METRIC DESIGN GUIDE
a precision measurement. However, since gage is a name and not a dimension, it is
acceptable to use 22-gage on metric drawings and specifications until an industry
converts sizes.
Rounding and Conversion
Simple Mathematical Rounding . This leads to many problems. An example is to take
an existing criteria dimension, such as 12 feet, convert it mathematcally to 3 658 mm, and
i
use this dimension. Builders, faced with entire drawing sets of awkward, nonrounded
numbers, will find that metric is more difficult. In projects to date, a number of builders
converted back to be able to measure with English tapes. They also made conversion
mistakes, causing rebuilding and delay. It is very important to make job site labor more
efficient by professionally rounding dimensions.
Professional Rounding . This technique takes the result of simple mathematical rounding,
and applies professional judgment. The basic module of metric design is 100 mm.
Following are two examples of professional judgment in rounding design criteria that have
already been included in GSA metric criteria in the Facilities Standards for the Public
Buildings Service (PBS-PQ100.1):
Example 1: Conversion of a code requirement.
Step 1. Determine the nonoffending direction.
1993 National Building Code Article 1011.3 requires 44 inches (1 118 mm) of
unobstructed pedestrian corridor width. However, 1 118 mm is not a round
number. It should be rounded to facilitate the cleanest construction possible.
Narrower doesn't meet the code. The nonoffending direction is larger.
Step 2. Select the largest feasible module.
1 200 mm is feasible, so this represents a choice however GSA corridors are
usually above code minimums. 1 500 may be more like current usuage.
Every effort should be made to keep design dimensions in increments of 100
mm.
In each case, the user must determine the acceptable choice, but the user is
encouraged to present clean, rounded metric dimensions as alternatives. Simple
mathematically converted dimensions will lead to an increase in project cost and
time.
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