By Karl Binder, Dip. Arch, Rob Pacholok, M.Sc., P.Eng., Gary Sturgeon, P.Eng.
The Alberta Legislature Building in Edmonton opened its doors on September 3, 1912. Serving as the permanent home to the Alberta provincial government, it has become an historic site. Decades after its construction, the two feature terra-cotta-clad domes needed restoration and replacement.
The building’s structural steel semi-spherical Major Dome is the most prominent and highly visible architectural feature. It features a ‘roof lantern’—cupola-type architectural element atop a larger roof that provides natural light into the space or room below. The Minor Dome, which has a cupola, is a low-rise saucer dome of structural concrete only readily seen from positions above the main roof level. When the iconic building was about 75 years old, the Major Dome underwent renovations because of de-bonding of the terra cotta on the dome, referred to as ‘bulging.’
To stabilize the terra cotta, steel angle iron was attached to existing steel members; to these members, threaded rod anchors embedded in epoxy were connected to the back face of the terra cotta. Along with including means to manage moisture penetrating the mass masonry and condensation formed on the dome’s interior surface, this served as a viable fix for issues of structure and water management for 20 years.
In 2010, the Alberta government committed to a massive project of full removal and replacement of the failing terra cotta masonry assembly on both domes. The pre-construction process began with condition surveys, followed by development of cost-effective solutions, issuance of architectural and structural drawings, and prequalification of masonry contractors. In honour of the 100th anniversary of the building’s opening, work commenced in 2012 with demolition of terra cotta on both domes. The restoration work is scheduled for completion this spring.
The restoration employed various technologies to preserve the building’s heritage. These include:
- building information modelling (BIM) to ensure details worked both in theory and in practice, and existing terra cotta layouts were accurately recorded for replication;
- shotcrete application to the Major Dome structure serving as structural backing to new terra cotta replicates;
- application of sprayed polyurethane foam (SPF) insulation to help manage heat, air, and moisture;
- engineering of custom masonry anchors to tie the new terra cotta units to the structural backing and facilitate drainage and drying using an included air space; and
- placement of a continuous waterproof membrane over the structural backing to prevent ingress of precipitation into interior space.
The Alberta Legislature Building was built between 1907 and 1913 and designed by architect Allan Merrick Jeffers in Beaux-Arts style—popular for public buildings during this period. The style was heavily influenced by Greek, Roman, and Egyptian architecture, suggesting power, permanence, and tradition.
Beaux-Arts buildings are characterized by:
- large central dome above a spacious rotunda;
- symmetrical T-shaped plan;
- doors and windows decorated with arches or lintels;
- a portico supported by massive columns;
- rusticated and raised first storey;
- arched windows;
- arched and pedimented doors;
- flat roofs;
- symmetrical layouts;
- co-ordinated sculpture, murals, and mosaics; and
- Classical architectural details.
The legislature building is often referred to as the ‘Leg’ (as in ‘ledge’). It includes a T-section with stem running in the north-south direction and extending 77 m (254 ft) in length and 24 m (80 ft) in width. The attached east-west wings each measure about 40 m (130 ft) long and 29 m (95 ft) wide. A structural steel skeleton supports the exterior cubic dimension stone masonry.
The first floor is clad with granite from Vancouver Island. The floors above are finished with Alberta Paskapoo sandstone quarried from the Glenbow Quarry west of Calgary—one of many local quarries serving the province at the turn of the last century. The building includes two terra cotta domes with Paskapoo sandstone bases. The terra cotta was manufactured in England by Gibbs & Canning, which is no longer in operation. The Major Dome is a structural steel semi-spherical dome and is the most prominent and highly visible feature of the ‘Leg,’ rising 54 m (176 ft) in height at its peak (Figure 1). The Minor Dome is a smaller, low-rise saucer dome of structural concrete. It extends 7 m (23 ft) above roof level, and rests on the building’s south wing above the ‘House’ where the legislative assembly meets when government is in session.
Terra cotta dome restoration
Due to masonry construction’s longevity, involvement in the removal and replacement of the original terra cotta domes was a once in a lifetime opportunity. This complex reconstruction required a highly skilled team of craftsmen to accurately replicate and fabricate the existing terra cotta profiles and units, and carefully lay the units, respecting the spatial orientation and positioning to suitably envelop the existing structure and accurately replicate the original.
Reproduction and fit began with the fabricator creating accurate setting drawings of the original terra cotta, by duplicating existing units and profiles using computer imaging, cutting, and moulding, and by replicating original terra cotta colours and finishes. Onsite, the masons ensured the installation satisfied terra cotta layout and unit placement, and all design details for structure and water management were suitably constructed in accordance with the approved shop drawings.
The Major Dome
The original Major Dome cross-section consisted of terra cotta architectural units fully bedded and bonded directly to the outer face of terra cotta masonry book tiles. The book tiles served as a structural infill, rigidly mortared together and against abutting continuous horizontal tee supports (located 483 mm [19 in.] centre-to-centre [c/c] vertically), without any inter-connecting mechanical anchorages. The horizontal tee sections were bolted to vertical steel I-beams—identified as primary and secondary steel ribs—curved to create the dome’s semi-spherical shape.
During demolition, a large area of terra cotta with supporting book tile was cut out, carefully removed, and presented to Alberta Infrastructure to retain for historical purposes. Original construction relied on geometry and masonry mass to resist the exterior precipitation penetration.
When the iconic building was about 75 years old, the Major Dome underwent repairs largely because of separations between the book tiles and the supporting tees, as well as between the architectural terra cotta and the supporting book tiles. The consequent ‘bulging’ of the finished surface measured 50 mm (2 in.) on the west elevation. At the time, the solution to stabilize and prevent further movement outward was to attach steel angle iron to existing steel members, and to these, connect threaded rod anchors embedded by epoxy into the architectural terra cotta’s back face (Figure 2).
Additionally, the work included means to effectively manage moisture penetrating the mass masonry, as described in the following sections. This renovation provided workable solutions to issues of structure and water management for two decades. In 2010, the Alberta Government committed to a massive project of full removal and replacement of the failing terra cotta masonry assembly.
In contrast to the original construction, the 2012 design required:
- terra cotta units be individually anchored to the structural backing (Figure 3);
- continuous air space be provided behind the terra cotta to facilitate drainage and drying; and
- continuous waterproof membrane be applied over the structural backing to prevent the ingress of moisture into interior space.
Mortar placed in the head and bed joints between new terra cotta units was relied on for structural continuity and to provide the first plane in the assembly to resist moisture penetration. Improved thermal resistance in the form of SPF insulation was also included to prevent condensation on interior dome surfaces, help maintain a controlled interior environment, and offer redundancy to the other means for resisting water penetration.
The redesigned dome assembly consisted of the following components:
- most inboard on the cross-section, a reinforced shotcrete structural backing tied to the original steel structure, applied and finished to follow the dome’s curvature (Figure 4);
- a single-ply styrene-butadiene-styrene modified-bitumen (SBS mod-bit) torch-on membrane installed directly over the shotcrete (Figure 5);
- a 38-mm (1½-in.) SPF insulation adhered to the membrane;
- a 25-mm (1-in.) continuous air space for drainage and drying; and
- anchored replicate architectural terra cotta on the outboard face.
This cross-section and material selection fully respects the modern ‘rainscreen principle’ to effectively control water penetration and condensation and provide a controlled interior environment. The dome’s double curvature meant a high degree of skill and attention to detail was required when laying out and installing the terra cotta.
Due to the limited cross-sectional thickness of the original mass construction and location of its supporting steel structure, and the need to retain the location and positioning of the finished terra cotta surfaces using units with the same thickness as the original, and with the need to include a 25 mm continuous air space, 38 mm of continuous insulation, and 113 mm (4.4 in.) of shotcrete, it was necessary to ‘grow’ the new assembly inward and to maintain tight control over the location and placement tolerances for the structural shotcrete and foam insulation finished surfaces.
Wood formwork was designed and constructed to bound and temporarily support shotcrete during its application and set. Curvature, and the required thickness of shotcrete, was accurately maintained over the dome surface by carefully positioning and contouring the formwork. The formwork consisted of two layers of 6.35-mm (1/4-in.) plywood braced by dimension lumber, positioned and formed in double curvature to neatly follow the curves defined by the existing primary and secondary steel ribs. The well-positioned and consolidated shotcrete served as the dome’s interior face for the attic space without additional finishing.
Terra cotta units
All original architectural terra cotta units were removed and replaced with replicates to preserve the building’s historical identity (Figure 6). Ashlar units were either 50 or 150 mm (2 or 6 in.) thick, with a typical bed height of 305 mm (12 in.).
On the Major Dome, rows of ashlar units abutted continuous vertical terra cotta ribs which extended from the dome base to the collar of the lantern above, and symmetrically divided the dome into octants. Rib units typically measured 600 mm (24 in.) in length, 305 mm (12 in.) in height, and varied in width from 535 mm (21 in.) at the dome base to 305 mm at the lantern collar above.
Unit weights were in the order of 0.11, 0.22, and 1.7 kN (25, 50, and 375 lb) for the 50-mm ashlar, 150-mm ashlar, and rib units, respectively. All head and bed joints between terra cotta units were a standard 10 mm (3/8 in.) width. Pre-construction testing and manufacturer pre-qualification ensured the new terra cotta’s physical properties exceeded those of the original material, particularly with respect to compressive strength and resistance to freeze-thaw deterioration.
The fabrication process first involved the selective removal from the dome of a full-size and whole unit of each original custom unit. These were delivered to the terra cotta manufacturer to serve as a replicate template. Models of the units were created in a process similar to 3D printing where multiple photographs were taken of the original pieces and imported into proprietary software that communicated directly with a computer-aided manufacturing (CAM) computer numerical control (CNC) machine that cut the models.
The models were used to create accurate plaster moulds for producing new terra cotta units. Concurrently, detailed setting drawings for the terra cotta were developed using onsite measurements of the existing structure and assemblies. These drawings identified boundary conditions, working points, critical elevations, plan dimensions, and each terra cotta unit’s location and spatial position within the matrix. Since the dome diameter varies with elevation, and because the units by specification could not be field-cut, each unit had to be prefabricated and individually marked to fit the detailed layouts (Figure 7).
After fabrication (and before site delivery), a number of full-scale sections of the Major Dome terra cotta were dry-laid at the fabrication facility to ensure proper fit and acceptable colour and finish.
The detailing on the more highly ornate units, such as the lantern column capitals and wreaths, was hand-carved (Figure 8). A BIM of the lantern was also created to ensure fit of units without interferences (Figure 9). All original column capitals were retained by Alberta Infrastructure.
Anchoring the terra cotta and accommodating movement
Full and complete terra cotta anchor layout drawings were created to ensure each unit was suitably supported and would resist the imposed live, snow, and dead loads (Figure 10). This comprehensive approach included the atypical terra cotta units and those at discontinuities and boundaries. Anchorage for such units can be missed when simply designing anchors for typical units located in the field of the matrix.
The architectural features and symmetry of the Major Dome conveniently divide it into eight equal octants. Each is bordered and defined by a continuous vertical string of terra cotta rib units which are, by design, positioned immediately above the underlying vertical I-beam primary steel ribs that form and support the dome. Rows of ashlar units abut the vertical terra cotta ribs and serve as infill.
Terra cotta masonry moves with changes in temperature and moisture. This must be accommodated, rather than restrained, in order to prevent or relieve stresses within the terra cotta. Similar to clay brick masonry veneer design in multi-storey construction, the terra cotta ashlar infill was therefore structurally isolated from the rib units by vertical movement joints along their junction. Further, the octant pie-shaped configuration of the ashlar infill was divided into three veneer panel walls by including two horizontal movement joints located roughly at arcs of 10 and 35 degrees from the horizontal.
Movement joints were suitably located to limit anticipated panel movement to less than ±5 mm (0.2 in.). These horizontal movement joints also extended through the vertical string of rib units. Similar to clay brick masonry veneer, the panel self-weight and live/snow load components were required to be supported by shelf angles along their base. As a consequence of introducing horizontal movement joints, fundamentally two types of anchors were needed and designed to support the ashlar and rib units on the Major Dome: tie-back anchors and dead-load clip angles.
Dead-load anchors—akin to shelf angles as they support vertical loads such as panel wall self-weight—were installed at all horizontal movement joint locations. However, dome curvature created a condition where a traditional continuous angle iron was unsuitable. For the Major Dome ashlar infill, the chosen alternative was to use discrete clip angles, each about 150 mm (6 in.) in length, and to site-weld the vertical leg of the clip angles to the exposed face of a continuous 9.5-mm (3/8-in.) thick stainless steel belt plate that had been well-positioned, embedded, and anchored into the structural shotcrete.
A clip angle was placed under the head joint of each ashlar unit located directly above a movement joint. The clip angle width and horizontal leg length were sufficient to ensure good bearing. Rib units were similarly supported, except the rib clip angles were field-welded to the primary steel ribs instead of the belt plates (Figure 11). An extended height of the belt plates facilitated vertical adjustment in the placement of the ashlar clip angles. The underside of the terra cotta units in contact with the clip angles were recessed along their bottom surface to accommodate the thickness of the horizontal leg of the clip angles to maintain a free unobstructed movement joint width of 10 mm (3/8 in.). All movement joints were sealed to prevent water penetration.
The tie-back anchor is a two-component anchor consisting of a 5-mm thick field-bent stainless steel plate and an 8-mm diameter stainless steel dowel (Figure 12). A single tie-back anchor was typically installed in the vertical head joint at mid-height of each terra cotta ashlar unit. On larger units, such as the Major Dome terra cotta ribs, and for units directly below horizontal movement joints, two tie-back anchors were installed at quarter points to provide stability. The 8-mm (0.31-in.) dowel passed through a vertical slot along the outer end of the tie-back plate and engaged the adjacent terra cotta units.
The base of the anchor was connected to the shotcrete structural backing using a wedge anchor. The tie-back anchor was designed to resist dead and live/snow loads acting normal to the surface of the terra cotta, and in this manner, the anchor ties-back the terra cotta veneer to the structural backing much like a multi-component clay brick masonry tie. However, unlike a brick tie, the anchor does not rely on mortar embedment to provide restraint. Rather, it firmly engages the terra cotta unit using the dowel. As a result, significantly higher loads can be resisted. Whereas the anchors restrain the terra cotta masonry against out-of-plane loads and movement, the anchors were specifically designed to allow in-plane movement of the terra cotta ashlar infill assembly.
The 8-mm dowel can slide along the slot in the bent plate, allowing for independent vertical movement between the terra cotta units and the anchors. To enable independent horizontal movement, the anchor was physically isolated from the adjacent terra cotta. The dowel sinkage (i.e. the void in the unit into which the dowel is inserted) on one side of the anchor was filled with sealant and on the other side with mortar. Closed cell foam tape was then wrapped fully around the anchor to prevent mortar or any other deleterious materials from filling the space between the anchor and the terra cotta unit that would otherwise offer resistance to movement.
In accordance with Canadian Standards Association (CSA) A370, Masonry Connectors, any component of an anchor engaging a masonry unit is required to be stainless steel. To meet the extended design service life demanded by this restoration, all terra cotta anchors were custom-manufactured from stainless steel Type S304L. The tie-back anchors were fabricated by an Edmonton-based manufacturer of masonry connectors, delivered to the site as flat plates, and field-bent using a hydraulic jack and die (Figure 12) to produce a clean and accurate bend similar to that obtained by plant manufacturing.
Field bending of the anchors was found to be the most efficient method to provide the needed out-of-plane adjustment (normal to the ‘wall’). Pre-bent anchors and multi-component ties connected by fasteners were impractical because of limited space. On the horizontal leg of the tie-back anchor, two parallel slots were punched to allow the mason to reposition the wedge anchor where shotcrete bar reinforcement obstructed its placement. All fasteners were double-sealed to prevent water penetration beyond the membrane.
As each ashlar unit on the upper portion of the Major Dome was set and engaged by the anchor system, vertical ribbons of mortar were laid over the foam insulation along the vertical webs of the hollow terra cotta units. These provided temporary support and stability at the time of setting the unit. When hardened, the orientation and partial filling maintained draining and drying of the air space. The anchor system was designed independent of any benefit to load resistance offered by the mortar under compression.
Redesigned minor dome
The original Minor Dome’s profile consisted of 40 decorative terra cotta rib assemblies mortared atop shingle-lapped terra cotta roof tiles, which in turn were fully mortar bedded and bonded without mechanical anchorage, directly to a cast-in-place concrete shell supported on a steel frame (Figure 13).
Original construction relied on geometry, shingle-lapped bonded roof tiles, and the mass of the concrete shell to resist penetration of precipitation from the exterior. However, over time, cracking of the roof tiles and concrete shell created problems with water infiltration.
To mitigate, original roof tiles between the ribs were replaced in the 1960s with new clay-fired roof tiles supplied by a Medicine Hat-based manufacturer. These efforts were unsuccessful. Figure 13 shows the 1960s construction at the time of a 2006 investigation of the Minor Dome roof. As Minor Dome sits directly above the ‘House,’ a carefully considered solution was required to prevent water movement into the building.
In 2012, the terra cotta tiles were stripped down to the underlying concrete shell. After some localized structural repair to improve its strength and with some surface preparation, the existing shell became the base substrate to receive the new materials and components required by the new design (Figure 14).
In contrast to the original construction, the 2012 design required:
- continuous waterproof membrane be applied over the structural backing to prevent moisture ingress;
- continuous air space be provided beneath the terra cotta to facilitate drainage and drying; and
- rib units be mechanically anchored to the roof tile assembly.
The redesigned assembly (Figure 15) consisted of the following components:
- more inboard on the cross-section, the original reinforced concrete shell repaired as needed;
- two-ply SBS mod-bit torch-on membrane installed directly over the concrete shell;
- 18-gauge Z-girts fastened to the concrete shell with wedge anchors, with all fasteners penetrating the membrane double-sealed to prevent water penetration;
- 38-mm (1 ½-in.) SPF insulation;
- 8-mm (5/16-in.) drainage mat;
- mortar scratch coat with stainless steel mesh fastened to the Z-girts with #12 stainless steel sheet metal screws;
- terra cotta roof tiles, manufactured with lugs, fully mortar bedded and bonded to a bed coat over the mortar scratch coat; and
- most of the outboard, architectural terra cotta rib tiles mechanically anchored to the terra cotta roof tile assembly with #10 stainless steel deformed dowels mortared into the rib unit cells and epoxied into the concrete shell.
With the addition of the waterproofing membrane, the 38-mm continuous insulation and 8-mm drainage mat, and by retaining the same thickness of terra cotta roof tile while maintaining the original concrete shell, it was necessary to ‘grow’ the new assembly outward.
The new shingle-lapped roof tiles provide the first plane in the assembly to resist penetration of precipitation, the polyurethane foam insulation provides a secondary plane of resistance, and the 2-ply SBS waterproofing membrane provides the primary resistance. The redesign retained the original look of the Minor Dome while ensuring it suitably manages precipitation (Figure 16).
As this unique, challenging, and rewarding project nears completion, it has become apparent overcoming the design and construction complexities of the restoration required the dedication of Alberta Infrastructure and the provincial government, design team, product manufacturers, terra cotta supplier, contractors, and masons. The completed project will be a testament to the abilities of those who worked so diligently to preserve the historic fabric of such an iconic building and to the durability of terra cotta masonry domes.
Karl Binder is a senior project manager with Gracom and holds a diploma in architectural and building engineering technology from British Columbia Institute of Technology. He began working in the masonry industry with Gracom in 2004 and has project management experience in residential, commercial, restoration, institutional, and industrial construction. Binder has also served as president of the North Chapter of Masonry Contractors Association of Alberta (MCAA) from 2011 to 2013. He can be contacted at firstname.lastname@example.org.
Rob Pacholok, M.Sc., P.Eng., is a senior engineer for Building Science Engineering Ltd., specializing in masonry construction and restoration. He is also project engineer for the Alberta Legislature Building domes waterproofing and recladding project. Pacholok is a technical committee member for CSA S304, Masonry Design for Buildings and CSA A371, Masonry Construction for Buildings, and jointly designed, developed, and patented numerous masonry connectors. He can be reached by email at email@example.com.
Gary Sturgeon, P.Eng., is a senior structural engineer who received his degrees through advanced research in masonry. He has 33 years of professional experience, serving as a technical consultant to building owners, design professionals, material suppliers, contractors, and various national masonry associations. He serves on the Technical Committees of all the CSA Masonry Standards as well as on the Standing Committee for National Building Code of Canada’s Part 5, Environmental Separation. Sturgeon can be contacted at firstname.lastname@example.org.