CASE STUDY: BROCK COMMONS TALLWOOD HOUSE, UNIVERSITY OF BRITISH COLUMBIA

Image courtesy of CADMakers | Photo courtesy of naturallywood.com

As part of Brock Commons’ design and preconstruction phase, a virtual design and construction model of the building was created. This VDC model was a comprehensive 3D model composed of all building elements, from the structure to interior finishes to the mechanical and electrical systems.

Vancouver, Canada

A Prefabricated Tall Wood Building: From Modeling to Construction

Located on a large forested peninsula on the west side of Vancouver, the University of British Columbia is at the forefront of the global movement to revitalize mass timber construction and be innovative in the use of engineered wood products in tall buildings. Among the large wood buildings already on campus are the Centre for Interactive Research on Sustainability, the Earth Sciences Building, and the Bioenergy Research and Demonstration Facility. The newest addition to the portfolio is the 54-meter-high (18-story) Brock Commons Tallwood House, featuring the first North American use of mass timber products in a residential high-rise.

Brock Commons is one of the University’s five high-rise, mixed-use, residential complexes that provide housing for students while acting as academic and recreational hubs for the campus community. While the overall design of the residences is similar, Brock Commons Tallwood House is unique in the use of a hybrid mass timber structure. The foundation, ground floor, second-floor slab, and stair/elevator cores are concrete, while the superstructure is composed of prefabricated CLT panel floor assemblies supported on GLT and parallel strand lumber (PSL) columns with steel connections. The ground floor of Brock Commons is enclosed by a glass curtain wall system. A three-layered CLT panel canopy with a double-folded, standing-seam metal roof provides coverage for pedestrians.

On the upper floors, the building envelope comprises a prefabricated panel system with an R-16 minimum thermal resistance. Each panel is composed of a structural steel stud system; fiberglass batt insulation; a wood-fiber, laminate-panel, rainscreen cladding system; preinstalled window assemblies; and a traditional SBS (styrene-butadiene-styrene) roof assembly on metal decking. The panels measure 8 meters wide (to span two structural grids) by 2.81 meters high (to span one story). The 127- by 127- by 13-millimeter steel perimeter angle, which is attached at each floor, supports the panels.

This prefabricated envelope system allowed the building to be rapidly enclosed as the structure is erected in order to protect the wood components from the weather. The prefabricated portion is composed of the rainscreen cladding system up to the steel studs. The vapor barrier, batt insulation, and the interior layer of drywall were applied on-site.

Due to the innovative nature of the Brock Commons Building, understanding how the building would be constructed—including prefabrication of components, trade sequencing, and required equipment—was critical to developing a realistic plan for delivering the project on time and on budget.

Design Phase: Virtual Design and Construction Modeling

As part of Brock Commons’ design and preconstruction phase, a virtual design and construction (VDC) model of the building was created. This VDC model was a comprehensive 3D model composed of all building elements, from the structure to interior finishes to the mechanical and electrical systems. CadMakers, the dedicated VDC consultant, worked from the consultants’ 2D drawings and 3D models concurrently with the development of the stamped construction documents. Every detail, including excavation, was included, along with precise geometries so that any construction process could be animated and any element or set of components could be exported in various formats. The VDC modeler was involved in the Brock Commons Building very early on and was tasked with collecting all relevant project information from the different team members in order to create a singular virtual model of the building with a very high level of detail.

VDC modeling is supported through building information modelling (BIM), which is a data-based project-delivery process centered on the collaborative, multidisciplinary development of an integrated digital model of the building and its components and systems. The model serves as a tool to support design decisions, coordination, and construction planning, and it can later be used to manage the building’s operation and maintenance as well as renovations and end-of-life decommissioning. While BIM tools are gaining in popularity around the globe, adoption and implementation are still limited in Canada.

Photo courtesy of naturallywood.com

The foundation, ground floor, second-floor slab, and stair/elevator cores are concrete, while the superstructure is composed of prefabricated cross-laminated timber panel floor assemblies supported on glue-laminated timber and parallel strand lumber columns with steel connections.

During the design phase, the virtual model was used primarily to assist in design development and decision making. The model was also used for coordination amongst the disciplines in terms of systems layouts, construction sequencing, and preparation for fabrication of certain building elements. The modelers worked in close collaboration with the design team, promptly incorporating design iterations and updates and notifying the team of any issues and conflicts that needed to be addressed, in order to ensure that the model was always accurate and detailed in its representation of the project. The VDC model also functioned as a tool for communicating with the construction trades prior to tender. It helped in describing the scope of work relative to the project as a whole and in demonstrating that the design, while innovative, was not complex or highly risky.

Preconstruction Phase: Full-Scale Mockup

During preconstruction, the VDC model was used to create a full-size, proof-of-concept mockup of part of two floors of the building. The mockup helped to validate the VDC model, as well as the design decisions, with the help of feedback from the trades. It also provided an opportunity to study constructability and installation feasibility, test communication procedures for prefabrication, select installation equipment, and identify options for efficiencies. These experiences and knowledge informed the construction planning, including sequencing and prefabrication of assembly packages. The VDC model was also the basis for the fabrication model that was used directly by the CNC machines for the CLT panel stress tests.

The full-size mockup of a portion of the building was built by the construction management and design-assist trades, using the virtual model as a template. The mockup is composed of a section of the ground and second floors, spanning three bays by three bays (approximately 12 by 12 meters). It includes the primary elements and connections that are in the final building, with the exception of the roof assembly (i.e., the cast-in-place concrete core wall and concrete ground floor, the CLT panel floor assembly, the PSL and GLT columns, the building envelope panel, and all the relevant connections). All of the engineered wood products were digitally fabricated using the VDC model.

The project team used the mockup to test and validate the viability of the design decisions and assess the constructability of the hybrid structural system components and connections between the columns and floor assemblies, between the CLT panels and concrete cores, and between the CLT panels and exterior envelope panels. The virtual design models and physical mockups were analyzed in advance of production to improve the accuracy of fabrication and the coordination of components and assemblies.

The mockup also provided an opportunity to test different finishes and cladding in real conditions, including the type of concrete topping and the wood sealer to be used to protect any exposed wood during construction. After viewing the panels at real scale, the university decided to change the exterior panel cladding from an originally specified metal cladding to the wood-fiber laminate cladding.

Preconstruction Phase: Construction Planning

The VDC modelers worked closely with the construction manager, Urban One Builders, and the rest of the team on the construction planning for Brock Commons. The schedule for the project was very aggressive, which, along with the small size of the site, placed importance on the coordination of the production, storage, delivery, and installation of all the building components. The planning was a highly collaborative process; it included input and feedback from all the specialized trades and personnel regarding the constructability and safety of building assemblies and the sequencing of specific activities.

As a key tool within the planning process, the VDC model was used to develop animated simulations that illustrated the sequences of installation and assembly outlined in the construction schedule.

Also known as time-based construction modeling, these animations were highly detailed and based on 1-hour increments. Animation was essentially a virtual construction process for the building, which allowed the construction manager and the trades to work through the installation procedures in 3D and confirm their feasibility prior to actual construction.

Some assumptions about the time required to complete tasks, for example regarding crane speeds, had to be made prior to the beginning of construction, and these were included in the original schedule. However, over the course of construction, the modelers recalibrated the simulations to reflect the actual durations of activities as they became known, thus improving the planning and scheduling of the remainder of the work.

The model was also employed in financial planning through the development of rapid budget prototyping. Due to the extensive detail available in the model, it was possible to create accurate material quantity estimates in real time. These budgets allowed different options to be analyzed and helped control the project’s finances.

Prefabrication and System Design Phase

Prefabrication of structural and envelope components was a key strategy in meeting the project’s timelines and budgets. The Brock Commons project used this type of construction approach—i.e., kit-of-parts prefabrication—to an extent not usually seen in high-rise residential building construction.

For the mass timber structure, models of the specific components were generated from the VDC model and validated by the architect and structural engineer then transferred directly to the mass timber supplier. This included the wood elements (CLT panels and PSL and GLT columns) as well as the steel connections and drag straps. The steel fabricator was subcontracted to the wood fabricator in order to streamline the process and achieve the tight tolerances. The supplier then modified the model to ensure that the tight tolerances were achieved while accommodating the mechanics of fabrication, such as saw thickness and drill-bit diameter, and then used the resulting fabrication model to operate the CNC machine to cut the pieces to size and drill the penetration holes. The tolerances for the mass timber components were ±2 millimeter, a requirement that would have been challenging to meet without the use of the VDC model.

The mechanical, electrical and plumbing (MEP) systems on this project were included in the VDC model. Typically, engineers design these systems and their specifications but leave the spatial layouts to the construction trades to decide at the site in conjunction with the construction manager or general contractor. For Brock Commons, the VDC modelers worked with the engineers and the trades to fully design and model the layout of the MEP systems within the building.

This level of detail was required for the prefabrication of the CLT panels so the cutouts for each system penetration could be made during fabrication rather than on-site. It also enabled the construction manager and trades to develop an accurate bill of materials and detail the sizes and dimensions of the system components to facilitate procurement, off-site preparation, and on-site assembly and installation. For example, the detailed modeling of the mechanical room enabled the cutting and welding of pieces to be done off-site, thus reducing the on-site construction time from the typical three to four months to less than one month.

Effects of the Model on the Construction Process

The detailed planning and sequencing, along with the prefabrication of major elements, enabled the project team to meet an aggressive schedule: The concrete foundation, ground floor, and second-floor transfer slabs were completed in three and a half months; the concrete stair and elevator cores were completed in three and a half months; and the mass timber structure, the steel roof, and the majority of the envelope were completed in about three months. Also, detailed planning, made possible by the VDC model, allowed construction processes to be standardized and streamlined.

Interdisciplinary cooperation among the project team during construction was critical. The VDC model was an important tool in facilitating communication between team members by giving everyone a common frame of reference. Construction planning, using the modeling of construction sequences, was a collaborative effort that helped to secure buy-in from the construction trades. The on-time performance required by the aggressive schedule was a challenge for some of the trades that were accustomed to more flexible timelines for on-site work. The trades’ input not only helped ensure a realistic work plan and schedule but also helped give them a sense of ownership in the project.

As part of the sequencing, the prefabricated components were sorted and loaded onto trucks to minimize on-site handling. Thus, when a truck reached the construction site, the components, could be craned out in the required order and directly installed on the building or positioned in staging areas on each floor. The model helped the construction team visualize the work, establish the loading and installation order for the components and facilitate the coordination between the different crews on-site.

The repetition of the structural and envelope design on each floor and the use of a standardized installation sequence also contributed to the trades’ learning efficiencies as the project progressed. For example, based on a productivity analysis of hook time, the first floor of CLT panels (floor three) took 7.3 crane hours to install, while the last floor (floor 18) took 3.1 crane hours. And the first floor of residential envelope cladding (floor two) took 12.7 crane hours to install, while floor 15 took only 4.4 crane hours.

Photo courtesy of naturallywood.com

Detailed planning and sequencing made possible by the VDC model, along with prefabrication of major elements, enabled the project team to meet an aggressive schedule and allowed construction processes to be standardized and streamlined.

Prefabrication and just-in-time delivery decreased the extent of on-site assembly time, which was somewhat complex because of the limited size of the construction site. These processes also improved the quality and precision of the components, productivity of fabrication, and overall safety for the trades because the detailed work and critical tasks could be completed in the controlled environment of the factory rather than on-site by workers at significant heights in variable conditions. Prefabrication also reduced waste, both on- and off-site because the specific sizes and dimensions of components were determined in advance by means of the VDC model and the components were made or cut to the tight specific specifications, with limited trial and error being necessary.