Masonry Magazine December 1994 Page. 21
NCMA TEK
An Information Series from National Concrete Masonry Association
A national authority on concrete masonry technology
METRIC CONCRETE MASONRY
CONSTRUCTION
TEK 3-10
Construction (1994)
Keywords: construction, dimensions, metric, modular
coordination
INTRODUCTION
The modern metric system (Systeme Internationale or SI system) is the standard international system of measurement, and the system which has been mandated by the Metric Conversion Act (ref. 1) for use in the construction of all United States federal buildings. Building designs and construction drawings are to be submitted in metric units, and constructed according to metric specifications.
Metric conversion for some materials, like concrete masonry, means that the inch-pound dimensions are stated in metric (soft conversion), which does not require changing the material sizes currently in production. Hard conversion, on the other hand, refers to a physical change in the manufactured size of a product.
METRIC UNITS
The metric system uses several base units of measurement, with various prefixes. For example, the base unit for length is the meter, and the prefix kilo means one thousand. Therefore, a kilometer is one thousand meters. Table 1 lists the metric decimal prefixes and their magnitudes.
Just as the inch-pound system has preferred units of measurement (i.e., building dimensions are measured in feet, not in yards), the metric system also follows some conventions for preferred units. For design and construction in the United States, typically only the prefixes milli and kilo are used. For example, lengths are given in millimeters, meters, or kilometers, not in centimeters or hectometers.
Table 2 lists common inch-pound units used in building design and construction, their standard metric unit equivalents, and the factors necessary to convert from one system to the other. The metric units listed in Table 2 are the preferred units.
Understanding the basis of the metric system helps to visualize metric units. For example, 0°C is the freezing point of water; 100°C is the boiling point of water; and one kilogram equals the weight of one liter of water. Comparison benchmarks to the inch-pound system include: a large paper clip weighs about one gram; one meter is the length of a yardstick plus about 3 inches; 1500 psi is just over 10 MPa; 100 lb is about 45.5 kg.
As with any language, there are some basic rules that apply to the use of metric so that communication is consistent and clear, thereby minimizing the potential for errors during construction. The following summarizes the conventions applicable to the metric system.
Abbreviations
The third column of Table 2 indicates the proper abbreviations for metric units. Along with the proper abbreviations, the proper use of capital and lower case letters is also important. For example, megpascal is abbreviated MPa. If mPa were written instead, it would indicate millipascals rather than megapascals.
Symbols
When using inch-pound units, the use of symbols to represent feet and inches is second nature. No such symbols are used in the metric system; only the abbreviations listed in Table 2. However, in surveying, the symbol for degrees applies to metric as well inch-pound systems. The symbols for minutes and seconds may be used, or may be replaced by the decimal equivalent.
Stating Metric Units
While mixing feet and inches is common practice, a similar practice is not used in the metric system. For example, if dual units are shown on a set of plans, the metric equivalent
Table 1-Metric Decimal Prefixes
| Prefix | Symbol | Order of Magnitude | Expression |
|---|---|---|---|
| milli | m | 10⁻³ | one thousandth, 0.001 |
| centi | c | 10⁻² | one hundredth, 0.01 |
| deci | d | 10⁻¹ | one tenth, 0.1 |
| deca | da | 10¹ | ten, 10 |
| hecto | h | 10² | one hundred, 100 |
| kilo | k | 10³ | one thousand, 1000 |
| Mega | M | 10⁶ | one million, 1,000,000 |
TEK 3-10 © 1994 National Concrete Masonry Association
MASONRY-NOVEMBER/DECEMBER, 1994 21