Originally written: Jan 2004
This is a report on where we currently are in our never-ending search for the lowest cost building envelope. Be advised that is an ongoing project, and that our position may shift at any time. Like anything else in the World of Design – Mother Nature included – when you go far out in one direction, the other elements of the design become “warped” from their usual aspect. If you design a tree that can live in the desert, you get a cactus. If you design a vehicle for maximum fuel efficiency; space, load, and speed performance are usually severely compromised. This is certainly true in architecture. What we are talking about here may have limited application because it produces a form/envelope that appears very different from anything in your lumberyard plan book. These buildings are more barn-like than anything else which should not be a surprise, as their historic roots are in fact, well – barns!
Those who have investigated what the storage/commercial building industry is putting out – or those of us who just observe what is in the built landscape – are well aware of the ubiquitous metal building. These structures consist of steel frames - or “bents” – on some large regular spacing dimension called a grid or a bay. These may run anywhere between 6 to 16 feet. The steel bents are sort of hockey-stick-like; that is to say that the “business end” of the hockey stick is vertically positioned to be the building’s wall and the handle angles up to be the roof “rafter.” They are thickest at the wall-to-roof joint because that connection is designed to be rigid and not “hinge” at all.
In a conventionally gabled and roofed box we know that the outward thrust from the rafters wants to push to walls out at the top plate. We usually resist that in one of two ways. The most common solution is residential design is to tie the top of the walls together with ceiling joists. This creates our common attic. When this floor plane would go through a space that we wish to use, we remove it and put in a ridge beam. By holding up the roof at the ridge we eliminate the horizontal thrust and we may safely remove the ceiling joists. What the steel bents do is resist this thrust as a bending moment. That is why the deepest strongest section of the frame is at the top of the wall; this is the derivation of the “hockey stick” shape. (In fact, this is why hockey sticks have the shape they do.) Now we need neither collar ties or a ridge beam.
The ordinary steel building has these bents erected at a regular distance – the bay size. Steel “Z” section girts are installed horizontally connecting the bents. Similar members are installed along the “handle” part of the bent and are called purlins. Corrugated steel panels are then fastened to the girts and purlins becoming the walls and roof. For insulated steel buildings, these panels may be double-skinned with urethane foam cores, but usually fiberglass batting is fastened and hung from the inside of the girts and purlins. These buildings are one of the most cost-effective ways to enclose large spaces. That is their strength and that is why they have become so popular in the agricultural and commercial landscapes – places where profit margins are notoriously slim. This combination of large spans at low cost could not be resisted and the applications have spread. One may now find them as school gyms, retail space – you’ve all been to Lowe’s or Home Depot, right? – swimming pool enclosures (“natatoriums” to the well-heeled), other warehouses and factories. It’s no wonder that they shouldn’t be explored for residential construction. We are now at a point where the curve of ever-increasing home size crosses the curve of large span structural technologies.
We were approached by a local school to design their gym some years ago and were pointed by the building committee in the direction of these metal buildings. I suggested we forego the limited life expectancy (20 year finish warranty) inherent in this building type, spend a tad more for an envelope that would last much longer and give superior performance throughout to boot. In short, scrap the girts and purlins, scrap the corrugated steel panels and install SIPs directly onto the bents….well, not directly. We recommend wood plates be bolted to the steel and the SIPs fastened to the plates. There are now on the market self-tapping fasteners that are claimed to be able to be used directly into the steel bent top flange. As you may tell from my tone, we have no experience with them and doubt this would work well in the field. This gym was built in record time with out any difficulty and the cost was quite low.
We all know that SIPs are available as “jumbo” panels 8 feet x 24 feet. We set the bents at 12 foot centers to fully exploit the economy of this size. We used 6” SIPs for the walls and 10” for the roof (EPS core) for a very solid structure with excellent thermal properties. We had the steel building manufacturer verify the engineering for the bents and fabricate and ship them to us. The General Contractor had no trouble erecting them and “prepping” them with the wood plates.
We have a job now for a local sculptor who requires a building that has living quarters and workspace combined into a large barn-like structure. We are recommending this same type of solution for his program. The budget is tight, but the owner is no fool and wants an envelope that will deliver on comfort and energy efficiency. When you think about it, the larger your house becomes, the more you are mindful of the heating and cooling costs. The SIPs will receive a drywall finish inside for fire-resistance and an exterior weather-skin of HardiPanels. Special care has to be taken to detail the foundation/frost-wall/radiant floor/wall/bent connections. We will need a proper thermal break between the floor and the outside. Typical metal buildings on slabs do not usually consider this section of the building carefully and are erected on a monolithic pour of a thickened-edge slab which defeats the insulation of the walls and roof. We plan to utilize at least 3” of XPS rigid foam for our frost wall insulation all the way down to the footing and the same under the entire radiant slab. This is for a 5750 degree day climate.
Our client has need of the large clear span space that this system delivers. Normally, for most conventional residential plans, SIPs may be satisfactorily deployed without need of any other structural elements except for perhaps a few purlins or a ridge beam. However, when large spans – say 30 to 80 feet – are required SIPs alone don’t work. Timber framing can do this, but at a far greater cost than steel. Like most things in life, the greatest strength is also the greatest weakness. To the degree that these structures are cost effective for large spans, they are the reason that for the “normal” spans called for in most residential designs – 12 to 16 feet – they are uneconomical. Fortunately for us SIPheads, they are also structurally unnecessary.
Although this kind of system will have limited residential application, certainly with respect to “traditional” plans, there remain the few on the outer fringe who will find that this combo-package of SIPs on steel bents at 12 foot centers will meet their needs for large clear spans and a superior thermal envelope. All at a very modest price. For commercial applications, I think we are on the toe of an asymptotic up-swinging curve. While this is written during a period of record-breaking low temperatures, I can only hope that those who commission commercial structures will take note of this and not fall back on those steel buildings that, like fast food franchises, make no contribution toward regional architecture.
Originally written: Jan 2004
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