The 1965 film adaptation of the novel by John le Carre with the same title stars the ever charismatic but mysterious Richard Burton. While the film is based on the cold war spy industry and is steeped with figuring out who are the real undercover agents and possible defectors, our story is much simpler but hopefully does not have a tragic ending like the movie.
The S… coming in from the cold in our case is the structure that is continuous from interior to exterior and entails figuring how to limit the baggage it is trying to carry from the outside to the inside. The baggage is the transmission of the exterior cold to the inside conditioned space of the building. In reality the physics of the situation is that heat is being lost at such a rapid rate through the structure to the cold exterior that it lowers the temperature of the inside structure near the perimeter to the point where moisture in the air will condense on the surface and drip down on interior finishes. Bad situation.
We have studied 3 scenarios. The first is a laminated wood beam. The second is an exterior steel tube sleeve that slips over a stub of the interior structure that cantilevers outside. The third is a steel tube structure that has a thermal break connection right at the line of transition of interior to exterior. Keep in mind that the exterior portion of the beam is not decorative and must perform structurally in holding up a portion of the roof while cantilevering from the exterior wall line. This is how they play out.
Laminated Wood Beam
Wood has very low level of thermal transmission. Imagine holding one end of a stick and plunging the other end in liquid nitrogen. You could probably hold the stick with your bare hand. However with a metal rod, the entire rod would get cold very fast and you would need a protective glove to hold it. On the down side, the laminated wood beam has not proven itself to hold up well to the exterior annual Oklahoma weathering cycle of UV, moisture, heat and cold. One approach would be to clad the exterior portion of the wood beam but with what? Metal would seem the most effective, but with questionable results in achieving crisp corner lines and no oil caning (wrinkling) on flat surfaces. Would the wood under the metal cladding deteriorate over the years? The desired design effect is to have the same material seen inside be the material seen on the outside.
Steel Tube Sleeve
The interior steel tube must narrow down in cross section as it transitions to the exterior so that it can be inserted inside of the exterior steel tube sleeve. The narrowed down portion could be either steel or even wood. Narrowed down steel could be bolted or welded but then a thermal break material must separate the sleeve from the steel stub. A wood insert would have its own natural thermal break but would also have to be connected to the interior steel tube. The exterior sleeve must then be slipped over the stub, leveled and attached firmly.
The Thermal Break
In this scenario the interior steel tube comes up to the exterior wall line and has a steel plate welded on the end. The exterior portion of the tube has a matching steel plate where it meets the interior tube. An inert material is placed between the plates so that exterior steel is not touching interior steel creating a thermal break. The connection is made with bolts between the matching steel plates. The bolts themselves can have an inert washer which will reduce any thermal transmission through the bolt itself. Brilliant. The question is can the bolted connection provide the structural integrity needed to allow the cantilevered portion of the beam to carry its portion the roof load and transfer it back to the load bearing point at the exterior wall line. Calculations confirm that the bolted connection will perform.
The design team of architects, engineers and construction administrators has differing opinions on the pros, cons and acceptability of each scenario. It appears that all three could be made to work structurally but with different degrees of complexity and aesthetic results. We want a solution that will be long term. In the end, the Thermal Break approach seems the most simplistic, would be straight forward to construct, solves the problem of thermal transfer from interior to exterior and achieves the aesthetic goal of having the visible structural material be the same inside and out. We go with it.
This is what figuring out how a building goes together is all about for the design industry. Multiple disciplines working together in a collaborative setting, tackling problems and coming up with the best design solutions.