Using Soft Metric Reinforcing Bars in ... - Page 3

Thus, the end result of soft conversion of the inch-pound bar sizes is that, physically, the "new" soft metric bars are the same as the "old" inch-pound bars. Recall the definition of soft metric conversion ..... not physically changing the bar sizes.

Minimum Yield Strength. Initially, the minimum yield strengths in the ASTM Rebar Specifications were established as 400 MPa and 500 MPa.* The abbreviation "MPa" stands for "megapascals". The values 400 MPa and 500 MPa, which are equal to 58,000 psi and 72,500 psi, were intended to be equivalent to inch-pound Grade 60 and Grade 75, respectively.
As a result of revisions approved in 1995, the metric yield strengths were increased to 420 MPa and 520 MPa. The increased values are equal to 60,900 psi and 75,400 psi in the inch-pound system. The rationale for revising the minimum yield strengths in 1995 was to bring the metric values closer to the corresponding inch-pound values. And there was no justification in reducing the strength of a material (e.g., 400 MPa = 58,000 psi < 60,000 psi) just because of metrication.
Presently, the reinforcing steel industry is re-evaluating the minimum metric yield strength of 420 MPa. There is interest in submitting a proposed revision to ASTM to "tweak" the value to 415 MPa, which is equivalent to 60,100 psi. The exact soft conversion of 60,000 psi is 413.7 MPa. If the industry decides to pursue this small adjustment in yield strength, endorsement of the proposal would be sought from the user community.

Bars marked with a S and W meet both A615M and A706M
GRADE 420

GRADE 520

Figure 1 Marking Requirements for Metric Reinforcing Bars

Bar Marking. Soft metric rebars are required to be identified or marked distinctively for size and minimum yield strength or grade. For example, consider the marking requirements for a #25, Grade 420 metric bar. The ASTM Specifications require the number "25" to be rolled onto the surface of the metric bar to indicate its size. For identifying or designating the minimum yield strength or grade, the ASTM Specifications provide an option. A mill can choose to roll a "4" (the first digit in the grade number) onto the bar, or roll on a single longitudinal rib or grade line to indicate Grade 420. Marking requirements for metric rebars are shown in Figure 1 .

Substitution. The ASTM Specifications contain a provision for substitution of metric bars for corresponding inch-pound size bars and strength levels. For example, Section 20.3.5 is the substitution provision in the A615/A615M-96a Specification for billet-steel rebars. It states:

"20.3.5 It shall be permissible to substitute: a metric size bar of Grade 300 for the corresponding inch-pound size bar of Grade 40, a metric size bar of Grade 420 for the corresponding inch-pound size bar of Grade 60, and a metric size bar of Grade 520 for the corresponding inch-pound size bar of Grade 75."

The substitution clause in the A706/A706M-96b Specification for low-alloy steel rebars states:

"16.3.5 It shall be permissible to substitute a metric size bar of Grade 420 for the corresponding inch-pound size bar of Grade 60."

DESIGN ISSUES
Design Strength of Structural Members. The design strength of structural members is of prime importance. A pertinent question is:

How does the design strength of a structural member, reinforced with soft metric rebars, compare with the design strength of the same member reinforced with inch-pound bars of equivalent size and minimum yield strength?

TABLE 2 ASTM Standard Metric Reinforcing Bars

^a Equivalent inch-pound bar sizes are the designations enclosed within brackets.
^b The equivalent nominal dimensions of inch-pound bars are the values enclosed within brackets.

* ASTM Specification A615/A615M-96a also includes a Grade 300, which is equivalent to Grade 40. Since the production, and consequently usage of Grade 40 [Grade 3001 bars is diminishing, this strength level is not discussed in any detail in this report.