Timber has been considered as a promising building material because of its structural rigidity, environmental sustainability, and renewability nature. In Europe and Australia, timber materials have been used for many different types of construction such as residential, commercial, education, and industrial. However, in the U.S., the familiarity of timber products is gaining momentum. The construction practitioners are still reluctant to consider mass timber as a mainstream building material. A limited number of case study projects make it difficult for industry personnel to evaluate the actual construction feasibility of mass timber. As a result, a significant knowledge gap has been created that hindering the progress of mass timber material in the U.S. construction industry. To help solve the problem, this study aims to identify the existing awareness level among the U.S. building constructors regarding mass timber building materials. It further determines some of the major construction-related difficulties of mass timber buildings and recommendations overcome those difficulties to increase the acceptance of this material. The study performed a semi-structured questionnaire survey to carry out statistical analysis regarding mass timber building material. Analysis of descriptive statistics suggested that the level of awareness and involvement by the U.S. construction practitioners in mass timber building is still significantly low as 55% of the participants indicated no experience on mass timber building construction projects. Qualitative data analysis suggested that lack of experience in timber construction, poor coordination among the project parties, design-related difficulties, and high cost of mass timber panels are the biggest construction-related barriers to adopt this product. To overcome the existing difficulties, the study proposed an increasing number of timber building projects and manufacturing plants, effective early collaboration among the project parties, developing skilled workers, and a nation-wide promotion by the owners and the architects. The outcomes of this study will be helpful for the industry practitioners and the owners to adopt mass timber as a mainstream building material. The study will further increase the acceptance of this material in the U.S. construction industry.
The construction materials used in the building tall structures are responsible for extremely high carbon emissions. Therefore, to address this issue building designers are constantly looking at alternative sustainable construction materials. A new type of timber called MassTimber as a material for construction is now attracting the building designers because of its sustainability advantages. Mass-timber is an innovative type of engineered timber with improved structural properties making it suitable for the construction of tall and heavy structures. This paper is intended to study the performance of tall mass-timber buildings under the most severe dynamic loading conditions of India. Three models of mass-timber buildings are analyzed in ETABS under the seismic and wind loads according to the demands of most severe earthquake zone-V and one of the windiest regions at Bhuj, India. It is observed that the mass participation during seismic activities is considerably low and the wind loads are considerably higher than the seismic loads. It is concluded that with a suitable lateral load resisting structural system mass-timber buildings can perform adequately.
Long-span timber-concrete composite (TCC) floor systems have the potential to address the design challenges for conventional wooden floors in residential multi-storey timber frame buildings. The aim of this paper is to develop a design approach for long-span timber-concrete composite floor system of 6–9 m. A framework based on value-driven design approach has been developed for integration of results from graphical multi-objective optimisation, spreadsheet-based analysis, structural static and dynamic finite element analysis, and multi-criteria decision making. To verify the developed framework, a residential five-storey timber frame building as a case study has been studied. Optimal design includes optimised thickness of the concrete and optimised smeared stiffness of connectors for three different comfort classes A to C in descending order. TCC floor with span length 7.3 [m] belonging to comfort class A and TCC floor with span length 9.0 [m] belonging to comfort class C has been chosen as optimal solutions. The results indicate that proposed and innovative design approach is a promising tool for developers, architects and structural engineers when designing optimal long-span timber-concrete composite floor system.
We conducted a systematic literature search and meta-analysis of studies with side-by-side life cycle analysis comparisons of mid-rise buildings using mass timber and conventional, concrete and steel, building materials. Based on 18 comparisons across four continents, we found that substituting conventional building materials for mass timber reduces construction phase emissions by 69%, an average reduction of 216 kgCO2e/m2 of floor area. Studies included in our analysis were unanimous in showing emissions reductions when building with mass timber compared to conventional materials. Scaling-up low-carbon construction, assuming mass timber is substituted for conventional building materials in half of expected new urban construction, could provide as much as 9% of global emissions reduction needed to meet 2030 targets for keeping global warming below 1.5 °C. Realizing the climate mitigation potential of mass timber building could be accelerated by policy and private investment. Policy actions such as changing building codes, including mass timber in carbon offset crediting programs and setting building-sector-specific emissions reduction goals will remove barriers to and incentivize the adoption of mass timber. Private capital, as debt or equity investment, is poised to play a crucial role in financing mass timber building.
Society of Wood Science and Technology International Convention
The application of deconstructable connectors in timber-concrete composite (TCC) floors enables the possibility of disassembly and reuse of timber materials at the end of building’s life. This paper introduces the initial concept of a deconstructable TCC connector comprised of a self-tapping screw embedded in a plug made of rigid polyvinyl chloride and a level adjuster made of silicone rubber. This connection system is versatile and can be applied for prefabrication and in-situ concrete casting of TCC floors in both wet-dry and dry-dry systems. The paper presents the results of preliminary tests on the shear performance of four different configurations of the connector system in T-section glulam-concrete composites. The shear performance is compared to that of a permanent connector made with the same type of self-tapping screw. The failure modes observed are also analyzed to provide technical information for further optimization of the connector in the future.
This paper presents the results of long-term experiments performed on three timber-concrete composite (TCC) beams. An innovative fabricated steel plate connection system, which consists of screws and steel plates embedded in concrete slabs, was adopted in the TCC beam specimens. The adopted shear connection can provide dry-type connection for TCC beams. Steel plates were embedded in concrete slabs while the concrete slab was constructed in factories. The timber beam and concrete slab can be assembled together using screws at the construction site. In this experimental programme, the beam specimens were subjected to constant loading for 613 days in indoor uncontrolled environments. The influence of long-term loading levels and the number of shear connections on the long-term performance of TCC beams was investigated and discussed. The mid-span deflection, timber strain, and interface relative slip at the positions of both connections and beam-ends were recorded throughout the long-term tests. It was found the long-term deflection of the TCC beam increased by approximately 60% while the long-term loads were doubled. Under the influence of the variable temperature and humidity, the TCC specimens with 8 shear connections showed slighter fluctuations compared with the TCC beam with 6 shear connections. In the 613-day observation period, the maximum deflection increment recorded was 6.56 mm for the specimen with eight shear connections and 20% loading level. A rheological model consisting of two Kelvin bodies was employed to fit the curves of creep coefficients. The final deflections predicted of all specimens at the end of 50-year service life were 2.1~2.7 times the initial deflections caused by the applied loads. All beam specimens showed relative small increments in mid-span deflection, strain and relative slip over time without any degradations, demonstrating the excellent long-term performance of TCC beams using the innovative steel plate connection system, which is also easily fabricated.
As the construction industry shifts towards sustainability and owners seek to construct buildings that are sustainable - built from natural and renewable materials, and pleasing for their occupants to work in - mass timber is becoming the popular alternative to traditional steel and concrete buildings. An abundance of information is available on mass timber products and their properties and applications, but little information on the process of actually building a mass timber project. This report seeks to extend practical knowledge on building with mass timber. In order to accomplish this, this research will highlight specific differences and challenges related to building with mass timber; create general guidelines and recommendations for contractors tasked with building a mass timber project; and identify new areas of research. Through interviews with two commercial contractors who have built mass timber projects in the California Bay Area, specific challenges have been identified. These challenges include longer project duration; increased preconstruction time and complexity; difficulties getting timely plan approvals; differing design and material procurement methods; necessity of MEP coordination at the beginning of the jobs; unique transportation, storage, and handling requirements; and different installation procedures and requirements.
Construction of eco-friendly high-rise buildings using cross-laminated timber (CLT)-concrete composite (CCC) slabs is increasing. CLT and concrete, which are major component materials of the CCC slab, are significantly affected by moisture. In particular, the moisture content of concrete in the production process affects the quality of both materials. In this study, the effects of the wet construction method on CLT and concrete component materials are examined by monitoring the behavior of the CCC slab during curing time (28 d) and by evaluating the quality of the concrete and CLT after curing. When manufacturing the CCC using the wet construction method, moisture penetration from the concrete into the CLT during the curing time is suppressed by the shear bonding between the concrete and the CLT when an adhesive is used. This minimizes the effect of the moisture on both component materials, consequently yielding uniform compressive strength to the concrete after curing and preventing the deterioration of the CLT’s delamination performance. Therefore, the shear bonding method using an adhesive is expected to minimize the quality deterioration observed in concrete and CLT after curing.
Timber use is becoming more appealing in the recent years especially ‘exposed timber’; however, the information available on the performance of engineered timber after fire is limited. This paper explores the performance of timber elements exposed to well defined thermal boundary conditions and examines the extent of adhesive degradation after heating. Two different types of timber beams are explored; ‘glued laminated timber’ (Glulam) and ‘laminated veneer lumber’ (LVL). A subset of beams was exposed to radiant heat as per a modified ASTM E1321 heating procedure. An additional subset of beams also had an area of their cross-section carved away, equivalent to the char depth of the heated beams. The carved beams allow for the identification of degradation beyond the char layer, as theoretically both the carved and charred beams would have the same effective cross-sectional area. All beams were mechanically loaded to failure using a four-point loading setup. While the current allowance for degradation beyond the char layer is considered to be 7 mm for exposure times of 20 minutes and greater , the results herein indicate that for bending members this layer extends to at least a minimum of 11.7 mm for LVL and 12.3 mm for Glulam. The aim of this paper is to assess the post-fire performance of Glulam and LVL through looking at strength loss due to adhesive degradation, which may contribute towards enabling tall and unencapsulated engineered timber buildings.
This paper describes selected observations, measurements, and analysis from a series of large-scale experiments on cross-laminated timber (CLT) slabs that were exposed to fire from below, using four different heating scenarios, with a sustained mechanical loading of 6.3 kN m per metre width of slab. The deflection response and in-depth timber temperatures are used to compare the experimental response against a relatively simple structural fire model to assess the load bearing capacity of CLT elements in fire, including during the decay phase of natural fires. It is demonstrated that the ventilation conditions in experiments with a fixed fuel load are important in achieving burnout of the contents before structural collapse occurs. A mechanics-based structural fire model is shown to provide reasonably accurate predictions of structural failure (or lack thereof) for the experiments presented herein. The results confirm the importance of the ventilation conditions on the fire dynamics, burning duration, and the achievement of functional fire safety objectives (i.e. maintaining stability and compartmentation), in compartments with exposed CLT.
Recent interests in adopting sustainable materials and developments in construction technology have created a trend of aiming for greater heights with timber buildings. With the increased height these buildings are subjected to higher level of lateral load demand. A common and efficient way to increase capacity is to use shearwalls, which can resist significant part of the load on the structures. Prefabricated mass timber panels such as those made of Cross-Laminated Timber (CLT) can be used to form the shearwalls. But due to relatively low stiffness value of timber it is often difficult to keep the maximum drifts within acceptable limit prescribed by building codes. It becomes necessary to either increase wall sizes to beyond available panel dimensions or use multiple or groups of walls spread over different locations over the floor plan. Both of the options are problematic from the economic and functional point of view. One possible alternative is to adopt a Hybrid system, using Steel Plate Shear Walls (SPSW) with timber moment frames. The SPSW has much higher stiffness and combined with timber frames it can reduce overall building drifts significantly. Frames with prefabricated timber members have considerable lateral load capacity. For structures located in seismic regions the system possesses excellent energy dissipation ability with combination of ductile SPSW and yielding elements within the frames. This paper investigates combination of SPSW with timber frames for seismic applications. Numerical model of the system has been developed to examine the interaction between the frames and shear walls under extreme lateral load conditions. Arrangements of different geometries of frames and shear walls are evaluated to determine their compatibility and efficiency in sharing lateral loads. Recommendations are presented for optimum solutions as well as practical limits of applications.
The study presented herein proposes a retrofit method aimed at reducing the seismic vulnerability of reinforced concrete (RC) frame structures. The method consists in the replacement of the existing masonry infills with timber structural panels made of Cross Laminated Timber (CLT) fixed to the concrete frame by using a timber subframe and dissipative metal dowel-type fasteners. The first part of the research was carried out by performing nonlinear static analyses of finiteelement (FE) models of bare, masonry infilled and retrofitted single-storey single-bay frames. A large number of configurations was analysed considering different original conditions (e.g. in terms of geometrical characteristics, mechanical properties and loading) and several retrofit implementation approaches. Special attention was paid to the improvement of the seismic response of the beam-column joints, that represent a well-known structural vulnerability of existing concrete frame-buildings. The analysis results permitted to define a set of “general rules” to guide the implementation of the retrofit method depending on the characteristics of the original structure. Using these design rules, the proposed solution was then applied to the FE models of three case-study buildings, located in Italy and built in the period from 1950 to 1990. By comparing the seismic response of the pre- and post-intervention structures, it was observed that the proposed system could significantly improve the structural behaviour of the buildings, favouring the development of ductile mechanisms and reducing the vulnerability of the beam-column joints.
This research was conducted to discover how the U.S. building construction and forest products sectors could benefit from the development of tall, cross-laminated (CLT) and mass timber buildings. Barriers that may restrict such development were also investigated. The primary benefits were discovered to be eco-performance and job creation. Code restrictions and material performance misconceptions were found to be the largest obstacles. Case studies of Treet, Tamedia, and the WIDC were conducted to demonstrate the benefits of tall wood buildings and the various paths around potential barriers. Opportunities for tall wood buildings in the U.S. are also discussed. This research discovered that a tall wood movement is gathering momentum in the U.S. To fully realize this potential, accurate information regarding the use of wood and the performance capacities of mass timber systems needs to be disseminated. Co-operation between academia and industry will also be necessary.
In recent years, timber has been considered as an alternative source of building material because of its sustainability and design efficiency. However, the cost competitiveness of timber buildings is still under study due to the lack of available cost information. This paper presents a comprehensive cost comparative analysis of a mass timber building mainly developed with cross-laminated timber (CLT). The actual construction cost of the project is compared with the modeled cost of the same building designed as a concrete option. The result shows that the construction cost of timber building is 6.43% higher than the modeled concrete building. The study further investigated the change orders associated with the project and found that the total cost of change orders contributed 5.62% to the final construction cost of mass timber building. The study is helpful to provide insight into the construction cost of typical mass timber buildings. It also can be used as a guide for the project owners to make decisions regarding their initial investments on a mass timber project.
Due to the increasing environmental awareness, the transition pace to renewable materials has increased, and the use of timber in construction is no exception. However, using timber in high rise building applications comes with structural challenges, e.g dynamic issues originating from timber being lightweight compared to conventional building materials. Some of the structural challenges with timber can be resolved by the implementation of Timber Concrete Composites (TCC), which increases the effective bending stiffness by adding a concrete layer connected to the underlying timber floor. Furthermore, the higher self-weight of concrete contributes to improved dynamic performance.
Despite the fact that the TCC floor is a versatile and quite common structural design solution in Europe, the TCC knowledge in the Swedish construction industry is limited. The main scope of the thesis is to raise this knowledge of TCC by studying the structural behavior and develop applicable design methods. Both analytical design methods and FE-modelling are addressed. The content is limited to TCC floors with a 5-layer Cross-Laminated Timber (CLT) section, with use of notches or screws as shear connectors.
In CLT design, the Gamma method is commonly used and applicable to a CLT layup up to 5 layers. This method can, by a slight modification, be applicable for TCC sections with a 5-layer CLT as well. The concrete layer on top is regarded as an additional longitudinal layer, flexibly connected to the CLT section. The Equivalent gamma method and the Extended gamma method are two modified versions of the conventional Gamma method, valid for TCC floors with 5-layer CLT sections. Each method determines the effective bending stiffness accurately, compared to FE-modelling and laboratory test results. The Extended gamma method has a more solid theoretical base compared to the Equivalent gamma method, and is considered the recommended design method. The simplified methodology of the Equivalent gamma method is theoretically questionable, hence its recommended use is for preliminary calculations only.
The following concluding remarks can be drawn from the analysis of the structural behavior of TCC floors:
- The shear connectors should be concentrated to areas of high shear flow, i.e. close to support, for optimal structural performance.
- An increased ratio of timber in the longitudinal, load-bearing direction of the CLT section increases the effective bending stiffness of the TCC.
- The concrete layer increases the effective bending stiffness due to the high Young's modulus. However, the high density of concrete entails a thin concrete layer thickness to achieve a light-weight and structural efficient TCC system, and the decisive optimisation factor is the ratio of mass-to-effective bending stiffness, m/EI.
Biological durability issues in cross-laminated timber (CLT) have been majorly ignored in North America because of the European origin of the material and careful construction practices in Europe. However, the risks of fungal and insect attacks are increased by the North American climatic conditions and lack of job-site measures to keep the material dry. The methods to evaluate durability in solid timber are inadequate for use in mass timber (MT) for a number of reasons, such as moisture variation and size being critical issues. This study therefore proposes a method, which is suitable to evaluate the strength of MT assemblies that are exposed to fungal degradation. The objective of the study was to explore a controlled method for assessing the effects of wetting and subsequent fungal attack on the behavior of CLT connections. Two different methods were used to create fungal attack on CLT assemblies. Although they were both successful, one was cumbersome, left room for many errors, and was not as efficient as the other. In addition, a standardized method to evaluate and characterize key performance metric for the connections is presented.
Cross laminated timber (CLT), as a structural plate-like timber product, has been established as a load bearing product for walls, floor and roof elements. In a bending situation due to the transverse shear flexibility of the crossing layers, the warping of the cross section follows a zigzag pattern which should be considered in the calculation model. The Refined Zigzag Theory (RZT) can fulfill this requirement in a very simple and efficient way. The RZT, founded in 2007 by A. Tessler (NASA Langley Research Center), M. Di Sciuva and M. Gherlone (Politecnico Torino) is a very robust and accurate analysis tool, which can handle the typical zigag warping of the cross section by introducing only one additional kinematic degree of freedom in case of plane beams and two more in case of biaxial bending of plates. Thus, the RZT-kinematics is able to reflect the specific and local stress behaviour near concentrated loads in combination with a warping constraint, while most other theories do not. A comparison is made with different methods of calculation, as the modified Gamma-method, the Shear Analogy method (SA) and the First Order Shear Deformation Theory (FSDT). For a test example of a two-span continuous beam, an error estimation concerning the maximum bending stress is presented depending on the slenderness L/h and the width of contact area at the intermediate support. A stability investigation shows that FSDT provides sufficiently accurate results if the ratio of bending and shear stiffness is in a range as stated in the test example. It is shown that by a simple modification in the determination of the zigzag function, the scope can be extended to beams with arbitrary non-rectangular cross section. This generalization step considerably improves the possibilities for the application of RZT. Furthermore, beam structures with interlayer slip can easily be treated. So the RZT is very well suited to analyze all kinds, of shear-elastic structural element like CLT-plate, timber-concrete composite structure or doweled beam in an accurate and unified way.
The report aims to investigate norms, standards, guidelines and experience within the industry for how to design CLT (cross-laminated timber) regarding vibrations induced from humans. The following is being researched, ISO137, KL-trähandboken, Eurocode 5 and a new unpublished working draft of Eurocode 5 final working draft, Canadian CLT handbook and Cross-laminated timber structural design according to Eurocode from Austria.The conclusion is that the literature for CLT is non-existent in the current Eurocode 5 which only addresses timber floors with joists, however the new Eurocode draft suggests an update to include CLT which is similar to the norm CLT from Austria.The report contains a calculation part in which an analysis is conducted for a real project with calculations based on Eurocode 5 and the Eurocode 5 final working draft, the design tool Calculatis and FEM program RFEM. The calculations are compiled and evaluated.The calculation results show differences between the different standards. The natural frequencies are typically the same. The biggest difference is between the accelerations which is in direct relation to the modal mass, and the modal mass differs a lot between the calculations. It is understandable how Eurocode 5 final draft and RFEM calculate the modal mass, but not so for Calculatis as it doesn’t show any calculations in the technical documentation.There is a difference of the modal mass between Eurocode 5 final draft and RFEM, likely because EK5 calculate the modal mass for a rectangular floor simply supported at two or four sides. Whereas the RFEM model is not strictly rectangular nor is it simply supported everywhere, instead there are beams in some places. This suggests that caution should be regarded in calculations where floor structures have been simplified.
Wood-based mass-panels (WBMP) are emerging as an attractive construction product for large-scale residential and commercial construction. Australia is following the lead of Europe and North America with several recent projects being completed using predominately cross-laminated timber panels (CLT). These sawn timber-based panels offer some key advantages to the construction and sawmilling industry. However, veneer-based mass-panel (VBMP) systems could offer additional benefits including the more efficient use of the available forest resources to produce WBMPs that have equivalent to superior performance to CLT. Research to confirm the expected technical viability of veneer-based systems is required. VBMPs could provide a valuable contribution, alongside CLT, to the Australian timber products market.
This paper examines the performance and apparent temperature in cross-laminated timber (CLT) school buildings. The research presents empirical data on the performance and provides the first set of data on apparent temperature in CLT school buildings. The development is in the New England area of the Northeast of the US. The investigation was conducted in the summertime. The principal aim of the investigation is to evaluate the performance, occupants’ comfort, apparent temperature, and other thermal indices concurrently in CLT school buildings. The research intends to understand if occupants of CLT school buildings are susceptible to thermal stress in summer and assess whether apparent temperatures are consistent with sensation. The study also discusses other indices, practical implications, and applications of the outcomes. To achieve the research aim, the study considered the field measurements of variables. Occupants’ comfort is accessed using the PMV and adaptive methods of various comfort standards. During the survey, the development was occupied from 8am-6pm and partly operated from 7pm-7am. The mean temperatures during the occupied and non-occupied periods varied from 22.1°C-22.4°C. The overall RH was 59.2%. The PMV range and sensation showed the occupants were comfortable. Approximately 80% of the users were satisfied with the thermal environment. The temperatures were within the acceptable bands of ASHRAE-55, CIBSE TM52, and EN16798-1 thermal comfort models. The results showed that the apparent temperatures are consistent with the outcomes of the sensation at different periods. The mean indices ranged from 18.8°C-23.5°C. The study recommends that further research should be conducted on occupants’ comfort and heat indices in school buildings during the first few hours of occupation to understand changes that occupants can make to remove unwanted heat from the thermal environment. The study also recommends that various designers should consider heat stress analyses along with thermal comfort assessment at the design phase to determine possible interventions to improve the thermal environment of schools and other buildings.