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SI Units for AASHTO

SI Units for AASHTO

SI Units for AASHTO

I'm an engineer in Japan and have recently been studying AASHTO. I was wondering if there was a particular reason as to why they discontinued publishing the SI units.

Also I'm wondering if the conversions in the SI units are still valid. A good example to describe what I mean is the clause on concrete cover. I understand the section for concrete cover and the values listed for different structures have not changed since the last SI version was published. If you compare the values, the US units would say something like 2.0(in) for a particular structure type (50.4 (mm) technically) but in the SI units it would be 50 mm. If I applied a concrete cover of 50(mm) but was using the latest US units version, would this be deemed as not satisfied.

RE: SI Units for AASHTO

This comes from a decision to use either a "hard" conversion or a "soft" conversion to SI units. A soft conversion is a direct numerical equivalent based on, say, 25.4mm per inch. A hard conversion is when the nearest appropriate SI unit is used, thus the 50mm cover as compared to 50.4mm. AASHTO and many of the state departments of transportation decided to use hard conversions in their manuals to prevent confusion.

RE: SI Units for AASHTO

there was a push about 20 years ago to finally switch the US DOT to SI units. After a brief time, it was clearly not well received by the construction and engineering community. A decision was then made to remain with the current system and not switch. Currently, the US DOT uses U.S customary units so there is virtually no reason to be concerned with the converted SI units.

RE: SI Units for AASHTO

Two inches is 50.8 mm using the international foot. There is a very slight difference using the US Survey foot (39.37 inches per meter).

When metric was "used" in the US, the contract plans were metric, but the projects were actually being built using USCU. The practice was to "hard" convert most things as previously stated by Ron(Structural): 1 foot = 300 mm.

RE: SI Units for AASHTO

The use of "hard" and "soft" above for conversions is the opposite of how we used the terms at the last three places I have worked. I learned decades ago that a "hard" conversion is an exact one and a "soft" conversion is an approximation. This website agrees: https://www.csemag.com/articles/hard-and-soft-metr...
ASTM/IEEE SI10-2010 has abandoned the use of "hard" and "soft" when changing measurement values. Instead of a "hard conversion," the proper term for changing a quantity directly from inch-pound to SI units is "conversion." What was called a "soft conversion" that results in a rough equivalence is known as "substitution."

On the other hand, this website says the opposite: https://www.aspenational.org/blogpost/1657539/2956....

On the whole, I found more agreement with the former than the latter and I would trust an engineering source over an estimators source. And, it makes sense given the meanings of the adjectives "hard" and "soft".

"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

RE: SI Units for AASHTO

In California "soft" meant an exact conversion, "hard" meant maintaining a hard number. The only time I remember using these terms is for the few years that the California DOT switched to metric--and don't think I have seen them or used them in practice since.

RE: SI Units for AASHTO


I'm in California myself and was project manager and/or lead civil engineer for three federal prisons, two new buildings at a naval air station, and a fire protection pipeline and fire protection system upgrades at a marine corp air station, all of which used SI units. I also provided technical assistance and QA/QC reviews for a project at a federal prison in Colorado, which also used SI units. In every case, "hard" meant exact and "soft" meant approximate. These adjective meanings make perfect sense, while the opposite do not. For example, "hard" is simply a contraction of "hard and fast", which means strict or fixed, which in turn strongly implies exact. The antonym "soft" means variable or elastic or indefinite, which strongly implies an approximation.

The misuse in California of "hard" and "soft" may come from this Caltrans document: https://dot.ca.gov/-/media/dot-media/programs/desi.... Simply put, Caltrans made a huge error with word meanings.

"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

RE: SI Units for AASHTO

fel3 (Civil/Environmental)...

Yes, my work (back in the metric days) used the same terminology as the California DOT: their construction projects, their manuals, and their standards.

RE: SI Units for AASHTO

Yeah, AASHTO no longer does anything in metric. Hard or soft conversions, however you define them, are probably both equally valid for most calculations. Nearly all equations in the spec are somewhat approximate, anyway. Material properties, loads, etc. all have some (sometimes large) variation.

For the record, we always used "hard" conversions to be exact, and "soft" as the approximate.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: SI Units for AASHTO

The conversion type was nearly always obvious, and 99% of the time the conversion difference was negligible.

Since metric passed I only use the terms "exact" and "approximate", no need to swap the customary tried and true for the contrived. The unnecessary confusion is indicative of the whole metric fiasco.

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