Superstructure system

8.2.1 Description and construction

The six-storey superstructure is 22.1 m high, and the footprint is about 1000 m² with the major plan axis being about 2.5 times the length of the minor axis. As shown in Fig. 8.1, the building superstructure has three flat facades, and one curved and sculpted facade with exposed steel columns that are sheathed in timber at the lowest above-ground storey. The roof is flat with a parapet wall and few minor obstacles such as pedestrian access stairheads and physical services.

Primary superstructure elements are RC walls that act as shear walls and enclose fire escape and firefighting routes and glulam framework and horizontal diaphragm elements. The super-structure is anchored to a level RC slab that is monolithic with an underground three-level RC parking garage that forms the foundation.

The building was constructed in stages, wherein the RC wall elements were cast in situ, prefabricated glulam framework and diaphragm elements were added, and floors were com-pleted structurally. Although floors were added and comcom-pleted approximately in harmony with the addition of the framework for different storeys, the construction sequence was coor-dinated such that two-storey framework segments could be preassembled and then lifted into place (Fig. 8.2). These segments are located in vertical planes parallel to the minor building plan axis. Therefore, within the superstructure there is continuity of column and girder ele-ments across some connections. The framework functions such that main floor and roof gird-ers are connected to prefabricated frame segments. Once the system was completed, floors functioned as continuous diaphragms. All glulam elements were cut precisely to length and shape, and holes for dowel fasteners were drilled using computer numerical controlled (CNC)

8.2 SUPERSTRUCTURE SYSTEM 111

machines. This resulted in less workmanship skill sets and was essential for ease and speed of construction. Precise cutting of glulam elements avoided the possibility of distorting or damaging the superstructure by force-fitting parts. Columns have one piece, and girders are made from either one piece or two pieces of glulam. In some cases, girders fit together as composite elements that wrap around the column, thereby resulting in desirable connection and framework actions. As seen in Fig. 8.2, secondary framework elements were connected to primary framework elements by brackets; floor slab elements were connected to the frame-work elements to achieve composite action; and in some locations special frameframe-work details were adopted (Fig. 8.2f ).

After the structural parts were added to the superstructure, the roof was made watertight, and the exterior claddings were added. Finally the building was finished internally. The construction process is illustrated schematically in Fig. 8.3. In the finished state, the glulam structural ele-ments are visible inside the building.

 2 0. 2 m

   2   2  m

5 2 .6 m 

(a)

(b)

Fig. 8.1: Six-storey office building in Quebec City, Canada: (a) completed building (courtesy of FPInnovations); (b) framework elements (courtesy of Nordic Engineered Wood)

8.2.2 Glulam framework and diaphragms

From the structural performance design perspective, general points to note are that Quebec City is a location for which design roof snow loads are high; design wind pressures are moderate without threat of hurricanes and slight possibility of tornados; and the peak ground acceleration used in seismic design is moderate. Relevant to wind and snow loading is the building’s loca-tion which is the last in a row of abutting buildings of about equal height, in an urban setting at the bottom of a valley. Thus, the building facades are not highly exposed to wind. The roof has

(a) (b)

(c) (d)

(e) (f)

Fig. 8.2: Installation of glulam superstructure framework and slab elements: (a) two-storey segment; (b) interconnection of segments; (c) primary glulam framework elements; (d) glulam  floor slab elements; (e) interior column supporting girders in and transverse to the planes of  framework segments; (f) special framework of sixth storey on the curved and sculpted facade

(courtesy of FPInnovations)

8.2 SUPERSTRUCTURE SYSTEM 113

low parapet walls and minor obstructions (Fig. 8.3, bottom-right diagram), leading to the pos-sibility of snow accumulations, which is accounted for via standard provisions according to the NBC. Structural steel and RC framework buildings of similar general size and shape in Quebec

RC slab above below ground parking garage First storey RC walls cast

First level of glulam framework installed First elevated slab structurally complete and second level RC walls cast

Second level glulam framework installed Second elevated slab structurally complete and third level RC walls cast

Roofing and facades added to make the building weather tight

Fig. 8.3: Selected stages of the supertstructure construction sequence (courtesy of Nordic  Engineered Wood)

City can have superstructure elements sized for strength based on dominant effects of gravity or lateral loads within the design loading combinations. The overall superstructure shape, local topographical features, and the non-symmetric arrangement of superstructure elements of the discussed building indicate that it sways laterally and twists about its vertical axis under the effects of wind and would do so during seismic events.

The framework of glulam columns and girders are the primary parts of the above-ground system for resisting effects of gravity forces associated with the self-weight of building elements, roof snow load, and occupancy floor loads (e.g. office and document storage). As already discussed in Sub-section 8.2.1, the framework itself is structurally complex because of the in-plane continuity that exists in the prefabricated framework segments, and articulations/rotations that can occur where framework segments join together or are joined to RC substructures. Connections within and at the boundaries of framework segments are also complex in their design, construction, and behaviour.

Elevated floors and the roof incorporate glulam elements that are arranged to make those substruc-tures function as horizontal diaphragms capable of collecting the effects of external wind pressures or seismic ground accelerations (i.e. lateral load effects) and transferring them to RC shear walls.

8.2.3 Timber connection methods

As is discussed in Chapters 2 and 5, selection, design, and construction of connections are highly important aspects of the structural design of frameworks made from large glulam/timber elements. Framework and other connections were designed taking into account the implications for the stiffness and stability of the completed superstructure system. This led to adoption of framework connections made using steel dowels that fit tightly in pre-bored holes in glulam, steel plate linking elements with minimally oversized holes, and girder seating elements that tie glulam elements to adjoining superstructure components. Secondary connections were made using threaded steel bolts, self-drilling screws, and annularly threaded nails. All critical struc-tural connections are embedded within strucstruc-tural members for reasons of aesthetics and fire protection (Section 8.4), apart from structural efficiency requirements. Care was taken to avoid eccentric loading of columns under governing design load conditions. This was achieved by arranging steel linking components such that they were symmetric relative to column axes.

Figure 8.4 shows typical details of connections within the superstructure. The connections were designed such that glulam framework elements that are continuous through connections lock together (in the style of carpentry lap joints), and when glulam elements are not continuous through connections, their ends join precisely with steel plate linking and seating elements.

Dowel fasteners keep elements in place, besides having force transfer functions. Where pos-sible, transfer of forces is by direct bearing on glulam or steel plate elements rather than fasten-ers, which minimizes the possibility of splitting members. In all cases, the intimate nature of t he connection between elements means that force transfers are achieved by combination of glulam, dowel fastener-glulam, dowel-fastener-to-steel linking element, and glulam-to-steel seating element bearing and friction. Close attention was paid to avoiding situations where moisture content-related dilations of glulam members might strain connections. Specifically, care was also taken to avoid placing fasteners that joined glulam to steel plates too far apart in the transverse direction, in which glulam could shrink and lead to splitting (Subsection 8.5.1).

The Canadian timber design code [114] and similar international best practice specifications emphasize the need for such considerations.

8.2 SUPERSTRUCTURE SYSTEM 115

Connection Trp. SOLD

Connection Trp. SOLD

(a)

(b)

(d)

(e) (f)

(c)

Fig. 8.4: Typical primary connection between glulam girder and column elements: (a) scheme  for arrangement of interlocking glulam elements showing steel plate linking and girder seating elements at a location where an edge column is continuous between storeys (courtesy of Nordic  Engineered Wood); (b) two-storey column element precut and drilled to receive dowel

fasten-ers; (c) multi-bay girder element precut and drilled to receive dowels (one half of the complete built-up girder); (d) steel plate linking and seating elements ready to receive the preassembled  framework segment for the next two storeys; (e) shear keys inserted in slots to create compos-ite diaphragm action between glulam slabs elements in a floor; (f) attachment of glulam slab elements to RC shear wall using steel angle sections, self-tapping wood screws, and concrete anchor bolts (photographs (b) to (f) courtesy of FPInnovations)