Construction of buildings with wooden frames higher than two stories has been permitted in Sweden since 1994. As construction of multi-story buildings with wooden frames is relatively new, people in the construction industry are more likely to construct these buildings with concrete frames. The current research evaluates the factors influencing the choice of wooden frames for construction of multi-story buildings in Sweden. The purpose of this study is to explain which advantages and disadvantages construction companies in Sweden consider with wooden construction and to highlight the factors for why multi-story buildings are built with wood to a lesser extent than with other materials. The main goal is to investigate what factors or assumptions construction companies base their decisions on, and whether experience and competence in wooden frames for construction of multi-story buildings are considered in short supply in Sweden today. The chosen method for this research is a descriptive survey study with a qualitative and quantitative approach. The survey is based on respondents from five leading building companies in Sweden with regard to the companies’ revenue. The respondents had either previous experience in constructing multi-story buildings with wooden frames, experienced respondents (ERs), or no experience, unexperienced respondents (UERs). 63% of the respondents were ERs, while 37% of them were UERs. It is resulted that the respondents think there is a lack of competence and experience in wooden frames for construction of multi-story buildings in Sweden. Factors that have the greatest impact on decisions to construct with wooden frames are positive environmental and climatic aspects as well as production advantages. Factors that are considered as major obstacles to construct with wooden frames are cost, acoustics, and moisture problems.
The building industry is a large contributor to greenhouse gas (GHG) emissions and a vast consumer of natural resources. It is estimated that, in the next 40 years, around 415 Gt of CO2 will be released as a result of global construction activities. Therefore, improvements in construction technologies are essential to reduce GHG emissions and thereby attain national and international goals to mitigate climate change. Cross-laminated timber (CLT) has emerged as an innovative alternative material to steel/concrete in building construction, given its relatively low carbon footprint, not to mention its high strength-to-weight ratio, simple installation, and aesthetic features. CLT is a structural composite panel product developed in the early 1990s, and the contemporary generation of CLT buildings are yet to reach the end of their service life. Accordingly, there has been growing interest to understand and optimize the performance of CLT in building construction. In view of that, this paper presents an overview on the feasibility of using CLT in buildings from a life-cycle assessment (LCA) standpoint. The authors performed a brief review on LCA studies conducted in the past decade pertaining to the carbon footprint of CLT buildings. On average, the findings of these studies revealed about 40% reduction in carbon footprint when using CLT in lieu of conventional construction materials (steel/concrete) for multi-story buildings. Furthermore, the paper explores the challenges associated with conducting LCA on CLT buildings, identifies the gaps in knowledge, and outlines directions for future research.
Braced timber frames (BTFs) are one of the most efficient structural systems to resist lateral loads induced by earthquakes or high winds. Although BTFs are implemented as a system in the National Building Code of Canada (NBCC), no design guidelines currently exist in CSA O86. That not only leaves these efficient systems out of reach of designers, but also puts them in danger of being eliminated from NBCC. The main objective of this project is to generate the technical information needed for development of design guidelines for BTFs as a lateral load resisting system in CSA O86. The seismic performance of 30 BTFs with riveted connections was studied last year by conducting nonlinear dynamic analysis; and also 15 glulam brace specimens using bolted connections were tested under cyclic loading.
In the second year of the project, a relationship between the connection and system ductility of BTFs was derived based on engineering principles. The proposed relationship was verified against the nonlinear pushover analysis results of single- and multi-storey BTFs with various building heights. The influence of the connection ductility, the stiffness ratio, and the number of tiers and storeys on the system ductility of BTFs was investigated using the verified relationship. The minimum connection ductility for different categories (moderately ductile and limited ductility) of BTFs was estimated.
Cross-laminated Timber (CLT) is gaining popularity in Australasia as a building material for multi-storey structures. For multi-storey timber buildings located in seismic areas, designing strong but ductile hold-downs for CLT shear walls can be challenging and requires careful structural connection design. In this study, dowelled connections in New Zealand Douglas-Fir (D.Fir) CLT with inserted steel plates were experimentally investigated as a solution for hold-downs in multi-storey timber buildings. The dowel group spacing was varied for CLT3 (3-ply, 135 mm thick), CLT5 (5-ply, 175 mm thick) and CLT7 (7-ply, 275 mm thick) D.Fir CLT to investigate the spacing impact on ductility of the hold-down connections under both monotonic and quasi-static cyclic loading. These results were also compared with past similar testing of dowelled connections in 5-ply (150 mm) Radiata Pine CLT. A total of 12 monotonic and 36 quasi-static cyclic tests were carried out and it was observed that increased dowel spacing increases ductility with similar strength when compared to past more dense dowel spacing tests. Furthermore, to deter the onset of tension perpendicular to grain brittle failure, fully threaded screws and nuts were added to the dowelled connection and the impact of this is discussed.
This paper begins with an overview of the state of the art in the design of multi-story mass timber structures and their lateral systems in low to moderate seismic regions. Boston, MA has been chosen as the location for a feasibility analysis of 8-, 12-, and 18- story mass timber structures. These building prototypes are used to compare the structural and environmental efficiencies and tradeoffs of replacing conventional concrete cores with mass timber braced frames and steel-timber hybrid frames. The lateral resistance of prototype configurations is evaluated through numerical analyses to understand in more detail the characteristics of an efficient mass timber lateral system. Finding an optimal timber gravity system configuration is followed by examining lateral resistance of the prototypes. The resulting designs demonstrate a practical approach to assist designers in selecting a lateral system during the early stages of conceptual design. This research was conducted in parallel with a related study for implementation of mass timber in affordable housing in Boston, enabling a comparison between composite systems and all-timber structures.
This dissertation introduced a new hybrid building system in which the post-tensioned rocking CLT panels were coupled with traditional light-frame wood constructions. The initial study showed excellent self-centering and energy dissipation capacities of the hybrid walls. A finite element model was developed for rocking walls/columns and allowed to perform a global-analysis for structures using rocking elements. The model was validated by 3-D FEM models in ABAQUS and MATLAB, and an experimental test. A direct displacement-based design check procedure was proposed for CLT-LiFS buildings and illustrated by designing a six-story CLT-LiFS building. The FEM model for rocking elements was utilized to implement 2-D non-linear static analysis and non-linear time history analysis to check the design. After that, pseudo-dynamic hybrid simulation tests at three hazard levels were conducted for the six-story CLT-LiFS building, in which a two-story CLT-LiFS building was built and served as the physical substructure of the test. The tests showed minor damages and very small residual drifts to the building, even after MCE level. Finally, an optimization problem was developed for mid-rise to tall CLT-LiFS buildings using evolutionary algorithm. The variables including number of stories, hybrid wall length, CLT panel width, number of CLT panels and cable arrangement were considered so that the buildings were optimized in cost while still met their technical performance expectations. The normalized cost (for frame work) of optimum building configurations were in the range of 15.88 – 21.44 USD/sft. The study also archived several figures that will help select the building configuration in the design process of CLT-LiFS building.
The timber industry has experienced in the last decades a relevant increase in terms of high performance buildings. Despite these advancements and the favorable properties of building with wood, the traditional position of "choosing by costs" still finds wooden building as more expensive than concrete or steel ones. In order to be competitive in the market against these two main building materials and meet the expectations of modern and large-volume wood based constructions, new improvements based on standardization and prefabricated systems have to be implemented. At the same time a full collaborative work between all the participants on a project is needed to redefine and optimize the construction and design processes through sharing specific and detailed information and extended know-how at a very early project stage. Through this approach, the high potential of combining off-site construction, Building Information Modeling (BIM) as a work methodology and lean management practices will be investigated, involving architects, engineers, BIM users in the timber industry, timber manufactures, contractors and all the stakeholders with the aim of reaching the most effective and productive design and construction process in multi-story timber buildings.
16th European Conference on Earthquake Engineering
The NHERI TallWood project is a U.S. National Science Foundation-funded four-year research project focusing on the development of a resilient tall wood building design philosophy. One of the first major tasks within the project was to test a full-scale two-story mass timber building at the largest shake table in the U.S., the NHERI at UCSD’s outdoor shake table facility, to study the dynamic behaviour of a mass timber building with a resilient rocking wall system. The specimen consisted of two coupled two-story tall post-tensioned cross laminated timber rocking walls surrounded by mass timber gravity frames simulating a realistic portion of a building floor plan at full scale. Diaphragms consisted of bare CLT at the first floor level and concrete-topped, composite CLT at the roof. The specimen was subjected to ground motions scaled to three intensity levels representing frequent, design basis, and maximum considered earthquakes. In this paper, the design and implementation of this test program is summarized. The performance of the full building system under these different levels of seismic intensity is presented.
This paper presents a numerical study of the influence of varying story strength on the seismic performance of multi-story wood-frame shear wall buildings. In the prior FEMA P695 studies of these buildings, the non-simulated collapse limit-state was exceeded primarily in the first story . This observation raised interest in quantifying the influence of varying strength from story to story on seismic response. In this study, four different distributions of strength are used as bounding cases. The Parabolic strength distribution (1) is based upon the ELF method in ASCE 7 and assigns lateral forces to each level based on weight and story height. The Triangular strength distribution (2) is based upon the simplified procedure in ASCE 7 and distributes lateral forces based on the seismic weight at each level. The Constant strength distribution (3) assumes the same shear wall design was used on all levels. The Baseline strength distribution (4) is from actual designs provided in the FEMA P695 wood-frame example and represents the practical implementation of the ELF method for designed shear walls. The FEMA P695 methodology, which quantifies seismic performance via adjusted collapse margin ratios, is employed in this study. The analytical models include P-Delta effects and utilize the 10-parameter CASHEW hysteresis model. Based on the analysis of a subset of index models from the FEMA P695 wood-frame example, it is observed that the Parabolic strength distribution, which facilitates dissipation of energy along the entire height of the building, has larger adjusted collapse margin ratios (lower collapse risk) than other strength distributions studied and reduces occurrence of concentrated inelastic deformations in a single story from the onset of an applied lateral force.
Steel-timber hybrid structural systems offer a modern solution for building multi-story structures with more environmentally-friendly features. This paper presents a comprehensive seismic performance assessment for a kind of multi-story steel-timber hybrid structure. In such a hybrid structure, steel moment resisting frames are infilled with prefabricated light wood frame shear walls to serve as the lateral load resisting system (LLRS). In this paper, drift-based performance objectives under various seismic hazard levels were proposed based on experimental observations. Then, a numerical model of the hybrid structure considering damage accumulation and stiffness degradation was developed and verified by experimental results, and nonlinear time-history analyses were conducted to establish a database of seismic responses. The numerical results further serve as a technical basis for estimating the structure's fundamental period and evaluating post-yielding behavior and failure probabilities of the hybrid structure under various seismic hazard levels. A load sharing parameter was defined to describe the wall-frame lateral force distribution, and a formula was proposed and calibrated by the time-history analytical results to estimate the load sharing parameter. Moreover, earthquake-induced non-structural damage and residual deformation were also evaluated, showing that if designed properly, desirable seismic performance with acceptable repair effort can be obtained for the proposed steel-timber hybrid structural system.