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December 2020 #12257 Page #104
The Speed and Safety of Roof and Floor Cassettes By Greg Watson
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n the quest for site speed and safety, panelized systems for components, such as roof trusses and floor joists, are the natural progression from installing single frames. In the United Kingdom, these panelized systems are becoming increasingly popular for volume house builders and Social Housing Associations. This allows for rapid erection of the unit’s super-structure – often in hours. Many of the benefits and challenges are shared with prefabricated wall panels, which I discuss in my previous article, Integrated Closed Wall Panel Systems. However, I would like to highlight specific considerations to these panelized systems, or “cassettes” as they are more commonly known, and I will distinguish between roof and floor cassettes.
Roof Truss Cassettes These tend to be constructed at the job site on the ground, then lifted up to wallplate level. However, this is not the beginning of the process. It begins with design and certain considerations must be taken into account. Roof Truss Design The design of the roof trusses is essentially the same in regards to architectural and structural engineering. The difference comes into play when we think about how we are going to raise this structure as a single unit or in predefined sections. The first consideration is the proposed lifting beam positions, and the most common place for this Roof cassette by Truro Timber Frames Ltd is the node points between a fink truss diagonal web and the rafter. In the truss design software, we need to remove the bearing at wall level and input bearings at these node points. When we then run the design, you will note the nail plates will increase in size to allow for the additional stress and forces now exerted at these points. Depending on how your software runs, we can design the lifting beams in a combined fashion. Please note that you should have two design files for the job site, one for the general design and one for lifting, but make sure that the correct nail plates are present in both. Benefits & Challenges of Roof Cassettes One major benefit of utilizing roof cassettes is increased safety, as all of the frames can be constructed at ground level. That means the majority of the work crew will not experience the risks related to working at heights (nor will they need the extensive fall protection equipment). In addition to this, there is a cost savings in the reduced requirement for scaffolding around the structure. Smaller cassette sections can be assembled PHONE: 800-289-5627
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