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April 2023 #15285 Page #94
Glued Roof Trusses By Tuomo Poutanen
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n my previous article, The Evolution of Glued Trussed Joists, in the August 2021 issue, I explain that glued trusses have potential in floors. The new G-joist has high resistance. The chord shear failure is eliminated as the web finger punches the chord. The web-chord glue area is big. The mean glue shear stress can be fixed at an exceptionally reliable level, less than about 44 psi (0.3 N/mm2). Such a small shear value is reliable even in earthquake loads. The G-joist can be used for roofs of residential and commercial buildings up to 100’ (30 m). Glue is the cheapest, stiffest, and often the strongest timber fastener. Trusses are the most effective structural model in horizontal structures with the least material and the best flexibility. Prefabrication and automation are demands for effective construction. This means that the horizontal structures, floors and roofs, are made of shallow joists with uniform height. The glued timber truss has the lowest depth-span ratio, and it is the most cost-effective, flexible, and ecological horizontal joist with uniform height. Currently, glued trusses are not applied to roofs. In this article, calculations are presented to show that glued timber trusses have potential in roofs. A recent master’s thesis shows that a roof made of parallel chord glued trusses is more cost-effective than a glulam roof or a metal plate connected (MPC) wood truss roof up to 100’ (30 m), which opens a promising new business possibility.
Glued Residential Roof Trusses The MPC wood truss dominates the residential roof market due to its low cost, sophisticated technology, and flexibility to accommodate multiple geometric and structural forms. However, the MPC wood truss has some deficiencies: •
The MPC wood truss needs height, which means that the prefabrication of the complete roof structure is not feasible. The roof must be constructed on-site from prefabricated trusses.
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The MPC wood truss is not effective for a house with an attic, because either the trusses will be very expensive or the attic space will be limited.
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Having a roof opening in an MPC wood truss roof is complicated and expensive.
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