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Structural Strength of 8-Storey CLT Wood Innovation Design Centre

Case Study by Chris van Daalen
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Structural Strength of 8-Storey CLT Wood Innovation Design Centre
photo credit: Michael Green Architecture
Materials
Structure
Mass Timber Construction
Cross Laminated Timber
Prince George, British Columbia
Trudy Rotgans, MAIBC, MCIP, BC Building Safety and Standards Branch
Univ. of N. British Columbia
Commercial-Institution
51882
Michael Green, Michael Green Arch.
Chad Kaldal, PCL Constructors Westcoast
Eric Karsch, Equilibrium Inc.
10/2013

Abstract

Located in Prince George, British Columbia, the six-storey, eight-level Wood Innovation Design Centre was the tallest multi-use wood building in North America at 29.5 meters (97 feet) tall.  It's structural frame is built from cross laminated timber (CLT) panels and other laminated wood members to demonstrate that tall wood buildings can be structurally sound, cost-effective and beautiful. The Provincial Government’s goal was to push the limits of innovation beyond what was normally allowed by BC Building Codes.

Permitting Process

For this project, the BC Provincial Government adopted a Site-Specific Regulation (SSR) specifically authorizing this one building to be built as a demonstration project, and directing the use of Alternative Solutions to meet structural building codes.  Keys to success were a clear vision for the project, and the use of “alternative solutions” including modelling and analysis to ensure structural building code compliance, while entrusting its execution to an experienced design team, a rigorous peer review process and a trusted general contractor. The project team used several innovations to satisfy structural loads and procedures requirements including:  the use of vertical cross-laminated timber (CLT) elements in the building’s structural core (mechanical, elevator and stair shafts) for lateral stability; the use of double layer CLT panels for the structural floor section; the use of load-bearing end-grain-to-end-grain glulam columns running continuously from foundation to roof; and finally high strength proprietary connectors, to achieve structural performance.

Code RequirementCompliance Path
Building Structural Loads and Procedures, Sec 4.1-4.3 Alternative Solutions compliance path authorized by Site-Specific Regulation SSR M.203 that specified building height and occupancy specifically for this building; substitute language and authority for alternative solutions adopted by signature of Minister.
Building Structural Design requirements, Sec. 4.1.8.9(1) and 4.3.1.4(1) Design team developed an alternative solution conforming to new section 4.1.8.9(6) in SSR M.203 that specified an R-factor of 2.0 and an Overstrength factor of 1.5 for construction with cross laminated timber panels with ductile connections.  Their solution was approved based on Engineer’s Letter of Assurance and verification of structural performance modeling.

Compliance Details:

Interior wood structureEncouraged by extensive design, engineering, and technical research developed by the Canadian Wood Council and FP Innovations (two wood industry associations), the BC Provincial Government wanted to push the limits of innovation with a building that went beyond the existing building code.  The current code allows no more than four stories for non-residential wood buildings, so they amended the BC Building code with a Site-Specific Regulation to increase the allowable building height to 30m.  The amendment also added a new section 4.1.8.9(6) for construction with cross laminated timber panels with ductile connections, specifying an R-factor of 2.0 and an Overstrength factor of 1.5.

Assembling Elevator Core
photo credits: Michael Green Architecture
Lateral load resistance in the building comes mainly from the structural core of elevator and stair shafts walls, built from 12m long CLT panels erected vertically, and connected end-to-end to create continuous shear walls.  These were anchored to the foundation with a combination of shear brackets and hold-down brackets, joined to the panels by the ductile HSK-System.  This proprietary system uses a perforated metal plate inserted into pre-cut cavities filled with an epoxy adhesive, fusing the two materials in a rigid bond that is nonetheless flexible under seismic loads, i.e. during an earthquake the steel plates would begin to yield.

The structural floor system consists of two layers of parallel CLT panels bonded together, with alternating space between them where electrical, plumbing and fire protection services can be hidden.  Upper and lower floor panels are joined using the HSK connection system with perforated plates inserted into a continuous vertical kerf filled with epoxy, creating a very strong and elastic composite structure.
Column Joints.
The vertical structure is a glulam post and beam system, with vertical columns superimposed one on top of the other with metal connectors at each floor and horizontal beams attached with aluminum Pitzl connectors.  CLT and glulam are similar engineered wood products, but unlike CLT, in glulam all wood grain runs parallel to the length of the beam.  Eliminating cross-grain in the vertical section of the building is critical because wood under a crushing load is 10 times stronger against force applied parallel-to-grain than when it’s exerted perpendicular-to-grain.  Thus, constructing vertical glulam columns running continuously from foundation to roof with end-grain-to-end-grain connections virtually eliminates vertical shrinkage of the building.
Diagram of floor assembly

10 different joint types including wall-to-floor connections, vertical joints between walls, horizontal joints between floor panels, wall to roof joints, etc. are embedded and concealed within the timber elements to protect them from fire damage; testing by Intertek Labs demonstrated that as designed, fire-susceptible steel connectors would be protected by fire-resistant wood elements, passing with a significant safety factor.
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The building is owned by the BC Provincial Government with the first 3 floors reserved for the University of Northern British Columbia with demonstration, classroom and laboratory space for a Masters in Wood Engineering program, and the upper floors for government office space and wood industry associations.
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Resources and Documents

"Wood Innovation Design Center:  A Technical Case Study" by Woodworks, Canadian Wood Council.  See pp. 12-15 for Fire Safety info

Going Against the Grain: Mass Timber Skyscrapers by Nadine M. Post, Dec. 14, 2015.  Engineering News Record.
Cross-Laminated Timber (CLT) Handbook, Canadian Edition by FPInnovations.  January 2011, The Case for Tall Wood Buildings, by Michael Green.  May 11, 2011

Motivations:

Developed as a demonstration project to show that tall wood buildings are cost-effective, efficient and beautiful, the 6 storey, 29.5 meter (97 foot) tall Wood Innovation Design Center was designed with a simple, replicable building form to show CLT could be an economical and code-compliant way to build.  By doing so, the design team hope to encourage other architects, engineers and developers to consider building with mass timber as an alternate to steel and concrete.  In fact, architect Michael Green and engineer Eric Karsch performed analysis showing the basic structural concept for WIDC could be used for buildings up to 20 and 30 stories in height with little modification, in a report they published in 2012 .

Compared to traditional steel and concrete structures, building with wood offers economic and environmental benefits as a locally-grown and manufactured renewable and easily recyclable natural resource that sequesters atmospheric carbon throughout the products’ life-cycle.

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Design / Build Process

Design of this building was a unique challenge because of all the unknowns, overcome through a fully collaborative approach that involved not just all the trades, professionals and testing labs, but also regulators as partners in the design-build process.  With clear design objectives at the outset, and consistent modelling from fabrication to installation, the Building Safety and Standards was able to accept a level of risk by sharing responsibility with an experienced design team, peer review group and trusted general contractor to ensure compliance with all fire safety requirements.

Lessons Learned

Although product testing of CLT and other laminated wood products has been going on for some time, no prescriptive codes have been developed in Canada or the US.  This necessitates the use of "alternative solutions" on each project, using a fully engineered design to show compliance with structural requirements.  Canadian research going back over a decade is not accepted by US Code Officials, so many universities, engineering and architecture firms are collaborating on the research and code development that eventually will lead to prescriptive codes for CLT structural strength. Model codes are not expected to be adopted until the 2021 International Building Code, at least 5 years away.  Until then, every tall wood building will be a code innovation, building upon the collective science and experience of product developers and building designers in both Nations.

Project Contacts
Designer: Michael Green, Principal Michael Green Architecture (604) 336-4770 Designer: Eric Karsch, Principal Equilibrium Consulting, Inc (604) 730-1422 Consultant: Barry Thorson B.R. Thorson Consulting Ltd. (604) 929-8520
Plan Reviewer: Keith Calder Jensen Hughes Engineering
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