Construction detailing in mass timber buildings
Construction detailing in mass timber buildings
The advanced Rothoblaas X-Rad connector
Samantha Choles of Frog Communications organised the event for SSA. Dr Schalk Grobbelaar of the University of Pretoria and Braam de Villiers of Earthworld Architects facilitated the online seminar that was attended by over 70 people.
For many years, architects and engineers in Europe, Scandinavia, the UK, Canada and North America have used mass timber to construct single, multi-storey, and even high-rise, commercial, institutional, industrial, and residential buildings.
However, assembling a multi-storey building is much more complicated than haphazardly hammering in nails or using connectors and screws from the nearest hardware store. Structural detailing must be incorporated from the first step of the design phase.
PROPRIETARY CONNECTORS
He said local conditions often determine the ground connection systems. For example, in Europe, they use hold-down brackets covered by insulation to ensure airtightness. In South Africa, the emphasis is on durability.
"The durability of timber structures is closely linked to their correct design and construction. Rothoblaas has solutions for every aspect of the structure, including air and wind tightness, fire mitigation, and connections to the ground, doors, floors and windows", O'Connor explained.
He explained and compared the application of various products like plates, round head screws vs ring shank nails, large head screws, slot connectors, half-lap and spline joints, synthetic breathable membranes, vapour control barriers, sealing tapes and foams.
With 28 floors and 96m high, the Burj Zanzibar will be the world's tallest timber apartment tower
In his presentation, Kloos drew on his hands-on experience designing, specifying, manufacturing and assembling mass timber buildings.
"There is an overwhelming number of ways to connect timber. The nature of the connections is different to concrete and steel, and it is often the weakest part of the building. If a timber building fails, it will fail at the connection, especially in a fire. If you don’t plan and design connections, you can make or break a building", explained Kloos.
SCREWS
"However, for mass timber, the axial capacity of nails is significantly less than screws", Kloos said. "There are many types of timber connections. However, I am describing the three types that are, in my opinion, the most specified for mass timber".
Partial-thread screw: the smooth shank lets the screw pull and clamp two pieces of wood together if the smooth shank is longer than the thickness of the outer timber member. It is handy during the assembly phase.
- Timber to metal screw: The shank is thinner than the thread, and although it does not fit snugly in plate holes, it is perfect for countersunk holes in steel. The shank thickens beneath the screw head to interlock the plate and the screw.
Without meticulous detailing by the architect the mass timber structure of Bjergsted Financial Park would not be possible. Pic by Jan Inge Haga
Architects Helen & Hard designed special wood dowels for their award winning Bjergsted Financial Park in Norway.
Kloos said plates outperform screws because of their higher strength and rigidity. They are used with nails or screws extensively in multi-storey mass timber buildings. The factors influencing plate strength are their size, geometry, thickness and number of nails or screws used.
DESIGNING CONNECTIONS
"The strength is governed by structural codes that give design methods for shear and axial resistance. It is best to consult the supplier and catalogues for design tables and to pay attention to the edge distance and spacing between screws", he explained.
Corrosion is a durability threat to steel. "Embedded screws are protected by the wood except for the head. In addition, the timber species and wood preservative can affect corrosion. Plates are more exposed than screws".
Kloos said steel is a good conductor and quickly loses strength in a fire. On the other hand, "the humble screw embedded in timber is more fire safe. Even if the screw head is exposed, the charring does not penetrate". Kloos said steel plates are very vulnerable, and the solution is to "encapsulate plates with timber or non-combustible board like gypsum".
An example of a multi-purpose bracket connector from MiTek
Kloos said an important consideration from the project's outset is to "Design for fire resistance. Know what the fire rating should be and use the right connectors".
Then, "Keep the design simple", Kloos stated. Try to use the same screws with the same spacing on-site so that less can go wrong and to save costs.
"Remember, the complexity of the connection is proportional to the size and complexity of the building. A useful rule of thumb for timber connections is that many small-diameter screws, dowels and nails perform better than a small number of large-diameter connectors".
"Costing of projects is challenging", Kloos remarked. "The data pool of mass timber projects in South Africa is small and not consolidated. The cost depends on the connection design and factors like screws versus plates and off-the-shelf or proprietary. There is large supplier cost variation".
He presented a hypothetical example of a case study:
- The total cost of connections (only screws), including the handling fee, is R700 per cubic metre, amounting to 4,1% of the CLT cost.
Kloos is preparing the newest course on structural engineering of CLT in South Africa that will be available online by the end of the month.
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The Department of Trade,Industry & Competition (dtic), the University of Stellenbosch and the University of Pretoria support the series. It forms part of the Forestry Sector Masterplan and the long overdue drive to promote the use of wood and engineered structural timber products.
Written By: Joy Crane
