FPS system is useful in construction. Working in units of 12 makes it easy to divide by 2 or 3. There is also the issue of field tolerance. In general a difference of 1/8" or less can be neglected and accommodated in the field fit up. Measuring in any unit less (mm) is not significant to the overall design. When an ironworker is welding, it is easy for him to measure to the nearest 1/16" of an inch rather than to the nearest mm which is not significant. As the engineer, we are very interested in making the units work in our formulas, but the information is not valuable unless the worker can read, measure, and construct the project quickly without error.
Measuring force and weight in units of pounds is also useful in construction. Everything we build (unless it is underwater or in outer space) is subjected to the force of gravity. So, the weight of a 15,000 lbs beam can be used to calculations for force, stress, and any other desired calculations and the crane operator can also determine if he can lift it. So while pounds are not technically a unit of mass, no conversion between Newtons and Kilograms needs to be made at any point in the process. All work that I do, from the shop drawings to the crane pick and the calculations I perform in between deals in terms of pounds. There is no chance for confusion between Newtons and Kilograms to occur.
In many scientific and engineering fields MKS units are very effective and preferable. In construction, they are not. I do not want to use the reason "it has always been done that way," but in cases where we are renovating existing buildings, the existing member sizes and drawings are in FPS. So, mixing of units would be troublesome.
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Chad Morrison P.E., M.ASCE
Professional Engineer
Greenville RI
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Original Message:
Sent: 02-27-2018 23:01
From: Partha Aich
Subject: Unit of calculation
Why does the USA and other countries use FPS units for calculation, when the originator of FPS units
(the UK) has shifted to the MKS system?
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Partha Aich M.ASCE
Director
Delhi East ND
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