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Q&A with Ged Finch - Circular Economy

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In September 2019 Ged Finch shared his PhD research findings to date in a PrefabNZ InnovationBites Webinar (watch it here). Ged's research looks at how prefabricated building systems can play a major role in achieving net-zero carbon through designing for end-of-life material recovery and reuse.

We received many questions during the Webinar and Ged has responded to these below:

1. Have you thought about steel screws to substitute concrete foundations for lightweight timber structures?

Yes! Steel screw pile foundations are an excellent recoverable, reusable foundation option for low-density prefabricated construction. A standardised screw pile cap can be bolted directly into the pre-routed holes in the X-Frame node plates.

2. Is there any research showing the reality of building life spans and actual reuse in practice? If the focus is on being able to reuse timber then I expect more emphasis on airtightness and moisture management to ensure it lasts for a very long time.

Yes, exactly, this is one of the key requirements of designing timber systems for reuse. The structure must be kept dry (or be allowed to dry out quickly) and there must be a record of the load that is applied to the structure over the life-span(s) of the timber product. We are using the Australian Interim Industry Standard for Recycled Timber – Visually Stress Graded Recycled Timber for Structural Purposes (PN06.1039) as a guide to understand the estimated lifespan of well-protected timber products. A research thesis at Victoria University carried out in 2017 also tested the structural properties of recycled pine from a range of different buildings and uses and found no structural degradation issues when compared against new timber (see: The structural reuse of Pinus Radiata in New Zealand by Nick Forbes).

3. Are there diagrams of the screw/bolts fitting together?

Sure, assembly diagrams for the connections and joints in X-Frame are available here.

4. Are Earthships net-zero or close to it? Any example of reuse in Aotearoa after the old USA EarthShip?

Earthships are a great example of net-zero architecture. They do however have their own problems. Earthships work best when they have a large northern elevation, a dry climate and lots of space around the building for supporting services (ground-source heat-pumps etc). The need for these site conditions, as well as the distinctive aesthetic characteristics, limit the mainstream applicability of Earthships. They also face further challenges in New Zealand due to seismic design requirements. A similar, and somewhat more conventional approach, is rammed earth construction. These buildings have very low embodied carbon due to the substitution of concrete for soil. Operational carbon is also reduced due to the construction having a large thermal mass (thermal latency). Again, however, rammed earth has its own set of limitations. The main issues being that construction is labour intensive and for that reason can be very slow. They also often require larger eaves to protect the walls from moisture ingress. Additionally, some modern methods of rammed earth construction will include cement in the earth mixture for added strength. Adding this product quickly reduces the net-zero capacity of the building. For those interested in the Earthship concept visit their website here

5. How will TAs view a recycled building - regarding BC and lifespan of reused products and materials?

This is a major issue that has, to date, been the key barrier to reusing building materials in a structural capacity. Currently, there exists no legislation governing how materials can be reused structurally. The consequence of this has been that when materials are reused engineering sign-off is required to effectively manage the risk. An example of this is when large ‘feature’ recovered bridge-beams are used in architecturally designed buildings. As mentioned earlier, Australia has attempted to counteract such issues by introducing a guideline for using recovered timbers. Yet the issues of risk/uncertainty mean that reuse remains very rare. Our proposed workaround is to use the CodeMark certification platform to validate the reuse capacity of the X-Frame system. This would work by having X-Frame tested in different re-use conditions by an independent CodeMark certified structural testing organisation.

6. How do you address the wiring/ plumbing within the system?

Electrical services can be installed using techniques similar to those used in conventional platform framing. The advantage of installing electrical services in an X-Frame building is however that the frame is internally porous. At each structural node, there is a 40mm by 120mm rectangular void for services to pass both horizontally and vertically within the structure. This eliminates the need to drill through timber members and makes modification to electrical services after the building is finished much more straightforward. Plumbing services can be treated in much the same manner except where the pipe diameter is greater than 40mm (i.e. 2nd-floor toilet drains). In this instance, it is possible to use a half-frame module and then frame around the drain as you would in conventional platform framing.

7. Do we have a comparison between conventional stud frame & x-frame for material, structural, and efficiency?

Yes, these are all key metrics in measuring the applicability and relevance of the X-Frame system. In material efficiency terms X-Frame uses 7.9kg of timber per metre square of the wall compared with 8.1kg when using 90mm studs at 400mm centres. It should be noted here however that in this instance X-Frame is creating a 120mm thick wall to allow for greater levels of insulation. If weight is compared by wall thickness (imaging that 120mm platform framing exists for calculation purposes) X-Frame is 36% lighter per square metre (7.9kg vs 10.8kg). This efficiency analysis is complicated further however by the fact that engineered timber products are much denser than solid timber. If the volume of timber is used as an efficiency measure (which is relevant when considering thermal bridging due to framing timber) X-Frame is 57% more efficient that platform framing (of equivalent thickness).

For structural integrity X-Frame archives 54 bracing units per metre of frame for earthquake loads (BUs/M.eq) when un-lined. As we do not test conventional framing for bracing when unlined it is difficult to compare the systems. For context, however, plasterboard lined wall will without specialist anchors will achieve approximately 60 BUs/M.eq and with engineered hold-downs approximately 90 BUs/M.eq. During testing to-date, X-Frame’s proposed hold down solutions were found to be inadequate and it this, therefore, hoped that with a revised design it will be possible to achieve a similar bracing capacity to platform framing with engineered hold-downs. 8. What are the best types of connections when designing for a circular economy?

Dry jointing is a key requirement of designing for deconstruction - gaskets, bolted joints, reversible clips, friction and pegged connections all ensure that materials are not contaminated or damaged when separated. Tapes, spray foams, plasters, fluid-applied sealants and nails all cause issues when attempting to separate materials at end-of-life.

9. How realistic is to have those materials disassembled and reused with sufficient performances and compliances? What are the strategies you figure out to encourage reuse and make it viable?

Regarding compliance, the ideal approach would be a CodeMark certified product (as mentioned above) that is tested for multiple use cycles. Failing that it would be necessary to test the components coming out of a building before they are reused. This is something that can be achieved through a visual inspection process (such as the Australian Interim Industry Standard for Recycled Timber – Visually Stress Graded Recycled Timber for Structural Purposes (PN06.1039)). If that is also an issue for regulators then components would be required to be re-tested for strength and durability in the same way new materials are today.

10. How are the authorities responding to the novelty of your proposal?

This is very challenging as you can imagine. We are yet to formally engage with a local authority and go through the consenting process as we need a willing client who is prepared to come on that (potentially slow and painful) journey with us, any volunteers?! We are however actively working to address concerns that we know will arise during the consenting process. For example, destructive structural testing is carried out regularly to create a trail of engineering evidence that gives the project engineer confidence in the system. Our biggest concern at the moment is the lack of a H1.2 treated plywood product available on the market. Without this, durability is likely to be a major hurdle. We could switch to H3.2 treated plywood, however, this goes against all of the circular economy concepts underpinning the proposal. Talking to plywood manufacturers in New Zealand it appears as though it is straight-foward to create a suitable plywood product and is only a question of volume to make this worth-while.

Fire is also a concern as plywood structures have very little charing capacity. If a fire rated wall was required there would be a need to protect the structure using a fire-retardant sheet lining (plasterboard would be acceptable). There is precedent for this with Facit Homes in the United Kingdom using such techniques.

11. I would be interested in seeing a more detailed comparison on the benefits of the new plywood system compared to standard timber construction in terms of environmental impact, as well as a cost comparison.

A life cycle analysis comparison between platform framing and X-Frame is available here (see page 246). And a cost comparison (some-what out-dated) between platform framing and X-Frame is available here (see page 142).

12. Do you have a video of the assembly of X-Frame?

Sure! Some frame assembly and disassembly time-lapse videos are available here. Updates on new prototype building projects can be found here.

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