The risk of complicated structures
A while back, I posted in Dreamers versus Plumbers about the comprehensive failure of the climate-control system at Harvard’s Otto Hall. The building is being demolished only seventeen years after its completion, because the ultra-sophisticated humidity-management system was too space-age for its own good.
“Missed it by that much.”
Architects should never have the final say in designing any building.
Property development is susceptible to what I call the ‘ribbon-cutting trap’ – the tendency to declare that completion of the building is a successful finish. In reality, it’s only a successful beginning, because building is easy, operating is hard. Maintenance is unforgiving to wishful thinking […] where I come from, water is a property’s greatest enemy. That’s why water and sewer systems are so critical to building integrity, and more fundamentally, why it’s inconceivable to us that housing could exist without water and sanitation.
I got a puckish email from faithful reader Matthew Healy, pointing me to some articles on condensation in vapor barriers by Tim Padfield, whom I presume to be the world’s foremost expert on the physics of building walls. [If Harvard or MIT hasn’t signed him to be an expert witness in their architect litigation, both should.]
A man who knows how glasses do or don’t fog up: Tim Padfield
Mr. Healy pointed me to this great line, which might be said to be the epitaph for Harvard’s Otto Hall: “complicated structures will never be built accurately enough to realise the intentions of their designers.” In fact, I’m going to christen it Padfield’s Law of Construction Complexity:
Padfield’s Law of Construction Complexity ought to be graven on every foundation stone, pre-printed at the bottom of every blueprint.
“Don’t design what you can’t maintain”
Like a certain blogger we know, Dr. Padfield is a prolific author on a narrow topic, publishing pieces with titles like The interaction of water with paper in small spaces –
Figure 10: The warm wall evaporates water from the paper of the menu, which condenses on the cold glass, runs down and is re-absorbed by the paper. Because the sides of the enclosure are also exposed to the cold, condensation is worse towards the edge of the glass, obscuring the price while revealing the culinary delights on offer to the hungry traveller.
– and How to design climatically stable museums, as well as what reader Healy found, his paper Condensation in the Walls of Humidified Buildings, which describes Otto Hall with painful clarity:
Many museums in temperate climates are humidified in winter to attain the unofficial standard of about 50% RH at a comfortable temperature around 20C [68º Fahrenheit – Ed.]. Air conditioned museums are also usually pressurised, to prevent unauthorised entry of air that has not been filtered and humidified. The inside air will tend to flow and diffuse out through the walls.
Otto Hall had both.
Otto Hall: you wouldn’t know the skin was crumbling
As the air, together with its burden of water vapour, passes through the wall it will cool down. The relative humidity will consequently rise, if one assumes that water vapour moves at the same pace as the other constituents of air. We can use the atmospheric moisture calculator to show that air at 50%RH at 20C will reach 100% RH [Dew point — Ed.] at 9C [50º Fahrenheit – Ed.].
On a cold winter day this temperature will be reached at some point within the wall. Water condenses there and may later freeze. The water will also dissolve salts and move them around in the wall and will accelerate corrosion of metal fittings within the wall. The process is shown in the diagram. The temperature gradient is drawn diagonally down through the wall section. The picture also hints at a process that partially counteracts this process: water can evaporate from the outer surface of the wall.
Deposition of water in the wall is therefore dependent on several competing processes: diffusion of air through the porous wall, flow of air through cracks in the wall, absorption of water into the hygroscopic brick and finally, mixing of the humid air originating from inside the building with outside air in the outer layer of the wall.
Engineers have responded to this threat to the durability of the wall by inserting a barrier to air flow through the wall, usually a polyethylene foil. Other professionals in the building trade then make holes in the barrier to insert electrical cables, bolts, pipes and just nails.
Just what happened in Otto Hall.
This has resulted in some spectacular failures of buildings that were designed with an attention to detail appropriate to constructing a jumbo jet but built in the more relaxed tradition of the building industry
You sure about the assembly here?
As he continues, low-tech means low-maintenance-risk:
I have never seen comparable phenomena in the simple, solid walls of uninsulated, porous brick that are typical of older houses that are used as museums. This must be because the fine balance of processes described [above] results in a wall that is seldom wet, even in very cold weather.
It seems to me that museum designers should be investigating the advantages of walls of homogeneous, porous material with thermal insulation distributed evenly right through the wall section. This will avoid the cracks, steep temperature gradients and impermeable layers that cause water to accumulate. It will also tend to make walls that have a large thermal inertia, which provides extra stability to the inside climate.
In another essay, The Window in Context, Dr. Padfield observes
“Designing a structure that is robust enough to [work even if] badly built and intermittently maintained is just as important to the life cycle costs as is the energy consumption from day to day.”
These principles apply equally well to financial structures. When I was young and foolish, I liked very complex financial rules, incentives, and compensation structures. As I’ve had more experience [you mean, aged — Ed.], I’ve simplified rulemaking extensively, believing that simple rules not only after a better chance of being followed – fuzzy boundaries are bad boundaries – but also, they serve as better motivation. That’s led me to this rule of thumb on joint ventures:
Rule number 1: measure twice, cut once
The best one says, “what’s mine is yours.”
That joint-venture-rule-of-thumb works well in my marriage, too J.
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