B.C. building insulates interior slabs from balconies with thermal breaks

by sadia_badhon | November 1, 2019 4:25 pm

By Dritan Topuzi, PhD, P.Eng., PMP, LEED AP

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At 52 storeys, 3 Civic Plaza is the third tallest building in British Columbia, and an anchor to the fastest growing metro centre in the province. It houses a luxury hotel, an urban campus for Kwantlen Polytechnic University (KPU), meeting rooms, a state-of-the-art fitness centre, a rooftop garden, and 348 residential condominium units, all accessed by an expansive central lobby linking the building’s many uses.

The design team, consisting of ZGF Cotter Architects, Fast + Epp structural engineers, and ITC Construction Group, was charged with “envisioning the civic plaza of the future, and determining a mix of uses appropriate to the existing city centre, while predicting and successfully responding to future requirements.”

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According to the architectural firm, the challenge was creating a cohesive design for structurally different uses. The solution comprised an external concrete shear wall, as opposed to the traditional concrete core, which provided more flexibility in designing the interior spaces, and more visual interest with the distinctive, guitar pick-shaped window openings marching down the façade.

Sustainable strategies for occupant comfort include in-slab radiant heating and natural ventilation systems relying on pressure differences between indoor and outside air to refresh the building without depending solely on mechanical systems. Close attention was paid to energy-saving measures at the building envelope to minimize costs and carbon footprint. Copious amounts of insulation were applied on the walls, roof, and other areas. Mineral wool was placed in the window wall assemblies. Foamed plastic insulation was sprayed on the cast-in-place concrete exterior walls. Foamed plastic insulation board was applied at the roofs, and cellulose insulation was sprayed at the slab soffits under occupied space in the underground parking area.

However, the 37 floors of condominium balconies presented a potential thermal bridging problem. If cast conventionally as cantilevered extensions of interior floor slabs, balconies would conduct heat away from interiors and dissipate it into cold environments like cooling fins, thereby wasting heat energy. Chilled floor slabs inside of the building envelope would cause discomfort for occupants. Condensation on cold adjacent interior surfaces would support mould growth, an increasing problem in today’s airtight, high-humidity buildings.

The design team studied options to prevent thermal bridging at the balconies, and ultimately decided to insulate them from the supporting slab by using structural thermal breaks.

Evaluating design options

The team had originally envisioned precast concrete balcony slabs extending from the sides of the building, but thermal isolation was needed to retain the energy indoors and not lose it through conduction to the exposed balcony slab.

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“We concluded structural thermal breaks would be a more constructible solution,” recounts the project’s engineer of record. “The owner and contractors overcame initial cost concerns once they understood how structural thermal breaks fit into the design strategy, which emphasized sustainability and human comfort.”

While relatively new to North America, structural thermal breaks have been used in Europe for more than 35 years.

“I have known about structural thermal breaks since 2007, when they first started appearing in North America,” the engineer explains. “We installed them on smaller projects before using them on 3 Civic Plaza, which is the largest project in British Columbia to date to incorporate structural thermal break technology.”

Design work for the project began in 2012 and ground was broken two years later by ITC Construction Group.

How structural thermal breaks function

The structural thermal breaks used in this project consist of a fabricated module of graphite-enhanced expanded polystyrene (EPS) insulation along with stainless steel rebar running through the material for tension and shear strength. The insulating block is approximately 98 per cent less conductive than concrete, and the stainless steel rebar is roughly one-third as conductive as carbon steel rebar, effectively reducing heat loss at the penetration.

The thermal break modules are positioned in line with other building envelope insulation, and tied into the rebar of the interior floor and exterior balcony slabs prior to casting in concrete. Thus the thermal break insulates the interior floor slab from the exterior balcony, significantly reducing heat loss, while transferring the loads imposed on the exposed slab back to the interior.

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Depending on the type of construction, structural thermal breaks may also be designed for steel-to-steel connections.

The rate of heat flow through a thermal bridge depends largely on the thermal conductivity of the material penetrating the envelope, with carbon steel exhibiting the greatest conductivity of the most common structural building materials, and stainless steel at one-third the thermal conductivity of carbon steel.

Structural thermal breaks for steel construction consist of an 80-mm (3-in.) thick proprietary insulating material, placed between stainless steel plates on each face with a stainless steel tube welded between them. The plates and tube impart the module with the requisite stiffness to transfer axial, shear, and bending forces, while minimizing or eliminating the risk of mould and corrosion by preventing interior surfaces from cooling and forming condensation.

The placement of the engineered unit between the endplates of steel beams minimizes the surface area where the loadbearing components of the thermal break cross the insulating layer and attach to the structural steel beams, satisfying both thermal and structural requirements. The use of stainless steel loadbearing components contributes to the insulating performance of the module, as a material with a conductivity of approximately two-thirds less than that of structural carbon steel.

As a loadbearing element, this thermal break is engineered to handle normal forces in addition to bending and vertical shear forces transferred by the steel beam. At minimum, two modules are stacked one atop the other, unless only low shear force is being transferred. Additional thermal breaks can be incorporated vertically and/or horizontally to handle higher loads at the connection points.

Team specifies four types of thermal breaks

The residential units in 3 Civic Plaza begin on the 15^th floor, with balconies ranging in size from 4.7 to 9.5 m² (51 to 102 sf).

To insulate and support them, the design team specified 1755 structural thermal break modules consisting of:

• modules providing flexural strength for cantilevered balconies (Figure 1);
• modules providing shear strength only for supported balconies (Figure 2); and
• insulation filler modules (Figure 3).

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Since British Columbia has a history of seismic activity, the design team also specified thermal break modules to provide lateral strength for earthquake load transfer in concrete balconies. They transfer horizontal shear parallel to the insulation layer as well as uplift forces. This type of break is used in addition to the linear connection models (Figures 1 and 2) mentioned earlier.

Totally, 1928 linear m (6325 ft) of structural thermal breaks were installed, equating to 5.5 m (18 ft) per balcony on average.

“All of the thermal breaks were placed on the slab formwork toward the end of laying the rebar for the slab. Concrete for the floor slabs and balconies was poured at the same time,” the engineer continues.

Co-ordination among the trades ensured the project’s success. Blair Marriott, senior superintendent of ITC Construction, says, “We organized site meetings with the sub-trades, particularly with shoring contractors to ensure they understood the proper load paths. The team commitment resulted in success.”

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Thermal performance

According to the thermal break manufacturer, their modules can reduce heat loss through the balcony by up to 90 per cent.

They also increase the warmth of interior floors adjacent to the balconies by up to 19 C (34 F), and prevent condensation and subsequent mould formation adjacent to cold balcony penetrations.

As a result, structural thermal breaks can help comply with increasingly stringent building codes and expected thermal comfort levels.

Multiple energy-saving measures minimize costs and carbon emissions

As further energy enhancements, the structural engineer says, “In-slab hydronic radiant heating/cooling tubing mats distribute heating and cooling to all of the units. The energy originates from the City of Surrey District Energy System, and is distributed through the building in a continuous ambient hydronic loop. Dedicated heat recovery chillers extract the energy from the loop and transfer it to holding tanks until it is called again to heat or cool the units.” The geothermal district energy system serves most of Surrey city downtown and is expanding.

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These energy measures at 3 Civic Plaza reduce capital and operating and maintenance costs and increase the building’s marketability.

The developers followed the 2012 British Columbia Building Code (BCBC) in the building’s design. The engineer adds, “The structural thermal breaks posed no problems in conforming with the code.”

Building codes in Canada set to require net-zero energy by 2030

Effective April 2019, the City of Surrey adopted the B.C. Energy Step Code, enacted to improve energy performance levels, and measure energy efficiency in all building systems including the airtightness of windows and walls, roofing, and mechanical equipment. The B.C. Energy Step Code will help reduce greenhouse gas (GHG) emissions and increase the energy performance and comfort of new buildings in Surrey. There is also a pan-Canadian effort by the National Energy Code of Canada for Buildings (NECB) to require all new buildings to be ‘net-zero energy ready’ by 2030.

This expected performance will require minimizing thermal bridging in structures, which would otherwise have a considerable impact.

[8]Dritan Topuzi, PhD, P.Eng., PMP, LEED AP, is the product manager of Schöck North America. He is also an adjunct faculty member at Norwich University, Vermont. He received his PhD from the University of Waterloo in 2015. Topuzi is a member of the American Concrete Institute (ACI) and the Canadian Standards Association (CSA). He can be reached  via e-mail at dritan.topuzi@schock-na.com[9].

Source URL: https://www.constructioncanada.net/b-c-building-insulates-interior-slabs-from-balconies-with-thermal-breaks/

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