were calibrated using a dataset of 114 bidirectional accelerograms recorded in Chile between 1985 and 2020, with event magnitudes ranging from Mw 7 to Mw 8.8. A single functional form was selected for the four resilience parameters used, having an average coefficient of determination of R2 = 0.62. Results suggest that the selection of the fundamental period of vibration of a mid-rise seismically isolated CLT building can have a significant impact on the potential economic and environmental losses likely to occur when subjected to severe seismic events, and therefore, its selection should be based not only on technical aspects but also resilience and sustainability parameters.

A Numerical Study of the Stiffness and Strength of Cross-Laminated Timber Wall-to-Floor Connections under Compression Perpendicular to the Grain

The use of cross-laminated timber (CLT) in multi-story buildings is increasing due to the potential of wood to reduce green house gas emissions and the high load-bearing capacity of CLT. Compression perpendicular to the grain (CPG) in CLT is an important design aspect, especially in multi-storied platform-type CLT buildings, where CPG stress develops in CLT floors due to loads from the roof or from upper floors. Here, CPG of CLT wall-to-floor connections are studied by means of finite element modeling with elasto-plastic material behavior based on a previously validated Quadratic multi-surface (QMS) failure criterion. Model predictions were first compared with experiments on CLT connections, before the model was used in a parameter study, to investigate the influence of wall and floor thicknesses, the annual ring pattern of the boards and the number of layers in the CLT elements. The finite element model agreed well with experimental findings. Connection stiffness was overestimated, while the strength was only slightly underestimated. The parameter study revealed that the wall thickness effect on the stiffness and strength of the connection was strongest for the practically most relevant wall thicknesses between 80 and about 160 mm. It also showed that an increasing floor thickness leads to higher stiffness and strength, due to the load dispersion effect. The increase was found to be stronger for smaller wall thicknesses. The influence of the annual ring orientation, or the pith location, was assessed as well and showed that boards cut closer to the pith yielded lower stiffness and strength. The findings of the parameter study were fitted with regression equations. Finally, a dimensionless ratio of the wall-to-floor thickness was used for deriving regression equations for stiffness and strength, as well as for load and stiffness increase factors, which could be used for the engineering design of CLT connections.

Exploring the synergy between structural engineering design solutions and life cycle carbon footprint of cross-laminated timber in multi-storey buildings

ABSTRACT Low-carbon buildings and construction products can play a key role in creating a low-carbon society. Cross-laminated timber (CLT) is proposed as a prime example of innovative building products, revolutionising the use of timber in multi-storey construction. Therefore, an understanding of the synergy between structural engineering design solutions and climate impact of CLT is essential. In this study, the carbon footprint of a CLT multi-storey building is analysed in a life cycle perspective and strategies to optimise this are explored through a synergy approach, which integrates knowledge from optimised CLT utilisation, connections in CLT assemblies, risk management in building service-life and life cycle analysis. The study is based on emerging results in a multi-disciplinary research project to improve the competitiveness of CLT-based building systems through optimised structural engineering design and reduced climate impact. The impacts associated with material production, construction, service-life and end-of-life stages are analysed using a process-based life cycle analysis approach. The consequences of CLT panels and connection configurations are explored in the production and construction stages, the implications of plausible replacement scenarios are analysed during the service-life stage, and in the end-of-life stage the impacts of connection configuration for post-use material recovery and carbon footprint are analysed. The analyses show that a reduction of up to 43% in the life cycle carbon footprint can be achieved when employing the synergy approach. This study demonstrates the significance of the synergy between structural engineering design solutions and carbon footprint in CLT buildings.

Mass-customisation of cross-laminated timber wall systems at early design stages

Low profitability of the building industry has driven the adoption of industrialised production methods in this sector. In an industrialised context, mass-customisation (MC) is a set of strategies aimed to offer tailored products at prices close to those of mass-production. MC can be achieved through integrating market knowledge and supply-chain processes into the building design. Nonetheless, the MC-related literature is slim in optimising designs for such an integration. The goal is to find optimal design configurations for the load-bearing spanning systems that incur minimum product waste and manufacturing process waste. The paper uses Cross-Laminated Timber (CLT) buildings as cases of highly prefabricated systems. The preliminary layout of a building is first mathematically modelled. Then, genetic algorithms are adapted to optimise wall dimensions and billet cutting plan. The example case demonstrated significant waste reduction compared to the building industry benchmarks. This highlights the practical benefits of the systematical implementation of MC from the early design stages. The present study contributes to the body of knowledge by introducing a novel design support tool that facilitates the adoption of MC in the building industry.

Assessment of Summer Overheating in Concrete Block and Cross Laminated Timber Office Buildings in the Severe Cold and Cold Regions of China

The aims of the paper were to clarify whether office buildings in the severe cold and cold regions are overheating, especially those with natural ventilation, and whether potential overheating is related to the building materials. The severe cold and cold regions of China were considered to be cool regions during summer. However, with global warming, improvements in the thermal performance of the building envelope and the urban heat island effect, office buildings in these regions are showing different degrees of overheating during summer. Two office building materials commonly used in this area, cross laminated timber (CLT) and concrete block, were simulated in this study. With reference to the overheating standard, the degree of overheating in six cities in the severe cold and cold regions was quantitatively analysed and the extent of overheating for the two building materials was compared. Finally, the influence of thermal insulation on building overheating is discussed, and some suggestions are put forward to improve the relevant national regulations in China. The results show that office buildings in the severe cold and cold regions experience overheating during summer, and CLT buildings are more prone to overheating than concrete buildings during summer. This is attributable to the different thermal mass of the materials. Thick insulation does increase the risk of building overheating, and the effect on concrete buildings is more pronounced. Concrete buildings with an insulation layer can experience overheating for 27–71 h more than buildings without an insulation layer. Insulation on CLT buildings only results in an increase of 11–37 h. When considering the current situation with summer overheating in the severe cold and cold regions, relevant codes should also be modified and improved accordingly to guide building design, so as to achieve low-carbon and energy-saving goals.

An upgrade of existing practice-oriented FE design models for the seismic analysis of CLT buildings

Practice-oriented finite element (FE) modelling strategies represent a fundamental tool for the seismic analysis and design of Cross Laminated Timber (CLT) structures. Although substantial research has been undertaken concerning the seismic behaviour of CLT buildings, practice-oriented FE modelling strategies are still at an early stage. This paper presents an upgrade of an existing practice-oriented FE design model for the seismic design of CLT structures. The upgrade is supported through the same modelling strategy presented by Christovasilis et al. (2020) adding some features of analytical equations models presented by Casagrande et al. (2016).2D elements are used both for the modelling of CLT panels and for mechanical connections, which are represented by a horizontal strip with a height smaller than 7% of the height of the panel. Analytical equations for determining the elastic modulus of elasticity and the shear modulus of the horizontal strip are reported accounting for both rocking and sliding behaviour for single- and multi-panel CLT shearwalls, including the effect of the vertical loads by using a secant stiffness. The validation of the proposal is carried out in terms of the shearwall lateral stiffness through the results of experimental tests on full-scale shearwalls published by other authors and through a validation/comparison between a detailed non-linear model and the Christovasilis et al. (2020) strategy on two multi-storey seismic-resistant lateral systems configurations with different amount of vertical loads. Finally, a proposal for the initial layout of connections to reduce the iterative seismic design and analysis process for CLT buildings is presented through a case study.

Model updating of seven-storey cross-laminated timber building designed on frequency-response-functions-based modal testing

Based on the experimental estimation of the key dynamic properties of a seven-storey building made entirely of cross-laminated timber (CLT) panels, the finite element (FE) model updating was performed. The dynamic properties were obtained from an input-output full-scale modal testing of the building in operation. The chosen parameters for the FE model updating allowed the consideration of the timber connections in a smeared sense. This approach led to an excellent match between the first six experimental and numerical modes of vibrations, despite spatial aliasing. Moreover, it allowed, together with the sensitivity analysis, to estimate the stiffness (affected by the connections) of the building structural elements. Thus, the study provides an insight into the as-built stiffness and modal properties of tall CLT building. This is valuable because of the currently limited knowledge about the dynamics of tall timber buildings under service loadings, especially because their design is predominantly governed by the wind-generated vibrations.

Airtightness of cross-laminated timber envelopes: Influence of moisture content, indoor humidity, orientation, and assembly

Decrease in the relative humidity (RH) of the indoor air causes cracks in the cross-laminated timber (CLT) surface resulting in air leakages in the CLT panel. The effect of construction moisture on CLT air tightness properties is not clearly defined, which is important for moisture safe construction. The aim of this study was to investigate the effect of high MC on CLT air tightness properties compared to conventional factors such as indoor RH change, insulation type and façade orientation. An air permeability test was carried out on 12 different test walls with CLT as the airtight layer exposed to indoor environmental conditions. Test walls were constructed from CLT panels with two different initial moisture content (MC) values, ≈13%, and ≈26%, and in addition, different insulation materials were used in the wall assemblies: mineral wool, cellulose wool and polyisocyanurate (PIR) plates. Air leakage measurements were carried out at two different times, first in mid-autumn and second at the end of winter. Based on the results of this research it was concluded that high initial moisture content in the CLT panels significantly weakens the airtightness of the external wall. CLT exposure to water and wetting the CLT panels during construction carries more than a risk of moisture damage, and the planning and implementation of moisture safety in the construction of CLT buildings is therefore essential if the CLT is to be considered as an airtight layer. The effect of a change in indoor RH is small on the air permeability properties of the CLT wall. Therefore the 5-layer CLT panel can be considered and used as an air-tight layer in external wall construction as long as its initial low moisture content (about 13%) is maintained during both construction and service life.

Finite element analysis of alternative load paths to prevent disproportionate collapse in platform-type CLT floor systems

• Non-linear model for alternative load paths in cross-laminated timber floors. • Calibration to experimental results and parameter variations. • Collapse resistance by catenary and arching action, and the platform joint itself. • Collapse resistance increases at lower storeys and decreases with longer spans. • Catenary action requires novel connections, arching action requires thick panels. Multi-storey buildings require mitigation of consequences of unexpected or accidental events, to prevent disproportionate collapse after an initial damage. Cross-laminated timber (CLT) in platform-type construction is increasingly used for multi-storey buildings, however, the collapse behaviour and alternative load paths (ALPs) are not fully understood. A 3D non-linear component-based finite element model was developed for a platform-type CLT floor system to study the ALPs after an internal wall loss, in a pushdown analysis. The model, which accounted for connection failure, timber crushing and large displacements, was calibrated to experimental results and then adapted for boundary conditions corresponding to typical residential and office buildings. Subsequently, five parameters (floor span, connection type, vertical location of the floor, tying level, horizontal wall stiffness) were varied, to study their effects on the ALPs in 80 models. The results showed that three ALPs occurred, of which catenary action was the most dominant. Collapse resistance was mainly affected by the floor span, followed by the axial strength, stiffness and ductility of the floor-to-floor connection, the weight of the level above and the floor panel thickness. This study provides an approach to model ALPs in a platform-type CLT floor system to design disproportionate collapse resistant multi-storey CLT buildings.

Withdrawal Properties of Self-Tapping Screws in Japanese larch (Larix kaempferi (Lamb.) Carr.) Cross Laminated Timber

With the increasing popularity of cross-laminated timber (CLT) constructions around the world, there have been attempts to produce CLT using local wood species in different countries, such as Japanese larch (Larix kaempferi (Lamb.) Carr.) in China. Thus, the need to investigate the connection performance also increases to support the design and construction of CLT buildings using local wood species. In this study, the withdrawal properties of three different types of self-tapping screws (STS), with a diameter of 6 mm, 8 mm, and 11 mm, were tested with Japanese larch CLT. The results revealed that the withdrawal strength of STS increased with increasing density and effective length, but decreased with an increasing diameter. With a density increment of 0.05 g/cm3, the withdrawal strength increased by an average of 9.4%. With an effective length increment of 24 mm, the withdrawal strength increased by an average of 1.4%. An empirical regression model was adopted to predict the withdrawal strength of Japanese larch CLT based on the results, which can be used for potential engineering design of CLT connections using STS.

The influence of connection stiffness on the dynamic properties and seismic performance of tall cross-laminated timber buildings

It has become common practise to use Cross-laminated Timber (CLT) panels in shear wall applications in buildings around the world. Herein, the influence of the hold-downs, vertical and horizontal shear connections between the CLT panels on the period and stiffness of buildings was investigated. Two prototypes, 12-storey and 18-storey tall, of balloon-type construction were designed. To address the local seismicity of the chosen site in Vancouver, Canada, a range of crustal, subcrustal and subduction ground motions was selected for assessment as per the 2015 National Building Code of Canada. A total of 22 three-dimensional finite element building models with different combinations of connection properties were developed. Modal and time history analyses showed that the horizontal connections have the most significant impact on the overall building stiffness, resulting in up to 47% lager drifts when compared to the baseline model with an assumed rigid connection. The hold-down and vertical connection stiffnesses have lower impact on the dynamic properties and seismic performance of tall CLT buildings. The influence of connection stiffness was found to decrease with the increase of building height for this particular CLT tall building system with the connections designed as in this research.

Numerical Analysis on Global Serviceability Behaviours of Tall CLT Buildings to the Eurocodes and UK National Annexes

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.