METRIC DESIGN GUIDE
Example 2: Conversion of an existing design practice.
Professional rounding used when converting conventional design dimensions.
Ceiling Height. A common office ceiling height is 9 feet. Simple mathematical
conversion yields 2 743 mm. This is an awkward dimension and can decrease
productivity in use. Since this is above code requirements, there is no close minimum
requirement.
Step 1. Determine the metric design tolerance.
If, instead of 9 feet, the installed height varies by a few inches, the visual and technical
requirements will still be met and cannot be detected by casual observation. This
variation in actual height becomes a "design tolerance." The selection of design
tolerance is a professional judgment.
Step 2. Determine the acceptable design range.
A range is a simple mathematical conversion, such as 2 743, plus and minus 50 mm.
Acceptable design range becomes 2 693 to 2 793.
Step 3. Select a preferred dimension.
2 700 and 2 800 are within an acceptable design range. 2 700 will cost less than 2 800
and is usually given first priority.
Example: Some roof flashing systems require fasteners at a minimum 24 inches on
center, which mathematically converts to 609.6 mm. More fasteners would probably
be acceptable at a slight increase in material cost. Selection of equivalent distance
yields 600, which will be easier to install.
PBS-PQ260
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