UBC Brock Commons tall building in Vancouver was the mass-timber superstructure which took only ten weeks from initiation to completion. Evaluating the possibility of designing the building using mass-timber cores could find out the benefit of laminated-veneer lumber cores and all-wood solutions. By Thomas Connolly, Cristiano Loss, Asif Iqbal and Thomas Tannert, University of Northern British Columbia
While wood, as the most important renewable structural material, has always been an essential part of the built environment, the importance of using sustainable construction materials is increasing with the rising global demand for housing and an increase in the understanding of the impacts our built environment has on climate change.
Multiple studies, for example, demonstrated the favourable environmental balance when using wood or its engineered wood product derivatives as building material when compared to concrete or steel.
As a consequence of the increasing push towards lower carbon footprint buildings, there is a growing effort worldwide to develop solutions for the key strategic market of mid-rise buildings and to prepare solutions for high-rise buildings.
During the last few decades, several innovative materials, connectors and systems have been introduced that have contributed to the tall-timber renaissance.
At the material level, the introduction of engineered mass-timber products which can be used for floors or walls such as Laminated Veneer Lumber (LVL) and Cross-laminated Timber (CLT) changed the industry significantly.
Multiple studies also provided guidance for designing CLT lateral load-resisting systems (LLRS). At the connector level, Self-tapping screws (STS) and glued-connections provide alternative methods that go beyond the limitations of traditional dowel-type connectors.
At the system level, novel hybrid solutions such as pre-stressed self-centring systems, steel moment frames with CLT infill panels, mass-timber balloon-frames with steel links, the timber-concrete jointed-frame concept and timber-concrete composite floors or timber-steel hybrid systems are attracting more attention due to their advantage in solving design problems.
Construction typologies with prefabricated elements made of engineered wood products are preferred since the process can be quickly industrialised and the on-site mounting time can be reduced; prefabricated elements can also be equipped with additional layers to complete the building envelope, including services and finishing
UBC Tall Wood Building
The ‘Tall Wood Building Demonstration Initiative’, launched in 2013 by Natural Resource Canada was implemented ‘To test the use of wood in larger and taller wood buildings’.
A call for expressions of interest to design and construct high-rise wood demonstration projects was posted with ‘The aim to link new scientific advances and data with technical expertise to showcase the application, practicality and environmental benefits of innovative wood-based structural solutions’.
The requirement was to use mass-timber products in structures of at least ten stories tall.
The University of British Columbia was successful with its proposal to design and build a wood-based student residence building, herein referred to as the UBC Tall Wood Building (TWB).
The 18-story building has a typical inter-story height of 2.8 m, a total building height of 58.5 m to the top of the elevator parapet, a story floor area of 840 m2 (15 m × 56 m) and provides 404 beds which are distributed as single-bed studios or four-bed units located on floors 2–18.
Due to the perceived increased fire risk, marketing a tall wood building to students and their families was seen as a challenge that was mitigated by a targeted media strategy.
Upon completion, ‘The project proved to be cost-competitive in the local marketplace, which was largely achieved by an integrated design team, real-time input from trades and structural discipline’.
In an effort to better understand the unique behaviour of this building moving forward, the structure is being monitored with accelerometers, moisture meters and string potentiometers.
The building belongs to Group C (residential) major occupancy which according to the governing British Columbia Building Code could only be of combustible construction if they are no more than six stories and 18 m high and have a maximum area of 1200 sqm.
For buildings to exceed these constraints, they must be of non-combustible construction, have floor assemblies with a fire-resistance rating no less than 2 h, be sprinklered throughout and use load-bearing elements that have a FRR no less than the supported assembly.
Given the code noncompliance, a performance-based approach was followed in the form of a Site-Specific Regulation (SSR) developed by the Authorities Having Jurisdiction (AHJ).
In this case, the AHJ was the Province of British Columbia’s Building Standards and Safety Branch (BSSB), authorized under the Building Standards and Safety Act. Obtaining approval was a challenge for this project, specifically considering the tight time-line.
Many design decisions were made to keep the project simple and ensure its approval. A key consideration was to get the AHJ involved and communicate the intent of the design solutions.
Given the noncompliance with the code, the SSR was developed by the AHJs. The outcome of the SSR was a regulation that is only applicable to the site that is concerned by the project.
Therefore, while setting an important precedent, this specific SSR does not allow for future tall wood buildings of similar design to be approved without a new process.
The SSR process involved both a structural review expert panel and a fire safety expert panel. The two panels met twice, with the final presentation being made by the design team in June 2015 to get approval by September 2015.
During the structural review, feedback was sought from the expert panels on whether the proposed design was too conservative, thus adding unnecessary cost to the project and which seismic load assumption to apply.
To mitigate the fire safety concern, it was decided to fully equip the building with sprinklers, including a reserve water tank to ensure sprinklers will work if the main water line is interrupted. Additionally, all structural timber components were encapsulated in gypsum wall-boards to comply with a 2 h FRR.
Building’s Structural System
The building’s structural system is a hybrid configuration and is supported by 2.8 × 2.8 × 0.7 m thick reinforced concrete spread footings.
Each core is supported by a 1.5 m raft slab that includes soil anchors and a 250 mm thick wall on a strip footing is located at the perimeter of the building’s foundation. The second floor, also made of concrete, acts as a transfer slab.
The building’s superstructure comprises of the concrete podium and cores, the mass timber columns and floors, as well as the steel roof structure.
The decision to go to a hybrid structural solution was taken early on in the project and was paramount in setting the direction for the project approval.
The concrete podium houses the ground level amenities and provides high clearances and large spans in the public spaces on the first floor of the building.
The 2nd floor acts as a transfer slab and takes the gravity load of the 17 stories above and allows the ground level structural grid to be independent from the grid of the wood structure.
The 450 mm thick cast-in-place reinforced concrete cores provide the rigidity to support lateral forces on the building as well as the vertical circulation, including the stairs and elevator shafts.
The primary framing consists of mass-timber columns (Parallam for the highest loaded zones and Glulam for the remainder) and a perimeter beam to support the building envelope. The typical column cross sections are 265 × 265 mm and 265 × 215 mm on the upper levels with the typical structural bay measuring 4 × 2.85 m.
Although there were only four different panel lengths, most panels on a single floor were unique due to the configuration of multiple openings. The secondary framing consists of continuous CLT panels, eliminating the need for beams and increasing the speed of erection.
One disadvantage of this solution was the fact that the columns had to be braced during construction, as they were free-standing until the top panel was in place.
The mass timber superstructure resulted in a building that is 7,648 tonnes lighter than an equivalent concrete building, therefore requiring smaller footings and resulting in lower costs.
However, in a high seismic zone such as Vancouver, this decrease in mass also resulted in lower resistance to overturning when compared to an equivalent concrete structure.
Some of the key challenges in designing the second level slab were the different reactions to lateral loads of the concrete and the wood components, the anchor bolt placement for the columns during construction and the coordination of the electrical and mechanical (e.g., water-lines) installations through the floors.
The connections between the CLT floor panels and the concrete core are another important interface, as it needs to account for the vertical shear transfer and the differential settlement between the wood and concrete structure, which was expected to be approx. 50 mm.
The solution was a steel ledger angle (203 × 152 × 13 LLH) welded to a 300 mm wide embedded plate cast in the core at every 1500 mm and then screwed to the CLT panels. The CLT floor panels were also connected at their edges using 140 mm × 25 mm cross-section plywood splines.
The construction process of the TWB displayed a significant difference in duration between the cores and superstructure. The construction of the cast-in-place reinforced concrete cores took fourteen weeks whereas the mass-timber superstructure, including the envelope, took just ten weeks.
The construction of the TWB with mass-timber cores could have further decreased the schedule leading to cost savings and environmental benefits.
Based on this idea, the research team was motivated to investigate the structural feasibility of the TWB with mass-timber cores instead of concrete cores. Specifically, the impact of substituting the core material from concrete to timber on the lateral response of the building was evaluated based on dynamic analyses.
As a secondary objective, a life cycle analysis was conducted to evaluate the environmental benefit of these all-wood solutions.
Feasibility Of Mass-timber Cores
For comparison purposes, the as-built solution with concrete cores has also been included and was labelled D-1. For layout 1, four alternative design solutions were studied: D-2 and D-3 considering CLT cores and D-4 and D-5 with LVL cores. Analogously, for layout 2, in design solutions labelled D-6 and D-7 CLT- and LVL-based LLRS were modelled, respectively.
The LLRS was designed considering two engineered wood products, CLT and LVL. In addition to the original solution with only cores as LLRS, a supplemental option with cores and mirrored ‘C-shaped’ walls, was considered in the analyses in order to provide additional lateral strength and stiffness to the building.
For D-3, D-5, D-6 and D-7, a pragmatic approach was adopted where the stiffness properties of the cores were reduced by 50 percent to account for the joints distributed along the borders of the timber panels and through the height of the wall. Such an approach is frequently applied in practice for preliminary design without prior knowledge of the exact connector layouts.
This simplified assumption of 50 percent reduction in core stiffness was taken as the lower bound while the monolithic core was taken as an upper bound for this case study on substituting the concrete cores in the TWB with mass-timber.
Modal Analysis (MA), Linear Dynamic Response Spectrum Analysis (LDRSA) and Dynamic Wind Analysis (DWA) were carried out for design options D1–D7.
The MA was executed to determine the natural mode shapes and free vibration frequencies of the building at different variations of the LLRS.
The objective of this work was to investigate the feasibility of using mass-timber cores (designed in CLT and LVL) and an option with additional ‘C-shaped’ mass-timber walls to replace the concrete cores of the UBC Tall Wood Building.
Based on the Finite Element, Modal, Linear Dynamic Response Spectrum, and Dynamic Wind, it can be concluded that:
(1) The side position of the cores induced a notable torsional component on the lateral response of the building, particularly evident in the case of mass-timber cores. The findings suggest the inclusion of additional shear walls to help reduce plan torsional sensitivity;
(2) Only design options D-4 and D-7 with LVL walls met all NBCC lateral design requirements; however, only the solution with additional ‘C-shaped’ walls appears feasible since it is unrealistic for timber panels to use completely rigid joints to build monolithic cores as assumed for D-4;
(3) The LVL design D-5, which had the stiffness of cores reduced by 50 percent to account for the joints between panels as per the authors simplified preliminary design assumption, exhibited seismic and wind performance close to code limits of 2.5 percent and 0.2 percent, respectively;
(4) Even for a model assuming monolithic cores, it seems unreasonable to pursue the solution with CLT for the TWB with the original layout. This recommendation is also conditioned by the commercially available CLT panel thickness, currently limited to 315 mm in Canada;
Life Cycle Analyses
A life-cycle assessment (LCA) analysis was conducted on the design options with mass-timber cores and additional ‘C-shaped’ shear walls and the original concrete building (D-1).
The environmental impact used the LCA software ‘Athena Impact Estimator for Buildings’, which provides an inventory of common construction assemblies and materials as well as their associated environmental impacts.
The software also considers the location of the construction by applying local energy grids to energy use and transportation methods and distances.
Several LCA approaches can be considered, with ‘Cradle-to-Gate’ or ‘Cradle-to-Grave’ the two most commonly used. The activities in a ‘Cradle-to-Gate’ assessment include production processes (material manufacturing including resource extraction and recycled content) and construction processes (installation; transportation).
In addition, a ‘Cradle-to-Grave’ assessment includes the impacts that occur after the building’s service life is complete, such as the effects of material recovery, recycling and carbon sequestration.
The environmental impact categories analysed were those included in LEED v4 building LCA: (i) global warming potential; (ii) stratospheric ozone depletion; (iii) acidification of land and water; (iv) eutrophication; and (v) depletion of non-renewable energy resources.
The material quantities for each layout are input as separate projects and the program outputs the comparisons of environmental impact categories. The material quantities were determined from the individual numerical models.
For the existing building, the core caps and staircases were constructed from 200 mm thick concrete, however, for the mass-timber layouts, these were replaced with 169 mm thick CLT panels.
The amount of reinforcing steel in the concrete was approximated using assumptions of two percent and one percent for the reinforcement in the walls and slabs, respectively, leading to approximately 28 cubic metres of steel with a mass of 218 tonnes.
When analysing the environmental impact of timber, including the effects that occur beyond the building life have a significant impact on the global warming potential.
This is because wood sequesters carbon from the atmosphere and effect which is taken into consideration at the end of its useful life in the building and the global warming potential for the mass-timber design is negative, that is, using mass-timber reverses the effects of global warming due to carbon sequestration.
For the concrete design, however, including the impacts beyond the building life increases the global warming potential. The Cradle-to-Grave impact assessment causes no significant change in the other four impact categories.
Analysing the results from Athena Impact Estimator for Buildings it is evident that the use of mass-timber cores in place of concrete has significant environmental benefits for the UBC TWB.
Regarding the Cradle-to-Gate impacts, the concrete core building has the highest impact in all but one category when compared to the mass-timber layouts.
Design D-3 with LVL walls has a 3 percent higher eutrophication potential when compared to D-1 with concrete, however, it is significantly lower in all other impact categories.
For the mass-timber layouts, the more material used, the higher the environmental impact of the building. Conversely, when looking at the effects beyond the building life (Cradle-to-Grave assessment), the larger volume of timber corresponds with a decrease in global warming potential could be found due to the carbon sequestration of the wood.
In conclusion, the LCA demonstrated that the use of a mass-timber LLRS for the UBC TWB would have significant environmental benefits in regard to the impact categories.