In 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 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 at the component manufacturer and then delivered to site; however, larger trusses may have to be erected at the job site due to transport limitations. It may be the case on some smaller jobs that an entire roof structure may be lifted at once. If site space is at a premium, however, there may not be sufficient space to construct the roof structure at ground level.
Floor Cassettes
Turning to the floor structure, here joists can be utilized in cassette form. One thing to note is that this method can be used for solid joists, I-joists, metal web joists, and floor trusses. Unlike roof cassettes, floor cassettes are much more friendly to transport to site from the component manufacturer. Again, let’s go to the start of the process and explore the design considerations.
Floor Joist Design
When looking at the design of a floor cassette, we need to consider certain aspects specific to the cassette itself. Factory-fitted lifting straps may be installed and we need to make sure that these positions are taken into account in the design. The joist orientation – such as upstanding load bearing beams and how these interact with the limited width of the cassette – also needs to be taken into consideration. In some instances, HVAC or plumbing can be factory-installed at the component manufacturer. Again, this is another factor to consider when designing the cassettes and how these runs progress though each cassette and through the building.
Benefits & Challenges of Floor Cassettes
The primary benefit of floor cassettes is the speed of erection. The entire floor structure can be completely manufactured offsite and stacked on the transport trailer, which allows entire floors in a multifamily building to be erected in a very short space of time. A shared benefit with the roof truss cassettes is the safety aspect of reduced time of contractors working at height. The addition, including services such as HVAC and plumbing reduces contractor time on site and thus reduces costs further. One of the main challenges of floor cassettes – certainly for the component manufacturer – is the labor time to assemble them and the added responsibility of including services.
Having briefly highlighted some of the considerations, benefits, and challenges of roof and floor cassettes, I think you’d agree that these cassettes ultimately reinforce the argument for offsite manufacturing in the construction industry. Speed and safety are the key advantages of these systems, from which the majority of house builders and construction companies would ultimately benefit.
With 16 years in the roof truss and timber engineering industry, working in the UK, Europe, and North America, Greg Watson currently lives in Scotland and is the Design Manager at a Northern Irish component manufacturer. He has a passion for large and complex timber structures and seeks to promote timber as a sustainable building material.