DETA IL ING IS SUE S The detailing of hybrid structures must reconcile the differentiated movement of steel and wood due to temperature and moisture. There are analytical programs available now to help set up the structure needed when combining the materials. A fabricator that accepts a hybrid timber and steel project should be familiar with this software, as it assists greatly in detailing. This view of the fit between the steel and wood sections on the Brentwood Skytrain Station in Vancouver, BC, Canada shows how much of the interface between materials is hidden inside the wood member. The timber has to be carefully cut to fit the steel insert.
Some detailing will require that movement is accommodated in the connection itself. In some cases, slotted holes in the steel can allow for some movement of the wood. This runs counter to most AESS work, where the tolerances are half standard and a high level of precision is required in the sizing of the holes. The expansion and contraction of the wood must still allow the connection itself to remain aligned. As the steel connections themselves will not move, it is critical that the connectors do not span the full depth of the timber members, as the timber will change shape over time and a restrictive connection could result in the splitting of the wood at the connection. It is paramount in creating a hybrid structural system to work with the strengths of each material and to appreciate the context in which each functions optimally. For example, if designing a simple truss where the individual web members, as well as top and bottom chords, are to take either compressive or tensile axial loading, steel would be a more appropriate choice for the tensile members and wood for the compressive members. This will allow the tensile members to be very thin — able to be fabricated as slender as rod elements. The timber can be heavier in cross section, thereby expressing its resistance of compressive loading. Left: The Gene H. Kruger Pavilion at Laval University in QC, Canada, designed by the consortium Les Architectes Gauthier Gallienne Moisan, uses light steel rods as the bottom chords of the wood trusses. The compression members have been constructed from timber. Right: The detail of the connection shows how the steel connection plates have been inserted to slots in the wood and bolted. The tension members connect to a rectangular steel ring that is simply bolted to the bottom of the truss post. This provided a means to neatly resolve the connection of the six rods to a single point. The wood members are free to expand independently of the steel. The hybrid trusses that clear-span across the wine production area at the Jackson Triggs Estate Winery in Niagara-on-the-Lake, ON, Canada, designed by KPMB Architects, illustrate a balanced combination of steel and wood. The steel members, more slender in nature, provide the tensile forces in the truss. This contrasts with the relative roughness and bulk of the sawn timbers.
As wood tends to expel and acquire moisture over its life, unprotected steel cannot come into direct contact with the timber or oxidation is likely to occur. The steel can be protected by being galvanized or through the application of moisture-resistant paint systems. It will help to use dry wood in the first place, which also assists in limiting differential movement. From the perspective of aesthetic balance in a hybrid AESS design, there should be enough of each material to result in a complementary use where the tectonics of each contributes to the overall design appearance.
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