Oregon and southwest Washington are poised as a manufacturing hub for the emerging Cross Laminated Timber (CLT) market in the United States. The region is bountiful with luscious forestland, a large percentage of which is designated as working forests. Thirty million acres of forest span across Oregon alone. As a value add product that has environmental and social co-benefits, CLT is economically competitive as a structural framing product for multi-story, even high-rise building construction: a market previously dominated by concrete and steel.
The research and outreach activities performed as part of this 2015-2017 study have played a vital role in continuing the advancement of the CLT market in Oregon & SW Washington. Eager regional stakeholders see CLT and other mass timber panel products as forest products capable of providing economic benefit to communities within our region that had grown around forest product industries.
A candidate CLT diaphragm analysis model approach is presented and evaluated as an engineering design tool motivated by the needs of seismic design in the United States. The modeling approach consists of explicitly modeling CLT panels as discrete orthotropic shell elements with connections between panels and connections from panels to structural framing modelled as two-point springs. The modeling approach has been compared to a developed CLT diaphragm design example based on U.S. standards showing the ability to obtain matching deflection results. The sensitivity of the deflection calculations to considering CLT panel-to-panel connection gap closure is investigated using a simple diaphragm example. The proposed modeling approach is also applied to the candidate floor diaphragm design for the Framework project, one of the two U.S. Tall Wood Building Prize Competition winners, currently under design. Observations from this effort are that the proposed method, while a more refined model than typically used during building design, shows promise to meet the needs of innovative CLT seismic designs where appropriate simpler diaphragm models are not available.
A candidate cross-laminated timber (CLT) diaphragm analysis model approach is presented and evaluated as an engineering design tool motivated by the needs of seismic design in the United States. the modeling approach consists of explicitly modeling CLT panels as discrete orthotropic shell elements with connections between panels and connections from panels to structural framing modeled as two-point springs. The modeling approach has been compared to a developed CLT diapragm design example based on the US standards showing the ability to obtain matching deflection results. The sensitivity of the deflection calculations considering CLT panel-to-panel connection gap closure is investigated using a simple diaphragm example. the proposed modeling approach is also applied to the candidate floor diaphragm design for the Framework project, a winner of the US Tall Wood Building Prize Competition, currently under design. Observations from this effort are that the proposed method, while a more refined model than typically used during building design, shows promise to meet the needs of innovative CLT seismic designs where appropriate simpler diapragm models are not available.
Ascent, a 25 story residential tower located in Milwaukee, WI (USA), will become the tallest timber building in the world upon completion. This paper discusses the project's structural system, permit process, groundbreaking project specific testing, and several of the challenges the team overcame, all of which open the door to future Mass Timber projects; particularly in the United States.
During the past few years, a relatively new technology has emerged in North America and changed the way professionals design and build wood structures: Cross-laminated Timber (CLT). CLT panels are manufactured in width ranging from 600 mm to 3 m. As such, fastening them together along their major strength axis is required in order to form a singular structural assembly resisting to in-plane and out-of-plane loading. Typical panel-to-panel joint details of CLT assemblies may consist of internal spline(s), single or double surface splines or half-lapped joints. These tightly fitted joint profiles should provide sufficient fire-resistance, but have yet to be properly evaluated for fire-resistance in CLT assemblies.
The experimental portion of the study consisted at conducting ten (10) intermediate-scale fire-resistance tests of CLT floor assemblies with four (4) types of panel-to-panel joints and three (3) CLT thicknesses. The data generated from the intermediate-scale fire tests were used to validate a finite element heat transfer model, a coupled thermal-structural model and a simplified design model. The latter is an easy-to-use design procedure for evaluating the fire integrity resistance of the four commonly-used CLT floor assemblies and could potentially be implemented into building codes and design standards. Based on the test data and models developed in this study, joint coefficient values were derived for the four (4) types of CLT panel-to-panel joint details. Joint coefficients are required when assessing the fire integrity of joints using simple design models, such as the one presented herein and inspired from Eurocode 5: Part 1-2.
The contribution of this study is to increase the knowledge of CLT exposed to fire and to facilitate its use in Canada and US by complementing current fire-resistance design methodologies of CLT assemblies, namely with respect to the fire integrity criterion. Being used as floor and wall assemblies, designers should be capable to accurately verify both the load-bearing and separating functions of CLT assemblies in accordance with fire-related provisions of the building codes, which are now feasible based on the findings of this study.
Cross-laminated timber (CLT) is a building system based on the use of massive, multi-layered solid wood panels. Although CLT as a construction system has been successful in Europe, only a handful of CLT projects have been built in the U.S. This manuscript presents the results from qualitative research, carried out with the objective of assessing the market potential and barriers to the adoption of CLT in the U.S. Insights from national and international experts were collected using semi-structured interviews. Topics included perceived benefits and disadvantages of CLT as a construction system, major barriers to its adoption in the U.S., and level of awareness about CLT among the architecture community.
One of the most recent innovations in Engineered Wood Products is Cross-Laminated Timber (CLT). The system is based on the use of multi-layered panels made from solid wood boards glued together, with the grain direction of successive layers placed at 90° angles. The cross-laminated configuration improves rigidity, dimensional stability, and mechanical properties. Structurally, CLT offers performance comparable to concrete or steel, with panels suitable for use as walls, floors, roofs, and other applications. While CLT as a construction material has been successful in Europe for the past 20 years, and more recently has made inroads in the Australian and Canadian markets, it is not yet readily available in the United States. To better understand the market potential for CLT in the U.S., this study aims to assess the level of awareness, perceptions and willingness to adopt the system by U.S. professionals. To achieve these objectives, (a) a series of 10 interviews were conducted to gather insights from national and international CLT experts; (b) a web-based survey to U.S. architecture firms was conducted to gather information about familiarity, perceptions, performance and likelihood to adopt the system in the near future; and (c) a multi-family residential building project was designed to explore the architectural possibilities of the material.This study identified that the use of wood, a natural and renewable material, was the main advantage of CLT. Another important benefit of CLT over traditional construction systems is the dramatically shorter on-site construction time needed. CLT is a prefabricated system, thus reducing labor requirements, on-site waste, and accidents, all of which translates into significant cost reductions. The most commonly cited disadvantages of CLT were its acoustic and vibration performance. From the study it was found that the level of awareness about CLT is low among U.S. architects. Building Code compatibility, availability in the domestic market and cost were mentioned as the main barriers to the implementation of the system in the U.S. Cross-Laminated Timber appears to be a cost-competitive alternative to concrete structures, especially for buildings over six stories high. Architects seem to be willing to adopt CLT for their near-future projects, especially for multi-family, commercial, and recreational buildings. Importantly, this willingness to adopt CLT was found to be positively correlated to the level of awareness with the system. Results show that diffusion of knowledge about CLT and the role of early adopters will be essential for the successful introduction of this new building technology into the U.S. market. The preliminary design created as part of this study allowed demonstrating the structural capabilities of CLT, by maximizing the spans between structural elements achieving open and fluid living spaces. CLT also enabled the design of wide terraces and the inclusion of window openings on outside walls without compromising the structural integrity of the CLT elements.
Southern Pine (SP) is one of the fastest growing softwood species in the Southern Forest of United States. With its high strength to weight ratio, SP becomes an ideal candidate for manufacturing engineered wood products such as cross laminated timber (CLT). Two batches of CLT panels were manufactured using visually graded SP lumbers in this study: pilot-scale panels in a laboratory setting and full-size panels in a manufacturing plant environment. The first batch of pilot-scale CLT panels was manufactured at Clemson University. The second batch of full-scale CLT panels (3m x 12.2m) was produced and CNC-sized by Structurlam in Penticton, Canada and shipped to Clemson University for testing. Four types of structural wood adhesives were selected in the panel production, namely Melamine Formaldehyde (MF), Phenol Resorcinol Formaldehyde (PRF), Polyurethane (PUR) and Emulsion Polymer Isocyanate (EPI). This paper presents the manufacturing process of SP CLT in a laboratory setting as well as structural performance verification of 3- ply SP CLT in terms of rolling shear and bending properties. The obtained performance data of 3-ply CLT in both major and minor strength directions is verified against PRG-320 Standard for Performance Rated Cross Laminated Timber. Tested results are presented and discussed.
Tall building (higher than 8 stories) construction using Cross laminated timber (CLT) is a relatively new trend for urban developments around the world. In the U.S., there is great interest in utilizing the potential of this new construction material. By analyzing a ten-story condominium building model constructed using building energy simulation program EnergyPlus, the energy efficiency of this emerging building type was evaluated and compared with a light metal frame building system (currently viable construction type for this height based on the U.S. building code). A sensitivity analysis was also conducted to study the impact of different weather and internal load conditions on building energy performances. It was concluded that efficiency of CLT envelope is high for heating energy savings, but its energy performance efficiency can be greatly affected by other factors including weather, internal loading, and HVAC control.
Prior research showed that inward moisture diffusion from absorptive claddings such as brick veneer, stucco, or manufactured stone veneer can be significant in wood-frame walls. The inward migration of moisture is greatest when the cladding is heated by the sun after being wetted by rain. The same phenomenon is likely to occur in CLT walls with these types of claddings. General guidance on CLT building envelope design was published in chapter 10 of the U.S. CLT Handbook, which cautions that inward diffusion of moisture from absorptive claddings could lead to moisture accumulation in CLT based on initial computer modeling predictions. Experimental measurements are needed to provide a stronger basis for design of CLT exterior walls.
The objectives of the project are to measure moisture conditions in CLT walls with absorptive claddings under exposure to simulated rain and sun and to identify design and construction practices that minimize the risk of moisture accumulation in different U.S. climates.