Project Background

Canada’s Earth Tower: Balcony Mock-up

July 29, 2021

Executive Summary

Balconies are an important consideration in many jurisdictions for multi-unit residential buildings both as a functional extension of the living space and a key component in the overall building thermal performance. Also, they are essential design components that pose significant challenges for the successful execution of competing structural, fire and thermal requirements. These challenges are amplified in mass timber construction due to added new design challenges regarding structural connections, building enclosure detailing, and proper waterproofing membrane application. Overcoming these barriers and providing solutions to address them will improve the acceptance of timber as a viable structural system. For thermally broken balconies with mass timber components, the strategies developed as part of this objective are applicable as lessons learned to other projects and the focus of this research report.

The balcony mock-up design is based on Canada’s Earth Tower, a proposal to create a 38-storey, 31,494 square meters (340,000 square foot) high-performance mass timber residential mixed-use sustainable development targeting Passive House certification. This research project has selected a two-story balcony structure from Canada’s Earth Tower design proposal to use as a prototype to design and construct with a focus on investigating key research questions identified in the ‘Introduction‘ section of this report. While this is not a comprehensive comparison or examination, the balcony mock-up studied what it means to have a face-mounted, projecting and thermally broken timber balcony system to a glulam primary mass timber structure through a unitized CLT wall panel. The balcony structure was designed to utilize a steel and timber hybrid strut that acts in two ways: in compression under normal loading, and in tension to counteract wind uplift. The balcony decks are supported by two custom steel plates at the inner corners, and by the timber compression struts at the outer corners. Steel knife plate connections combined with compression struts work together to simplify the envelope detailing, and minimize the extent of the envelope penetration and thermal bridging, which can be further improved with the use of a thermally broken connector. The compression member prevents the balcony from buckling at the steel connections and reduces the size of the structural connection back to the building’s primary structure.

The balcony decks are prefabricated from the combination of a structural steel frame, exposed mass timber struts, dowel laminated timber (DLT), and cross-laminated timber (CLT) decks, and explore two types of waterproofing membranes as well as two types of finish decking material. Balcony decks come as a prefabricated modular unit to accelerate construction speed while minimizing construction waste and ensuring superior quality assurance and control. Unlike conventional balcony construction with a simple floor slab extension, the mock-up explores dissociation of the interior floor structure from the balcony through the steel connection details and allows for the height of the balcony to be designed to minimize the stepover height and to accommodate a zero-threshold condition.

Building code requirements and industry standards related to mass timber construction are continuously evolving and being updated. Overall, the balcony mock-up research and process demonstrates the viability of thermally broken balcony solutions in mass timber wood buildings.