At last, we’re finally ready to put together the design for our house.
A fully specified design, with every detail and system described, is well beyond the scope of this newsletter (not to mention the scope of my knowledge). But we can give a high-level schematic design, laying out the basic building systems and how they’ll work together to satisfy our exhaustive design criteria.
Foundation
The foundation supports the weight of the house, transferring its load into the ground below. It won’t get replaced unless something has gone catastrophically wrong, so we’ll need it to last the entire 1000 year lifespan of the house.
Typical single family home foundations consist of reinforced concrete, but reinforced concrete is a poor choice for a long lifespan building due to its susceptibility to corrosion. Unreinforced concrete is a much better choice. Unreinforced concrete was used for the foundations of the Roman Pantheon and Colosseum, and is still used today for temple foundations or other structures with extremely long design lives. We’ll want to use a specially designed, slow-curing mix to prevent the accumulation of thermal stresses – unlike conventional concrete which reaches its design strength in 28 days, ours will need at least 90.
Houses are light enough that their foundations can almost always bear directly on the soil. But this leaves the house susceptible to soil settlement and erosion – to prevent this, we’ll set our foundations on piles extending down to bedrock. These will be unreinforced as well (though corrosion will be less of a problem so far below ground).
Even though the load bearing elements are only on the exterior perimeter, we’ll extend the foundation under the entire footprint of the house. This should help prevent the dampness that’s often an issue with crawl space foundations (though we’ll want to make sure that this gets detailed in such a way to prevent water from accidently accumulating).
Structure
On top of the foundation sits the framing. Our framing must do several jobs:
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It must resist normal gravity loads, as well as potentially extremely heavy lateral loads (earthquakes, tornados) or other extreme events (a tree falling on it, etc.)
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It should be strong enough to not sag significantly over centuries of life
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It must be resistant to corrosion and noncombustible
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It must interfere as little as possible with future rearrangement of interior partitions, or of building additions
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It must accommodate changes to building services as technology advances
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It must be relatively simple to repair or modify
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It should be legible – it should be easy to understand what it is and how it works in the absence of drawings or other information
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It should last the lifetime of the structure
It’s not trivial to accommodate all these requirements. After consideration, I’ve opted for a steel moment frame using built-up stainless steel plate girders and columns.
With this system, we can make the load bearing capacity essentially as high as we want it to (high enough that we should see very little sag, even over the long term). For lateral resistance, we’ll use a bolted fuse system (something like a Durafuse) at the joints, which will make repairs easier in the event of an earthquake or similar event. Because stainless steel has extremely high corrosion resistance, it’s unlikely to corrode even in the event of a severe building envelope failure.
This system will allow us to keep the interior of the house completely free of load bearing elements like walls or columns, making it trivial to rearrange the interior layout at a future date. And because it has a relatively small number of exterior columns, it gives a great deal of flexibility for exterior renovations, making it easy to expand add an addition, enlarge windows, or move doors.
We’ll also use a crawl space design, with the first floor supported by structural framing instead of the ground – this will allow us to easily repair or replace services in the future, as well as giving us some small degree of flood protection. To add to the flexibility, we can fabricate our beams and columns with openings in them to allow the future passage of services.
The flexibility and durability of this system should make it easy to justify renovating it. Substantial renovations of old buildings tend to be more expensive than simply tearing the old building down and putting a new one up, but steel frames are easy enough to reuse that keeping them is often the cheapest option. You frequently find them being repurposed even in buildings with no preservation value, like warehouses and industrial facilities.
Stainless steel plate girders is an unusual system for any building (much less a house), but conceptually it’s no different than conventional steel construction, and it should be understandable by future builders. And it’s highly tolerant of modifications – new components can simply be welded or bolted to it as needed. The one complication is that attaching dissimilar metals can lead to galvanic corrosion, so they’ll need to weld stainless steel or attach items using a non-conductive spacer. But this seems like an acceptable risk.
The main drawback of this system is fire resistance. Stainless steel is noncombustible, but it’s not fireproof – at high temperatures steel quickly loses strength, and stainless steel is no different (though stainless does have better high temperature performance than mild steel).
We have a few ways of dealing with this. Using exceptionally thick flanges and webs should add some fire resistance, and will get us sufficient margin of safety that the building can remain standing even if the steel loses a substantial fraction of its strength. On top of this, we’ll protect the steel with something with a similarly long lifespan (which rules out spray-on fireproofing or intumescent paints). The best option here is something simple – we’ll encase the steel where possible with a thick layer of brick masonry or lightweight concrete panels.
These should limit the potential for fire damage – but we’re still taking a risk. For the truly paranoid, we could substitute the stainless steel for something like Inconel, a Nickel-based “superalloy” that maintains its strength at much higher temperatures (in addition to being even more corrosion resistant than stainless). But not only would this be phenomenally expensive, it would add extra complexity that would make future modifications harder – the Venn diagram of “builders” and “people who understand how to repair or modify Inconel” looks like an eight. So we’ll stick with stainless.
Another drawback is weight – ideally our sections would be light enough that they could be manipulated by hand, but our steel sections are likely to weigh in the neighborhood of hundreds of pounds per foot. Since the steel is designed to be as permanent and unobtrusive as possible, this also seems like a worthwhile tradeoff.
We’re also taking something of a risk using something as valuable as stainless steel – a common failure mode for buildings is for valuable material to be ripped out and repurposed. This can range from looters ripping the copper piping out of a house to sell for scrap, to Londoners reusing the stones from ruined Roman buildings, to countries at war melting down building components to make munitions. I don’t see an obvious way of addressing this problem – the risks of corrosion we’re avoiding with stainless steel seems like it’s worth the tradeoff, and covering it wit
