November 30, 2020
By Dominic Lemieux, P.Eng.
For the last decade, concrete and steel structures have been archrivals for multi-storey buildings requiring fire resistance. While cast-in-place concrete buildings offer an inherent fire resistance and slim floors, steel structures provide speed and precision due to their prefabricated nature.
Prefabricated composite structures allow designers and contractors to leverage the benefits of both concrete and steel to erect at a fast pace fire-resistant buildings with slim floors.
The Luxenbourg phase II and III are multi-family residential projects located near downtown Québec City, Qué. The development comprises two identical buildings, each containing a total floor area of approximately 8000 m² (86,111 sf). There are 54 units in both buildings with five levels of residential and two levels of parking.
The challenge for the developer was to find ways to reduce the construction schedule in order to enclose the building before winter, thus significantly lowering the impact of having to heat a poorly insulated building as well as snow removal during construction in the colder months.
Additionally, the site had a height restriction which meant a slim floor system had to be used to maximize the number of floors in the buildings.
To solve the challenges, the developer and his team of professionals introduced a couple of innovations to make the project viable.
Innovation 1: Using a prefabricated slim-floor composite structure
One of the objectives of the developer was to build a non-combustible structure that would be quick to install and would require as little manpower as possible. A combination of composite beams, composite columns, and hollow-core slabs were chosen to achieve the design solution.
The composite beams and columns are steel profiles containing shop-installed reinforcement. The profiles were later filled with concrete onsite to increase their strength and to provide fire resistance of two hours.
The precast hollow-core slabs also helped increase the speed of construction by minimizing the number of crane picks and by significantly reducing the amount of shoring required. Only two shoring posts per beam were needed during the erection which allowed for other trades to move in faster than with conventional cast-in-place construction. While the numerous shoring posts required for cast-in-place structures typically need to remain in place for weeks until the concrete cures, the few shoring posts utilized in a prefabricated structure are removed after just a few days since they are only required to stabilize the structure during erection. The faster the shoring posts can be removed, the faster other trades can get into the building.
The combination of hollow-core slabs and shallow trapezoidal composite beams created a floor assembly of only 250 mm (10 in.) while allowing for spans of over 11 m (35 ft). This compact floor assembly permitted the developer to maximize the number of floors he could build on this property.
Results obtained from using a prefabricated structure
Once the foundation walls had been poured, the five-man crew was able to erect a complete 1200-m² (12,917-sf) level every three to four days. The first day was used to install all the composite columns and the shallow trapezoidal composite beams. The second day was utilized to install the hollow-core slabs and the third day was used to grout the composite columns, the shallow trapezoidal composite beams, and all the joints between the hollow-core slabs. On the fourth day, the cycle could be started over on the level above. After 20 working days, the crew topped off the five floors of residential levels. The five-man crew consisted of one crane operator and four steel erectors.
Faster access to other construction trades
In addition to a faster erection time with a small crew, the prefabricated structural components offer another major benefit: Minimal shoring. Additionally, the shoring posts can be removed days after they are installed instead of weeks as it is typically seen in poured-in-place concrete construction.
The reason why the shoring is minimal with a prefabricated composite structure, such as the one used in the Luxenbourg projects, is that all the components do not require on-site curing time. The composite columns and the shallow trapezoidal composite beams are structural steel profiles which can carry the weight of the hollow-core slabs. The precast slabs are already cured when they arrive onsite and do not require shoring onsite. Prefabricated structures of this type only require shoring during erection to prevent the rotation of the composite beams when loading all the slabs on the same side of the shallow trapezoidal composite beams. This shoring usually consists of only one 100 kN post at each end of the beam. Once the slabs are loaded on the other side of the trapezoidal beam section, the rotation at the column is annulled. As an additional precaution during erection, the shoring posts are removed a few days after the floor above is grouted instead of several weeks as it is typically seen in cast-in-place structures.
Innovation 2: Eliminating the need for fireproofing
Due to their concrete infill, composite structures can provide a fire resistance of two hours or more. Even if their capacity does reduce in the case of a fire, like any other type of structure, the design safety factor applied for the live load is reduced from a multiplicator of 1.5 to 0.5 in the case of fire. In other words, if the live load used for the design of the structure is 4.8 kPa (100 psf), for ambient temperature this value is multiplied by 1.5 so one would use 7.2 kPa (150 psf) for the design of the structure. When designing a structure in case of fire, the 4.8 kPa value is multiplied by 0.5 so the applied design load on the structure would be 2.4 kPa (50 psf).
The shallow trapezoidal composite beams have been tested at the Underwriters Laboratories of Canada (ULC) based on CAN/ULC-S101, Standard Methods of Fire Endurance Tests of Buildings, and has obtained a ULC fire rating of four hours. Even with the structural steel remaining exposed, the fire test demonstrated the concrete infill along with the additional reinforcement placed inside the beam at the production facility is sufficient enough to provide the required strength to support a floor during a fire for several hours.
The National Building Code of Canada (NBC) provides guidance on how to design composite columns—a hollow structural section (HSS) with a reinforced concrete core. These provisions were followed when designing the Luxenbourg project.
As a result of the non-resistant attributes of composite structures, the fireproofing trade was eliminated from the project which led to significant cost and time savings.
Innovation 3: Reducing the floor-to-floor height by using flush beam assemblies
The Luxenbourg project, like most new construction in urban areas, was subject to a height restriction imposed by the municipality. To maximize the number of floors in the building, a flush beam combination was selected in lieu of a conventional steel or precast beam system. The shallow trapezoidal composite beams and hollow-core slab assembly allowed spans of approximately 11 x 8 m (36 x 26 ft) with a total depth of only 300 mm (12 in.). If wide flange steel beams would have been used to support the prefabricated concrete slabs, the beam would need to be 400 mm (16 in.) deep for a total floor assembly of 700 mm (28 in.).
Having beams that did not protrude under the slab also gave the design team complete flexibility for the interior layout. After finishing the first phase of the Luxenbourg project, the developer realized the units with smaller total floor area were easier to rent. Therefore, in the second phase of the project they decided to have 11 apartment units per floor instead of 10. Since the structural system requires few interior columns (six in a floor plate of over 1200 m²) the only change to the structure was the addition of one cantilever balcony on a perimeter pre-stressed slab. All the columns remained in their original location which facilitated the co-ordination with the parking on the level below.
Innovation 4: Designing cantilever precast balconies without a thermal bridge
One of the biggest structural challenges in residential construction is designing cantilever balconies without a thermal bridge. Since the slab is partially inside and partially outside, eliminating heat loss through the concrete and reinforcement is challenging.
In the Luxenbourg projects, the thermal bridge was eliminated by using pre-stressed solid slabs with an integrated 900-mm (35-in.) cantilever. Once the slabs were installed, urethane was sprayed under them and polystyrene foam was used over the slabs, thereby significantly reducing the thermal bridge. The polystyrene was then used to anchor the pipes for the radiant floor system.
The Luxenbourg project provided experiences that will lead to many new innovations which will be applied again in the future phases of the development.
Lesson learned 1: The more prefabricated elements the better
The Luxenbourg phase II & III building envelopes were almost completely prefabricated. The only part of the exterior envelope that were not prefabricated was the corners of the building and it consisted of a field installed curtain wall. These small sections of the envelope took longer than the rest of the building to install and led to condensation issues for the occupants during winter. The next phase of Luxenbourg will have corner windows like the Luxenbourg phase I. Also, with the current labour shortage situation, having building techniques requiring less manpower onsite is usually simpler to manage.
Lesson learned 2: Long spans equal flexibility
The combination of hollow-core slabs and composite beams created a large open space giving full flexibility to the interior design team. Originally, the Luxenbourg phase II and III were supposed to have units of similar areas as phase I. However, after a year of occupancy of the Luxenbourg phase 1, the owner realized the yield was greater with smaller units, so he instructed his design team to try to fit one more unit per level in phase II and III.
Lesson learned 3: Designing precast cantilever balconies without a thermal bridge is possible
The precast cantilever balconies of Luxenbourg phase II and III turned out to be a success over the previous design of an independent structure for the balconies of the Luxenbourg phase I. Although the first phase also eliminated the thermal bridge, having to bring a crane back to the jobsite just to install balconies proved to be costly and time consuming.
As explained above, the Luxenbourg projects have been an accomplishment because of the successful implementation of construction innovations and overall quality of the building. The next four phases will incorporate the best practices from its first three phases but one could simply sum it up as: the more prefabrication the better.
Dominic Lemieux, P.Eng., is a licensed structural engineer in Ontario and Québec and has introduced shallow composite beams to over 150 projects in North America. He studied civil engineering at the Universitat Politècnica de Catalunya in Barcelona, Spain, and graduated in civil engineering at L’École de Téchnologie Supérieure in Montréal. Lemieux is Peikko North-America’s vice-president, Precast Concrete Institute’s (PCI’s) total precast committee chair and, PCI’s precast hollow-core slab committee’s vice-chair. He can be reached at via e-mail at firstname.lastname@example.org.
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