Trusting The Weatherman

Designing to a Standard

It is an interesting fact that many projects are "over-designed".  This is nothing especially new but it seems that we are seeing more of it lately.  As an example we are currently working on a project that will be located north of Detroit but is being designed to operate at temperatures that exceed the ASHRAE 0.4% design criteria for Phoenix.  On the surface this seems to be overkill in the extreme.  The increase in capital costs for equipment that will probably never have to perform to that level could easily drive the project over budget.

The psychology behind making design decisions of that type basically indicates a lack of confidence.  The end-user chooses to ignore the ASHRAE climactic weather data and recommended design points because he or she lacks confidence in the data.  Personal experience of temperatures that exceed the published design conditions add to the lack of confidence in the recognized standard.  ASHRAE has tried to address this by also publishing the 10, 20, and 50 year maximum (or minimum) recorded temperatures.

This criteria is similar to the "100 year flood" criteria that civil and site planning engineers use.  Many of us have seen, or experienced, times when the "100 year flood" line has not only been crossed but crossed multiple times.  At a recent meeting that I chaired we had a presentation by a well regarded environmental and site planning engineer.  The presentation showed how the location of various coastal high water design lines have changed over the last few years...moving further inland and changing the flood insurance status of existing structures that were originally well outside of the potential flood area.

Could it be that the climate is actually changing as many people suggest?  Do we need to revisit our temperature design criteria more often?  The alternative is to ignore the standard and add an arbitrary "risk premium" to the design criteria...adding costs that might not be necessary.

When I was a young consulting engineer many years ago I was told to design to the ASHRAE design points.  One reason was that by using a recognized standard I could always fall back on that point as evidence that I had used proper engineering practices in my design.  The nature of mechanical equipment was such that most systems ended up over-sized anyway and could throttle their performance to meet the criteria.  If owners today add a "risk premium" to their design criteria...and then the mechanical equipment also ends up over-sized...then the capital costs and system capacities are doubly over stated.  As we move towards a market where building operating characteristics are posted by the front door, much like an automobile's gas mileage rating, the practice of arbitrarily over-sizing systems will put some owners at a disadvantage when it comes time to lease the space.