Installation of structural steel is nearly complete at the University of Toronto Daniels Faculty of Architecture, Landscape + Design.  The addition to the heritage building at 1 Spadina Crescent is primarily a concrete structure, except for its steel-framed roof and mechanical penthouse.  The roof is a signature architectural feature of the project:  it spans over 110 feet (34m) between two service cores, across a column-less hall that will house the Faculty’s graduate design studios when the building opens later this year.  A series of 3 cantilever trusses form the geometry for a modified “sawtooth,” composed of clerestory windows that will admit high-quality northern light into the studios below.  The roof will eventually support a ceiling composed of gypsum board, forming a subtle ruled surface between the clerestories as the steel members angle upwards toward the roof’s “spine”.

Above:  Rendered view of the raked seating area situated below the level 03 graduate studio hall. Roof clerestory windows above will admit diffuse north light. (rendering by NADAAA)

Above: View of the level 03 graduate studio hall, facing north. Sructural steel erection is nearly complete, and the triangular shape of the clerestory apertures are evident. (Photo credit: Richard Lee of NADAAA)

The “spine” follows the central axis of the building, which shares a significant urban alignment with Spadina Avenue and the adjacent heritage building.

Above: Rendered view of the column-less graduate studio hall. (Rendering by NADAAA)

The bow-tie configuration of the steel trusses allow for 3 discrete clerestory windows.  The two larger windows are oriented north, however, the smaller keystone apertures along the central  “spine” face south and filter direct sun with a honeycomb glazing insert.  The trusses themselves do not comprise a true span, in fact, they are 3 distinct structural components: two cantilevers and a link beam.  As such, the trusses function like a cantilever bridge such as the Forth Bridge in Scotland (see also illustration below), or the Confederation Bridge which connects New Brunswick with Prince Edward Island.  Cantilever bridges are characterized by greater structural depth aligned with the vertical supports, tapering to thin cantilevers at opposite ends between two adjacent spans.  These twin cantilevers establish an equilibrium about the vertical support, balancing equal and opposite overturning forces.

Above:  Axonometric view of bow-tie Truss #1  (Courtesy Entuitive Corporation)

Above: “Living model illustrating the principle of the Forth Bridge”

At the Daniels, however, there is only a single span.  This means that the vertical supports — the service cores — must function to anchor the bridge both vertically and laterally.  In order to resist the overturning moment of the cantilever, the design of the cores themselves must be assymetric, analogous to a contrapposto to establish counter-balance.  This is accomplished by a deeper footing below the core walls, configuration of reinforcing bar, as well as the use of the concrete floor diaphragms below to brace against the cores.

Above Left: Donatello’s David, in contrapposto — analogous to service core design supporting the roof cantilevers.

Above Right: Full building structural axonometric view of the Daniels Faculty Addition.  (Courtesy Entuitive Corporation)

Above: Construction webcam view of the Daniels Faculty Addition, looking south.  Snapshot taken during crane pick of truss #2.   (Click the image for a live view).

Above:  View from the northeast corner of the addition, looking down to the steel fabrication and staging area, prior to truss crane picks.  (Photo credit:  Michael Lukachko, Adamson Associates Architects)

Above:  View from the penthouse level looking east, as connections between truss #1 and core walls are completed.  (Photo Credit:  Michael Lukachko, Adamson Associates Architects)

Above:  View from Spadina Ave. looking south, with all 3 bow-tie trusses in place.  (Photo Credit:  Rich Lee, NADAAA)

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Courtesy of the Daniels Faculty. Photo by Peter MacCallum.

Installation of Bubbledeck has commenced at 1 Spadina Crescent, as part of ongoing construction activities for the University of Toronto Daniels Faculty of Architecture, Landscape + Design.   Bubbledeck is proprietary type of biaxial voided slab, an innovative structural concrete system.  It is structurally similar to a conventional concrete waffle slab, with a few key differences that I shall expand upon below.

1)  Put structure where you need it

The goal of bubbledeck is to produce a floor slab that can achieve longer spans with a continuous, “flat plate”  underside.  This is achieved by reducing concrete weight at the center of the slab’s section, where it is least helpful structurally.  Bubbledeck slabs resemble many “I” beam shapes stitched together when viewed in section cut in either orthogonal direction.  Concrete mass is concentrated at top and bottom of the section, where compressive and tensile bending stresses are greatest.  Mass at the center (or “neutral axis”) of a conventional flat plate slab is essentially free of bending stress, and voided slabs offset much of this “dead weight” with cast-in hollow plastic spheres or “bubbles.”  A “cage” of reinforcing bars help to keep the spheres in place during the forming process, and also help stitch the slab’s top and bottom together.

Bubbledeck panels placed on site. Bubbles have been omitted around the zone of influence of columns, where punching shear forces are high, and additional reinforcing is required. (photo courtesy Bubbledeck North America)

Rejected plastic kayaks (normally sold at Walmart) were recycled for the batch of “bubbles” at the Daniels Faculty project

2)  Precasting a smooth, flat ceiling

As in a conventional waffle slab, the tension component of bending stress is handled by a grid of steel reinforcing bars at the bottom of the slab section.  Concrete is cast around the bottom bars mainly to provide cover, but in the case of bubbledeck, also provides a smooth, architectural ceiling.  This ceiling surface is actually precast in a shop, along with the rebar cage and plastic voids.  Precast units can be as large as the truck used to ship them to the site, and can be formed on a smooth metal casting bed to ensure a high quality architectural finish.

Voided slab’s longer, beam-less spans, combined with its smooth ceiling finish, allowed the design team to transform spaces that would have otherwise been cluttered with concrete beams and drop panels into clean architectural volumes.  This is evident in the views above.

UofT Daniels Faculty Structural design BEFORE and AFTER voided slab.  (Renderings by NADAAA).

Examples of smooth voided slab suspended soffits.  (photos courtesy Bubbledeck)

3)  Making sausage:  Radiant concrete + electrical distribution

The smooth, flat underside of the voided slab system actually helped to streamline the project’s mechanical distribution.  The client, and our consultant, Transsolar, both recommended thermo-active radiant concrete ceilings in keeping with the sustainable mission of the project, which mandated water rather than air systems for mechanical heat transfer.  However, the initial structural design complete with drop panels and beams, interfered with even distribution of hydronic tubing (by Klimatrol), meant to sit consistently at 1.5” above the concrete ceiling.  Voided slab eliminated this problem while also permitting the tubing to be pre-installed and pressure-tested at the precaster’s shop.

At the precast shop: 1. Radiant PEX tubing attachment to bottom reinforcing mesh (top left), 2. Placement of plastic bubble voids, lattic girders and top welded wire mesh (top right), 3. Precasting bottom deck of panels (above).  (photos courtesy Bubbledeck North America)

Plan of typical 18″ deep bubbledeck panel.  (courtesy Bubbledeck North America)

However, electrical and data disruption on this project is anything but even:  all cast-in conduit serving floor boxes, lights, and other devices originate in a bottleneck at the electrical or IT rooms on each floor.  The pre-fabrication of voided slab panels required the trades to coordinate this work early and comprehensively.  Plastic bubbles were omitted in locations of high congestion, to ensure both structural performance and to reduce conduit bends.  Bubbles were also omitted under floor box locations, and all ceiling junction boxes were cast in the shop to match early coordination drawings.  A handful of boxes were missed, and installed in the field by removing a few bubbles.

Detail showing typical placement of floor boxes, ceiling boxes, and electrical/data conduit runs. (courtesy Bubbledeck North America)

A shop-cast conduit sleave, with bubble removed, in preparation for data distribution to partitions below.  Field-placed conduit routing between bubbles. (Photos courtesy Bubbledeck North America)

4) Less labor on site

There are obvious advantages to performing sensitive work in a climate-controlled shop, following rigorous coordination drawings:  architectural concrete finish, placement of radiant tubing, placement of junction boxes, etc.  Less obvious is the reduction in site labor, and particularly formwork construction.  The bubbledeck precast panels arrived on site by truck and were craned directly onto shoring, followed by conduit installation and placement of additional rebar.  These panels then serve as permanent formwork for the final pour on top.  Slab edges are formed and shored with steel plate edge forms that were cast in the shop, and coordinated to accept curtain wall anchor pockets.

Shoring erected in preparation for delivery of bubbledeck panels. (Courtesy the Daniels Faculty. Photo by Peter MacCallum)

Flatbed truck delivery (top left), Bubbledeck precast units craned on to shoring (top right), Final concrete pour over permanent precast formwork (above).  (photos courtesy Bubbledeck and Adamson Associates Architects)

5) What’s next

Our next project incorporating radiant slab ceilings might attempt to optimize thermal transfer to spaces below by manipulating the architectural surface.  This might include, for example, the use of textured form liners to create a series of ridges or other features on the concrete surface, which would multiply the area available for convective heat transfer.  This is similar in concept to the design of finned tube radiators or heat sinks, and presents significant architectural possibilities.

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Photo by Tom Beresford of NADAAA.

Phase 2 construction is officially underway at the new Daniels Faculty of Architecture, Landscape + Design / 1 Spadina Crescent at the University of Toronto.  Over the last 6 months, various 20th-century-vintage additions have been demolished around the north courtyard of the original 1875 heritage building to clear space for new construction.  Since the spring, excavation and shoring activities have been proceeding steadily, and reinforcing bar for the new mat footings are being placed (see above).

Drawing courtesy Entuitive Corporation.

The mat footings play several roles in the project. First, by tying together columns that land around the interface with the heritage building, the footings help to distribute loads eccentrically away from the shallow existing foundation walls. Second, the foundations are thickened into benches to shore up soil pressure around the perimeter of a depressed basement area below the centre of the building–home of a future formal gallery space. Third, for economy, the mat foundations double as floor slabs along the high level basement.

Plan showing higher and lower level basements, separated by a concrete bench that is integral with the building’s mat foundation. Drawing courtesy Adamson Associates.

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Another dispatch from the 1 Spadina jobsite in Toronto: structural demolition work is now complete along the original north wall of the Knox College heritage structure (constructed 1875).  At the time of our site visit, a few existing openings remain to be infilled with masonry; attachment of weather protection (see in bright orange) at recent Phase 1 concrete and block walls around the central stair block was ongoing.   The site is now ready for shoring and excavation activities in preparation for Phase 2 construction.

Panoramic view of the north side of 1 Spadina: all additions to the original heritage building have been demolished.

View of the central stair block in the foreground, and heritage masonry work occuring in the background. The removal of existing additions has revealed original ruble foundations, as well as new concrete foundation walls completed as part of the Phase 1 renovation work.

Openings that will connect to the new Phase 2 addition have been hoarded with sheathing; existing openings have been infilled with block and occasionally support lintels for new openings, as seen at left. The dark exposed masonry at far left is the remaining wall of a former airwell that was formed by the demolished Connaught laboratory addition.

Sawcut rubble foundations and 2 to 3 wythe masonry walls have been exposed at the location of the demolished military hospital wing. Holes in the existing fabric will be filled at the time that access is enabled by the Phase 2 addition concrete slabs.

Formwork was being released at the Phase 1 concrete foundations formed to support new janitor, electrical and IT closets surrounding the central heritage stair block.

Existing interior finishes are now exposed on the exterior in several locations.

Weather protection has been installed along the roofline, in anticipation of the Phase 2 addition roof tie-in.

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