Cross-laminated Timber Buildings Research Articles

Influence of the Floor Diaphragm on the Rocking Behavior of CLT Walls

In the design of cross-laminated timber (CLT) buildings in earthquake-prone areas, a crucial role in energy dissipation is played by the panel-to-panel joint. Such a connection, theoretically, could be designed for three different types of behavior: coupled, uncoupled, and monolithic. Coupled and uncoupled behaviors provide a certain amount of energy dissipation, whereas monolithic behavior does not. Currently, no specific design rules to attain a given condition are provided in any code. Furthermore, no information on the dependency of the wall behavior upon other variables, such as the out-of-plane stiffness of the floor diaphragms or the stiffness of other metal connections (e.g., hold-downs and angle brackets) can be found in the literature. In an attempt to fill in this gap, this paper presents the results of numerical analyses carried out using a commercial software package. In these analyses, the influence of the upper floor diaphragms on the rocking behavior of a two-panel wall assembly is investigated. Fully reversed displacement-controlled cyclic tests are simulated, while varying the geometrical properties (aspect ratio of the wall panels), mechanical properties (types and number of connectors used for the panel-to-panel, wall-to-foundation and wall-to-upper floor connections, out-of-plane stiffness of the floor panels), and gravity load applied on top of the assembly. The rocking capacity of the walls is investigated, together with displacements and global behavior of the assembly. The results obtained highlight the important role played by the stiffness of wall-to-floor diaphragm joints, whereas the out-of-plane flexural stiffness of the slab has a negligible effect on the overall response of the assembly.

Finite element analysis of alternative load paths in a platform-framed CLT building

Multi-storey cross-laminated timber (CLT) buildings are a comparatively recent construction type. Knowledge concerning the performance of CLT buildings regarding the prevention of disproportionate collapse after unforeseeable events (e.g. accidents or acts of terrorism) is not as refined as that for concrete and steel buildings. In particular, alternative load paths (ALPs) after the removal of a wall panel in platform-framed variants have not yet been studied in detail. The goal of this work was therefore to study ALPs in CLT buildings. An eight-storey bay of an existing building was evaluated by conducting a non-linear static pushdown analysis in a finite element analysis on three representative storeys. The analyses accounted for single fastener behaviour, timber crushing, friction, brittle failure and large deformations. The force–deformation behaviours elicited under the pushdown analyses were subsequently inserted in a simplified dynamic model to evaluate the transient response of the entire bay. Four ALPs were identified in this case – shear resistance in the floor panels, arching action of the walls, catenary action in the floor panels and hanging action from the roof. The dynamic analysis did not show a collapse, unless the inter-compartment stiffness was significantly reduced. The resistance mechanisms are described in this paper, which may provide information for improved building design.

Achieving airtightness and weather protection of CLT buildings

CLT buildings - a critical view concerning airtightness and weather protection during building time. Wood constructions are gaining a bigger and bigger share in the building industry as well in bigger constructions. Mainly CLT (Cross-Laminated-Timber) elements are due to it’s stability, fast building up time, easy installation and high share of prefabrication a key point for this success. In this presentation I will take a critical view on the building physical properties like diffusion resistance and airtightness. Especially according to the normal building material properties of wood like swelling and dwindling of timber. As well we will take a closer look to different glue technologies which are used on adhesive tapes to seal the CLT elements airtight. There are different technologies available in the market which offer different quality levels. Another point to solve is the temporary weather protection of timber building due to an enlarged building time. Especially when we have visible surface in the walls or in the ceiling they must be protected against rain water – there are new self sticking membranes available in the market which have proven their ability to protect ceilings against rain in a large number of realised constructions. The performance of these products will be presented in some already realised projects and we will take a closer look at the requirements and problems which have been found in these projects.

Use of cross laminated timber (CLT) in industrial buildings in Nordic climate – A case study

In recent years there has been a greater interest in developing new more sustainable solutions in the construction of buildings and in particular of large commercial and industrial buildings. This study analyses the feasibility and degree of sustainability of using cross laminated timber (CLT) as building material in industrial buildings in Nordic climate. An industrial building located in Eastern Norway, MAXBO Bjertnestangen, has been used as case study for the analysis. Two scenarios have been studied: i) the first analyses the existing industrial building (Scenario 1) built in steel, and ii) the second implies that the building components are replaced with CLT-elements (Scenario 2). For the structural analysis a commercial finite element method (FEM) code has been used and the results confirm that the CLT building achieves approximately equal mechanical and structural properties. For studying building physics in the two buildings, a commercial numerical simulation tool that couples hygrothermal with energy performance and uses the finite volume method (FVM) has been employed. The results show that it is possible to achieve a total energy saving of 3.3%, for the industrial building consisting of CLT-elements compared to the existing building. Furthermore, the life cycle analysis (LCA) shows that the total emission of CO2-eq is 16.7% lower in the CLT building, however the building’s construction costs are higher 13% compared to the existing industrial building. Finally, an optimized solution has been proposed in which sandwich panels in the roof are combined with CLT in rest of the building. In this case, the difference associated with costs is narrowed to 3.3%, while the difference in the total emission of CO2-eq stays still significant, i.e. 13.6%.

Identification and improvement of critical joints in CLT construction without weather protection

Wetting of timber structures during erection can have a harmful effect on their durability and could lead to adverse health effects. The probability of dampness related problems is very high when timber is exposed to free water. However, it is not always possible to implement full weather protection and thus there is a need for cost optimal solutions to increase the moisture safety of precipitation-exposed timber construction. In this study we observed the construction works and monitored the timber moisture content (MC) of a cross-laminated timber (CLT) building and proposed a set of activities and designed connection details that could help to avoid moisture ingress during the installation of CLT panels. Our findings showed that the most sensitive area to wetting is the end-grain on the CLT panel and the MC remained within critical limits in structures where drying was prohibited. Therefore, the most vulnerable section of the CLT structure is the foundation connection. We suggest using liquid-applied membrane coating on the cut edges of CLT panels to protect the end grain and to cover the horizontal CLT panels with self-adhesive membranes and vertical CLT panels with temporary clear weather protection foils.

Mechanics-based analytical models for balloon-type cross-laminated timber (CLT) shear walls under lateral loads

Besides platform-type construction method, a rarely investigated balloon-type method, in which the walls are continuous for the entire height of the building and the floor panels are attached to the walls at each floor, can also be used to construct Cross Laminated Timber (CLT) buildings. The balloon-type method endures several key advantages to the CLT buildings, such as reducing connections and avoiding accumulating compression perpendicular to grain in the floors, thus being a viable solution for high-rise buildings. In this paper, two mechanics-based analytical models (rigid- and elastic-base models) were developed for predicting the deflection and resistance of balloon-type CLT shear wall system. The developed analytical models incorporate the contribution of the wall bending and shear deformation, as well as contribution from the wall rocking and sliding, and the slip in the vertical joints between panels, on the overall wall deflection. The resistance of this system was assumed to be governed by the hold-downs, shear connectors, and vertical joints, if present. The analytical models were validated against the test results of four balloon-type CLT shear walls in two configurations tested under monotonic or cyclic load. The verified analytical models were used to investigate the influence of vertical load, wall aspect ratio, and vertical joint on the structural performance of the balloon-type CLT shear wall. The developed analytical models provide a mechanics-based approach for researchers and engineers to predict the structural performance of balloon-type CLT shear walls. The wall test results and the parameter analysis results gave a valuable insight into the structural behavior of the balloon-type CLT shear walls.

Methods for practice-oriented linear analysis in seismic design of Cross Laminated Timber buildings

In the context of innovative seismic-resistant structures, Cross Laminated Timber (CLT) construction holds an important position at a global level, supported by important full-scale tests and studies of the non-linear behaviour of these structures for severe events, demonstrating excellent performance. The present study aims to transfer current knowledge in the form of modelling approaches applicable to design engineers presenting tools and methodologies pertinent to the daily procedures of professional practice, thus relating to linear static and dynamic analyses. Two different approaches of structural modelling are presented, validated and compared; one that is based on a distributed representation of the connections between the structural panels and a second that is based on a discrete representation. Both models are validated by comparing the time-history responses with the full-scale shake-table test results of a 3-storey CLT structure under three earthquakes, showing in both cases a reliable estimate of the structural response. Finally, a parametric study for the damping coefficient of the structural response is presented and the typical value of 5% is evaluated. It is shown that values greater than 5% can be more applicable in order to compensate for the influence of the friction developed primarily between wall and floor panels on the global energy dissipation capacity of the structure.

Seismic shear and acceleration demands in multi-storey cross-laminated timber buildings

A realistic estimation of seismic shear demands is essential for the design and assessment of multi-storey buildings and for ensuring the activation of ductile failure modes during strong ground-motion. Likewise, the evaluation of seismic floor accelerations is fundamental to the appraisal of damage to non-structural elements and building contents. Given the relative novelty of tall timber buildings and their increasing popularity, a rigorous evaluation of their shear and acceleration demands is all the more critical and timely. For this purpose, this paper investigates the scaling of seismic shear and acceleration demands in multi-storey cross-laminated timber (CLT) buildings and its dependency on various structural properties. Special attention is given to the influence of the frequency content of the ground-motion. A set of 60 CLT buildings of varying heights representative of a wide range of structural configurations is subjected to a large dataset of 1656 real earthquake records. It is demonstrated that the mean period (Tm) of the ground-motion together with salient structural parameters such as building aspect ratio (λ), design force reduction factor (q) and panel subdivision (β) influence strongly the variation of base shear, storey shears and acceleration demands. Besides, robust regression models are used to assess and quantify the distribution of force and acceleration demands on CLT buildings. Finally, practical expressions for the estimation of base shears, inter-storey shears and peak floor accelerations are offered.

Comparative LCA of a concrete and steel apartment building and a cross laminated timber apartment building

In this paper an LCA is carried out on a concrete and steel apartment building and a cross laminated timber apartment building to compare the greenhouse gas (GHG) emissions from the two buildings. The buildings are built by Veidekke Entreprenør AS and they are almost identical except for the structural system and the number of floors. They are connected by an underground car park of reinforced concrete. The product stage (A1-A3), transport to the building site (A4) and operational energy use (B6) is examined. Results show that the cross laminated timber building has 25% lower GHG emission compared to the concrete and steel building when looking at the production stage, and 13% lower emissions when looking at all stages. The results also show that the material that contributes to the most GHG emissions is reinforced concrete, and that the underground car park has a high GHG emission because it consists of a lot of concrete. What is new in this paper is that there are two real buildings close together that can be compared to find out which has the lowest environmental impact. The paper is valuable for people designing environmentally friendly buildings with a low carbon footprint.

Energy retrofit analysis of cross-laminated timber residential buildings in Seoul, Korea: Insights from a case study of packages

Cross-laminated timber (CLT) is a potential option to achieve reductions in carbon emissions and energy consumption. CLT construction systems are environmentally friendly compared to more energy-intensive materials. In this study, the mid-rise apartments with CLT proposed by ASHRAE 90.1 were analyzed for energy consumption. First, the energy consumption of CLT buildings during heating and cooling was analyzed using four types of insulation. For CLT buildings, the airtightness was approximately 0.61 ac/h, (about 0.2 ac/h lower than that of the steel-frame buildings), suggesting that air-tight performance can reduce energy consumption. Energy-saving technology packages were applied to CLT buildings using rock wool insulation, which showed the highest total energy savings of 82.71 MWh, indicating economically advantageous hybrid-insulation. After the analysis of energy consumption according to the type of insulation, the energy retrofit was applied. Second, five energy-saving packages (including energy-saving, heating and cooling, and renewable energy technologies) were applied to two selected CLT buildings to further study energy consumption and energy efficiency. For CLT building using hybrid insulation, Package 3 reduced the total energy consumption by 14.14%, while for CLT building using rock wool insulation, Package 3 had the highest total energy consumption at 12.81%.

Environmental benefits of using hybrid CLT structure in midrise non-residential construction: An LCA based comparative case study in the U.S. Pacific Northwest

In this study, the cradle-to-gate environmental impact of a hybrid, mid-rise, cross-laminated timber (CLT) commercial building is evaluated and compared to that of a reinforced concrete building with similar functional characteristics. This study evaluates the embodied emissions and energy associated with building materials, manufacturing, and construction. Two alternative designs are considered for fire protection in the hybrid CLT building: 1) a ‘fireproofing design’, where gypsum wallboard is applied to the structural wood; and 2) a ‘charring design’, where two extra layers of CLT are added to the panel. The life cycle environmental impacts are assessed using TRACI 2.1 and the total primary energy is evaluated using the Cumulative Energy Demand impact method.Results show that an average of 26.5% reduction in the global warming potential is achieved in the hybrid CLT building compared to the concrete building, excluding biogenic carbon emissions. Except ozone depletion, where the difference in impact between scenarios is <1%, replacing fireproofing with charring is beneficial for all impact categories. The embodied energy assessment of the building types reveals that, on average, the total primary energy in the hybrid CLT buildings and concrete building are similar. However, the non-renewable energy (fossil-based) use in the hybrid CLT building is 8% lower compared to that of the concrete building. As compared to the concrete building, additional 1,556 tCO2e and 2,567 tCO2e are stored in the wood components of the building (long-term storage of biogenic carbon) in the scenario with fireproofing and with charring, respectively.

A case study on the construction of a CLT building without a preliminary roof

This paper focuses on cross-laminated timber (CLT) and how it is affected by the dynamic properties of moisture during installation in the cold climate of Estonia. The moisture safety principles are designed using a case study of comparable activities with 4D principles and on-site water content monitoring. On-site water content monitoring was done on the CLT elements that were installed and a parallel polygon specimen. Polygon testing was arranged with reduced size CLT elements subject to different conditions, with some exposed to the climate, some protected from precipitation, and some covered with film.&#x0D; The moisture content (MC) of the uncovered horizontal CLT element that was exposed to the climate reached over 25% after higher precipitation and the MC after prolonged direct exposure can reach up to 40% in a week, giving a clear signal of high risk areas for moisture safety. Installing a partly weather protected CLT element without a preliminary roof is a high-risk arrangement, but is essentially possible in a cold climate. Moisture safety pre-planning and a lean strategy must be applied with timber construction.

Monitoring Moisture Performance of Cross-Laminated Timber Building Elements during Construction

There are currently no standards regulating water management for mass timber elements during construction, little knowledge of impacts of moisture exposure (wetting and drying performance, dimensional stability, checking), and few precedents serving as guidelines for monitoring moisture response of mass timber. To address these gaps, a hygrothermal monitoring study was devised to track moisture performance of U.S. made cross laminated timber (CLT) and glulam at a three-story mass timber building. This paper discusses moisture measurements that were collected during the first six months of construction at a CLT rocking shear wall and a timber floor connection. Despite the limited number of structural systems monitored during construction, the distribution and number of sensors in these elements allow to draw some important conclusions. The data confirmed that moisture distribution and wetting/drying rates varied based on local conditions and details (aspect, coatings, connections, etc.), with measurements at an uncoated, north-facing area showing the highest moisture levels (reaching fiber saturation at multiple ply depths and locations). Most locations rarely exceeded 16% moisture content for more than a few months. Certain moisture-trapping details consistently showed higher moisture levels (i.e., above 16%) and poorer drying. Some interior plies continued to show slow increases in MC even after months of drying conditions. These observations suggest preventative approaches implementable in the design (e.g., avoiding moisture trapping details), during fabrication (e.g., localized coating), and construction (e.g., sequencing installation to minimize exposure and allow drying).

Paths of innovation and knowledge management in timber construction in North America: a focus on water control design strategies in CLT building enclosures

ABSTRACTMass timber construction systems, such as cross-laminated timber (CLT), which originated in Europe, are gaining acceptance within North America as a viable alternative to multi-story steel-frame or concrete structures. The purpose of this study is to analyze types of innovation in the design of the building enclosure precipitated by the introduction of CLT in the North American market. Additionally, knowledge management and transfer mechanisms supporting the use of CLT in the field are investigated through the analysis of case examples. A number of factors are identified that characterize the implementation of innovation in mass timber building enclosure, among which is the interplay of structural and enclosure design in those elements at higher risk of moisture intrusion. These system innovations required concurrent design collaboration dynamics and effective knowledge management among the different project’s stakeholders and partners. This analysis can contribute to a better understanding of the current state of practice in architectural design, as well as in the mechanisms supporting knowledge management and R&D initiative in an emerging construction market.

Thermal bridging analysis of connections in cross-laminated timber buildings based on ISO 10211

Cross-laminated timber (CLT), developed in Europe for the utilization of low-grade wood, is an engineered wood product (EWP) with high carbon storage and low carbon emissions. CLT wooden houses have advantages such as reduction of greenhouse gases, shortening of construction period through precast and modular construction, improved of insulation performance and airtight performance, the realization of the high-rise wooden house. In this study, ply-laminate cross-laminated timber (ply-lam CLT) was prepared using domestic plywood as the core material of CLT, and the thermal properties and heat transfer performance of ply-lam CLT were analyzed. The thermal conductivities of the ply-lam CLT in the radial, tangential, and longitudinal directions were 0.104 W/(m K), 0.111 W/(m K), and 0.122 W/(m K), respectively. The results of the heat transfer analysis through ISO 10211 indicated that the ply-lam CLT walls had lower thermal bridges than wood frame structure walls. Moreover, the ply-lam CLT walls with exterior insulation had lower thermal bridges. In addition, the self-tapping screw (STS) joining method on all connections was able to maintain excellent insulation performance because of low thermal transmittance coefficient. In addition, it should be noted that the STS applied from inside overlapped with the geometric thermal bridges.

Tie-force procedure for disproportionate collapse prevention of CLT platform-type construction

The existing tie-force requirements for disproportionate collapse prevention, as prescribed in the current European codes and American guidelines, are unpractical for the design of Cross-laminated timber (CLT) buildings with platform-type construction. An approach towards meeting structural integrity for such buildings is presented in this paper, specifically an improved procedure to quantify the minimum longitudinal, transverse and vertical tie-force requirements. This indirect approach for disproportionate collapse prevention solely applies linear-elastic static principles of engineering mechanics to satisfy force and moment equilibriums and ensure that CLT platform-type buildings have sufficient strength, stiffness, and ductility to bridge over damaged walls. Cantilever action of the walls and catenary action of the floor panels are identified as the main collapse-resistance mechanisms. Case-study building analyses show that typical seismic design connection detailing can be used to trigger cantilever action, whereas novel connection detailing is required to meet the axial force and deformation demands required for catenary action. The procedure presented herein can aid in designing mid-rise CLT platform-type buildings for disproportionate collapse prevention.

Assessment of Energy Saving Potential by Replacing Conventional Materials by Cross Laminated Timber (CLT)—A Case Study of Office Buildings in China

This research evaluates the operational heating and cooling energy consumption of cross-laminated timber (CLT) office buildings in China. The evaluations involve a comparison of the energy consumption of a reference RC structure and CLT system office buildings. Computational simulation results are based on IES-VE 2019 and show that the estimated heating energy saving ratio of CLT buildings in Harbin, Beijing, Shanghai, and Kunming to the reference structure are 11.97%, 22.11%, 30.94%, and 23.30% respectively. However, the CLT buildings consume more energy for cooling in the summer. The results of the research show significantly higher heating energy reductions for CLT buildings in the Cold Region and Severe Cold Regions of China. Thus, the application of the CLT system is better suited to northern China than southern China. The results of the research can be used in further assessment of the use of CLT systems in different climatic regions in China.

Dynamic properties of cross-laminated timber and timber truss building systems

Incorrect prediction of dynamic behaviour of tall buildings can lead to discomfort for humans. It is therefore important to understand the dynamic characteristics such as natural frequency, mode shape, damping and the influence they have on acceleration levels. The aim of this study is to compare two timber building types, cross laminated panels and post-and-beam with stabilising diagonals, through a parameter study applying modal analysis. Empirical formulae for predicting the natural frequency and mode shape are compared to measured and numerical results. Tall building assumptions such as ‘line-like’ behaviour and lumped mass at certain points were evaluated for both systems. The post-and-beam system showed a stiffer behaviour than the cross laminated system where more shear deformation occurred. Empirical formulae should be used with care until more data is collected. For the post-and-beam systems an assumption of linearity may be appropriate, but for cross laminated systems the approximation can give results on the unsafe side. Finally, the relationship between stiffness and mass for cross laminated timber systems and its effect on dynamic properties needs to be further investigated.

A proposal for the capacity-design at wall- and building-level in light-frame and cross-laminated timber buildings

Due to the brittle nature of wood material, the dissipation of seismic energy in timber structures is typically ensured through yielding of the mechanical connectors, where plastic deformations are developed in fasteners that have adequate ductility and cycle-fatigue strength. These structural components are denoted dissipative zones, whereas all other structural elements are assumed to behave elastically. Such elements are designated as non-dissipative zones and are designed with sufficient over-strength, to meet the requirements of capacity-based design (CD). Despite the general consensus that using the principle of CD could lead to safe buildings, where brittle failures are avoided, the applicability of such approach to light frame timber (LFT) and cross laminated timber (CLT) buildings has lacked analytical expressions that depend on the structural typology and failure mechanism. The current paper aims to fill this gap in knowledge by proposing an analytical approach that incorporates the CD philosophy to LFT and CLT buildings at the substructure (wall) and super-structure (building) levels. A simplified approach is adopted for structures with a low-to-medium energy dissipation capacity whereas a more rigorous approach is presented for structure with a high energy dissipation capacity. An experimental comparison and design example is included to present the applicability of the proposal.

Carbon balances for a low energy apartment building with different structural frame materials

We analyse the life cycle carbon implications of a building, with structural frame of prefabricated concrete, prefabricated modular timber or cross laminated timber (CLT) elements, designed to meet the Swedish passive house criteria. The analysis covers non-biogenic carbon flows related to the building alternatives, over an assumed life time of 80 years. The building alternatives are all modelled to have the same housing service and operation energy demand. Substitution factors, showing the efficiency of CO2 emissions reductions when wood alternatives are used instead of non-wood alternatives, are calculated for the CLT and modular alternatives with reference to the concrete alternative. The results show that the CLT and modular buildings give less carbon emissions to the atmosphere during production and when the buildings are demolished at the end-of-life. Moreover, the wood residues from the production and end-of-life activities for the timber alternatives far exceed that for the concrete alternative. The substitution factors differ slightly between the CLT and the modular alternatives, and are significantly lower when fossil gas is substituted by wood residues instead of fossil coal. In summary, the life cycle carbon emissions are significantly lower for the timber alternatives.