NOTE: Make sure you read the first three posts (in order!) before tackling the rest, or it could be confusing: Post 1 is Designing the future, Post 2 is Setting up the problem, and Post 3 is Estimating basic requirements.


Wednesday, December 13, 2006

Global Warming meets Peak Oil Design

Continuing with the discussion of weather effects from a couple posts ago, there are a lot of other requirements we can define.

Jeff pointed out that we need to more carefully examine how much rain falls during the growing season, rather than just the whole year’s average. I’m looking for the data on that for our selected Iowa location and I’ll get back to you.

In the following draft requirements, the term “withstand” may need some further definition. The understanding of what withstanding something may vary from person to person – some might consider it to mean every part of the structure and farm stays intact, and some might imagine it means only the core structure must survive. Any thoughts on a better way to define this term?

The Homestead shall withstand temperatures of 115 ° F or greater.
The highest recorded temperature in the area is 102 ° F. Although the global warming models predict only modest increases in summer temperatures, I added a buffer to capture any error. If it gets hotter than this, we’ll be in trouble anyway. (See this -- courtesy of BigGav.)

The Homestead shall withstand temperatures of -30 ° F or less.
The lowest temperature recorded in the area is -28 ° F. In my estimation, it’s safe to assume we will not see temperatures lower than that over the course of the Homestead’s lifetime. In fact, we should consider whether -30 ° is too restrictive and could needlessly increase the cost of the design.

The Homestead shall withstand no less than XX lbs. of accumulated snow load.
I need to find the seasonal snow depth maximum.

The Homestead shall withstand sustained winds of no less than XX miles/hour.
This exact number is up for debate. Typical maximum winds top out at about 60 mph over any sustained period of time, but I haven’t found any models of increasing storm intensity due to global warming.

The Homestead shall provide shelter for no less than 4 people from tornadic winds (300 miles/hour).
This does not mean that the house has to be designed to resist tornadoes – that would be near impossible to achieve. Rather, the requirement implies some sort of storm shelter – a room in the basement, a storm cellar, or a standalone reinforced building.

The Homestead shall withstand no less than 50” precipitation per year.
This requirement is designed to capture the maximum expected yearly precipitation in a year at our location.

One thing that occurred to me in developing these weather requirements is that we haven’t specified how long we want the homestead to last. Climate models typically don’t publish results past 2100 – and some research predicts further dramatic warming after than, depending on how the anthropogenic (human-caused) forcings change. Do we want this homestead to survive for our grandchildren? Our great-grandchildren? 200 years? More?

The further out we place our target end-life for the homestead, the more uncertainty we encounter.


At 6:17 PM, Anonymous jeff said...

Suggestions on using terms:

1. Look to the science being used to see if it has a more refined definition.

2. Use a glossary to define terms.

3. Use more explicit terms.

In the way withstand is being used here, to me it means that something accomplishes was it was intended to. For example, a rain storm with high winds may tear off some shingles, but during the storm the roof won’t leak. I would think systems engineering has a way of dealing with terminology.

"Shall" is another one of those fuzzy words in that some people see as being optional and others see as non-optional.

Also, does "The Homestead" refer to the land and all the structures on it, or just the house itself? Do a roof and a thin, narrow fence both have to withstand the same XX pounds of accumulated snow load?

The end life of The Homestead Project. No matter how you slice it, this is a tricky one. You could strictly look at climate change forecasts with an eye just on temperature and precipitation, but that would be ignoring the huge *effects* of climate change we're already experiencing today from just a mild increase in temperature. The *rate* of change of the *effects* of global warming is for now going to continue to rise, and this can’t be overstated. I'll let smarter types come up with the appropriate way to design for an uncertain future. Personally, I wouldn't design unmovable things to last much more than 50 years. We have at least 10 more years of affordable oil, and in that time technologies will advance, better farming methods will emerge, and greener ways of living will come to light. I see projects like this being a work in progress for a very long time.

At 11:02 PM, Blogger PeakEngineer said...


"Shall" has very specific meaning in Systems Engineering -- see this link for a quick explanation. Using "shall" specifies a mandatory function.

A glossary should indeed be used for technical terms, but in this case we're looking for a common-language word not subject to misinterpration. An alternate option is to reference a specification with the requirement statement -- one that specifically defines what me mean by "withstand" for example. The goal here, as in all of Systems Engineering, is to avoid ambiguity as much as possible. Perhaps a spec reference is the route to go in this case since we are trying to capture a potentially complex idea.

In our context, the "Homestead" refers to the entire system that meets our goals with the given inputs and outputs. In that sense, yes, we are imposing the same snow-loading requirements on every part of the system -- from the fence to the barns to the cistern (if they exist in the design). Our system-wide requirements mandate what the system as a whole must meet.

At 12:41 AM, Anonymous jeff said...

Thanks for the link, looks useful.

It's odd that systems engineering defines the word shall and not the word withstand as they're both common-language words. Such is life.

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