The industry's quest for adoption of soft metric bar sizes was
endorsed by the Construction Metrication Council (CIVIC), a council of the National
Institute of Building Sciences.* The CIVIC is charged with coordinating the federal
government's metrication efforts.
In April 1996, the Board of Directors of the American Association of
State Highway and Transportation Officials (AASHTO) adopted a resolution to support the
reinforcing steel industry's initiative for soft conversion of the inch-pound bar sizes.
TABLE 1
Soft Metric Bar Sizes vs. Inch-Pound Bar Sizes
| Metric |
Inch-Pound |
#10
#13
#16 |
#3
#4
#5 |
#19
#22
#25 |
#6
#7
#8 |
#29
#32
#36 |
#9
#10
#11 |
#43
#57 |
#14
#18 |
What are soft metric reinforcing bars?
Referring to the definition: "Soft metric conversion" is
describing the dimensions of inch-pound rebars in terms of Sl units, but not
physically changing the bar sizes ... The relationship of the sizes and
designations of the soft metric rebars to the inch-pound bar sizes is shown in Table 1.
Are other construction materials being soft
or hard converted?
Soft metric conversion is the most popular. For example, of the major
construction materials, structural steel from the onset has been soft metricated.
Dimensions of inch-pound cross-sections have been soft converted to Sl units. The
designations of metric shapes reflect the soft conversion. For example, an inch-pound
"W" shape WEI x 31 has the metric designation W200 x 46.1. The "W200"
is the soft conversion of W8. A W8 x 31 has an actual overall depth of 8.00 in. Thus, the
"W200" is derived from (8.00)(25.4) = 203.2 mm and called 200. The weight 31
lbs/ft has been soft converted to kg/m, i.e., (31)(0.4536)(1/0.3048) = 46.13 kg/m and
rounded to 46.1 kg/m. Tensile properties have also been essentially soft converted. For
example, Grade 50 material (fy = 50 ksi) is Grade 345 (fy = 345 MPa). The exact conversion of 50 ksi is 344.8 MPa.
ASTM SPECIFICATIONS FOR REBARS
Format. The ASTM Specifications
for Reinforcing Bars, which have the designations A61 5/A61 5M-96a for billet-steel and
A706/A706M-96b for low-alloy steel, are presented in a "dual-units" format.** A
dual-units form at means that a single document, e.g., Specification A615/A615M-96a,
contains inch-pound units and Sl metric units, and the Specification prescribes the
requirements for both metric and inch-pound billet-steel bars.
Specifying Metric Rebars.
Specifying soft metric rebars for a construction project will require inclusion of a
provision in the project specifications or a note on the project drawings or both. The
provision or note will need to include a citation of the metric part of the ASTM
Specification designation including the year-date. Examples of such a provision or note
would be:
• For Grade 420, billet-steel rebars ...... "Reinforcing bars
shall conform to ASTM A615M-96a, Grade 420."
• Similarly for low-alloy steel rebars ...... "Reinforcing bars
shall conform to ASTM A706M-96b." [Specification A706/A706M-96b covers only Grade 420
bars.]
If either type of steel rebar is acceptable for the particular
project, the provision or note could state for Grade 420 material ..... "Reinforcing
bars shall conform to A615M-96a, Grade 420, or to ASTM A706M-96b."
When the metric part of the designation of a dual-units ASTM
Specification is cited, a provision in the Scope of the specification implements the
metric requirements throughout the document.
TECHNICAL PROVISIONS IN THE ASTM REBAR
SPECIFICATIONS
The main differences in the metric provisions in the ASTM Rebar
Specifications, as compared with the requirements for inch-pound bars, are concerned with:
• Designations of the bar sizes
• Minimum yield strengths or grades
• Bar marking
• Substitution of metric bars for inch-pound bars
Designations of Metric Bar Sizes
Major changes to the metric bar sizes in the ASTM Specifications were
adopted in 1996. Soft metric conversions of the eleven inch-pound sizes are now the
standard sizes of metric bars. The one-to-one correspondence of bar sizes was shown in
Table 1. Table 2 presents additional metric and inch-pound data for the bars. Each metric
bar size has the same physical characteristics as its corresponding inch-pound bar size.
For example, consider a #13 metric bar. The corresponding inch-pound bar size is a #4. The
#13 metric bar's nominal diameter, cross-sectional area, and weight per unit of length -
except for being expressed in metric units and its weight more properly called
"mass" in metric jargon - are exactly the same as the nominal diameter, area and
weight of its counterpart #4 inch pound bar. In addition, the height and spacing of the
#13 metric bar's deformations are the same as the #4 bar's deformations.
* The council's mailing address:
Construction Metrication Council
National Institute of Building Sciences
1201 L Street, N. W., Suite 400
Washington, D.C. 20005
** The other two ASTM Specifications are A61 6/A616M-96a for
rail steel and A617/A617M-96a for axle-steel reinforcing bars. The metric bar sizes
in these two specifications have also been soft converted. These specifications are
not discussed in this report, because rail-steel and axle-steel rebars are not
generally available except in a few areas of the country.
