Introduction

In 1960 the Eleventh General Conference of Weights and Measures adopted the name International System of Units (accepted abbreviation SI, from Syst me International d Unites) for a practical and consistent set of units of measure. Rules for common usage, notation, and abbreviation were adopted. Most industrial nations in the world have adopted and converted to SI. SI is not the old metric system (cgs or MKS).

A large number of code and industry design tables are required to carry out a timber structural design. Many of these tables now have dual units which includes both U.S. Customary units and SI metric units. However, the primary units are still U.S. Customary units, and the SI equivalent dimensions are generally provided for information purposes ( soft conversion). Most wood products are still produced in traditional U.S. Customary unit sizes, and a hard conversion to metric sizes has not taken place. Consequently most practical structural design in the U.S. is still done in U.S. Customary units.

However, the future trend will be toward the use of SI units. The following brief introduction to SI is included to aid the structural engineer in conversion between U.S. Customary (USC) units and SI units. Additional information can be found in ASTM E380 Metric Practice Excerpts.

E.2 Notation

SI is made up of seven base units, two supplementary units, and many consistent derived units. The following units are pertinent to structural design.