For these two homes, Habitat served as the architect and general contractor, while Scorpio Masonry/In-line Masonry provided masonry construction services and Williams Engineering donated structural engineering services to the project. A two-ply built-up 2×12 ledger was fastened to the concrete block wall to support the floor joists. The ledger and fasteners anchoring the leger to the wall were designed to structurally support the floor and roof loads. In the case of the attached homes, this was facilitated by using a 250-mm (10-in.) CMU on the basement party wall and moving to a 150-mm (6-in.) block for the floors above grade and partially supporting the ledger on the 50-mm (2-in.) ledge provided by the change in block size as can be seen in Figure 4.
On the main and second floors, it became more practical to use wood framing to support the second floor and roof trusses as the party wall was serviced on both sides (Figure 5). The 2×4 spruce-pine-fir (S-P-F) studs were cut to 1×4 actual thickness to provide additional usable space in the interior. The axial load on each stud was reduced by anchoring to the block wall with power-driven fasteners spaced at 406 mm (16 in.) on centre (o.c.). Again, the studs were checked structurally to ensure they held sufficient capacity to bear the floor joists.
The walls were serviced and sheathed with gypsum and this project was able to use the load-bearing capacity of both masonry and the wood framing.
Firewalls in multi-family residential buildings
Wood framing provides a cost effective alternative to other forms of construction in multi-family residential buildings. With the adoption of six storey wood-frame buildings into the 2015 National Building Code of Canada (NBC), more wood-frame multi-family residential buildings can be expected in the future. Masonry can once again complement this form of construction by providing places of refuge for firefighters and occupants. By constructing stairwells, elevator shafts and firewalls with concrete block masonry.
As mentioned earlier in the article, shrinkage between masonry and wood framing must be accommodated. Typically, the wood framing can be expected to shrink around 19 mm (¾ in.) per storey. The masonry shrinkage is typically less pronounced, resulting in differential movement of masonry relative to the wood framing of 9.5 mm (3/8 in.) per storey. With four storeys or greater, it is definitely important to consider the differential movement between the two materials.
The differential movement is most easily and practically accommodated by using load-bearing wood framing on which to bear the floor joints and to simply use the masonry as a firewall or fire separation for the stairwells and elevator shafts. Although current building codes only require a 1.5-hour firewall for stairwells, the use of masonry to achieve the fire rating for theses wall provides additional safety and a place of refuge and egress for occupants and fire fighters.
Figure 6 provides a typical section detail for load-bearing wood framing surrounding a 150-mm (6-in.) concrete masonry firewall intended for a large condominium project in Edmonton. Figure 7 illustrates a typical plan for wood framing around a 150-mm block stairwell.
For the floors to provide lateral support of the firewall, it may be necessary to provide long slotted connections (approximately 76 mm [3 in.]) into S-P-F wood blocking between the wood joists to provide for movement as can be seen in Figure 8. The advantage of the wood framing eliminates the need for ‘fire connectors’ (which break away to prevent unintended loads on the firewall from collapsing elements tied to the firewall). The wood framing surrounding the connection is consumed by the fire, preventing unintended additional loads on the firewall and remains laterally supported by wood framing on the protected side of the wall.
The additional cost of using masonry to construct the stairwell walls, elevator shafts, and two-hour firewalls on a four-storey multi-family building with 80 units in Alberta was estimated at approximately one per cent of the total cost of the building; this translated into an approximate cost of $2500 per unit in Alberta. This cost represents the addition of CMU without the removal of wood-framed, gypsum-sheathed stud walls surrounding the masonry and utilized for bearing floor joists and wall services.
The combination of timber framing and masonry has been used successfully for centuries in Europe and continues to be an excellent means of employing the strengths of both materials. The addition of CMUs in critical areas of wood-frame buildings—where masonry has a proven history of fire performance due to its non-combustible and impact-resistant properties—enhances safety while taking advantage of the lightweight, efficient ease of construction provided by wood framing. The combination of the two materials appears to be an effective way to construct multi-family wood-frame buildings in non-seismic areas like many locations in Alberta.
It is important to note some concerns have been raised by the Association of Professional Engineers and Geoscientists of British Columbia (APEGBC) in its recently revised “Technical and Practice Bulletin on Mid-rise Buildings” about the ability of the masonry walls to perform integrally with light wood framing regarding the relative shrinkage and behaviour of the integrated system in seismic areas.2
However, there appears to be insufficient field data and lack of consensus from masonry experts to substantiate these concerns. Part 5 of NBC makes no reference to the APEGBC’s Technical and Practice Bulletin on mid-rise buildings. Nevertheless, it is always important to consult the local building codes, and authority having jurisdiction (AHJ) to ensure the integrated wood-frame masonry wall systems comply with local building codes and requirements.
Mark D. Hagel, PhD, P.Eng., is the executive director of the Alberta Masonry Council. He holds bachelor’s degrees in actuarial science/applied mathematics and civil engineering, and a doctorate in civil engineering. Hagel was previously employed as a technical services engineer for the Canadian Concrete Masonry Producers Association (CCMPA) and a building envelope engineer and structural engineer with the Calgary office of Halcrow Yolles. Hagel can be reached via e-mail at firstname.lastname@example.org.