Using Soft Metric Reinforcing Bars in ... - Page 4

Soft metric bar sizes are identical to corresponding inch-pound bar sizes with respect to cross-sectional area (Table 2). The minimum yield strength, f_y, of metric bars is slightly larger than the corresponding f_y, of inch-pound bars. Thus, in all cases with respect to the reinforcement, the calculated design strength of a structural member, which is reinforced with soft metric bars will be slightly larger than the design strength of the same member with inch-pound bars of equivalent size and yield strength.

For example, compare the design strength of a structural member reinforced with 4 - #36 Grade 420 bars versus the design strength of the same member reinforced with 4 - #11 Grade 60 bars. From Table 2, the area of a #36 bar is 1,006 mm²; the area of a # 11 bar is 1.56 in². Carrying out the comparison with respect to the reinforcement:

* Expressing strength or stress values in MPa is inconvenient. For convenience in design calculations, the yield strength value of 420 MPa can be taken as 420 Newtons per mm² or 420 N/mm². To convert Newtons to kips, divide the value in Newtons by 4.45 X 10³. In this case, (1.69 x 10⁶) -- (4.45 X 10³) = 380 kips.
* * To convert ksi to Megapascals, multiply the value in ksi by 6.895 x 10⁶ to get Pascals. Then divide the result by 10⁶ to get Megapascals. In this case, (60) (6.895 x 10⁶) ÷ 10⁶ = 413.7 MPa.

The design strength of the member would be equal to the product of the strength- reduction factor, $, and the nominal strength of the reinforcement, A_sf_y , where $ = 0.90 for flexure, $ = 0.70 for compression in a tied column, and so forth. Since A_sf_y, for the metric rebars exceeds that of the inch-pound rebars, consequently $ A_sf_y of the member reinforced with the metric bars is greater than $ A_sf_y of the member reinforced with the inch-pound bars.

This straightforward example can also serve as a basis to discuss substitution of soft metric bars for inch-pound bars in a non-metric construction project. Suppose the pertinent question is re-stated:

A tied column in a non-metric designed building is reinforced with 4 - #11, Grade 60 bars; ties are #4. If 4 - #36 Grade 420 bars and # 13 ties are substituted, what is the effect on design strength of the column?

With respect to the reinforcement, the design strength, 4) A_sf_y of the soft metric bars is (0.70)(380 kips) = 266 kips. The design strength of the inch-pound bars in (0.70)(374.4) = 262 kips. Substitution of the soft metric bars does not have an adverse effect on the design strength of the column. Instead the soft metric bars provide a slight increase in design strength.

Mechanical Properties of Metric Rebars. Requirements for tensile and bending properties in the current ASTM Rebar Specifications are presented in Table 3. The metric bar sizes and criteria for tensile and bending properties in metric units comprise the requirements in ASTM A615M-96a and A706M96b. Similarly, the data for inch-pound bars, which are enclosed within brackets in Table 2, reflect the requirements in the inch-pound parts of the specifications.

The tabulated data in Table 3 serves to further illustrate soft conversion. Since the tensile property requirement "minimum percentage of elongation" is non-dimensional and related to bar size, the stipulated values are exactly the same for soft metric bars and corresponding inch-pound bar sizes. Note the gage length of 8 in. has also been soft-converted to (8)(25.4) = 203.2 mm. Minimum yield strengths have already been discussed. The minimum tensile strengths for Grade 60 and Grade 75 have been rounded slightly: 90,000 psi = 620.7 MPa, rounded to 620 MPa; 100,000 psi = 689.7 MPa, rounded to 690 MPa; and 80,000 psi = 551.7 MPa, rounded to 550 MPa. Since bend test requirements are a function of bar size, the pin diameters required for soft metric rebars are exactly the same as those for corresponding inch-pound bar sizes.

Development and Splices of Rebars. Section 1.2.1 in both ACI Building Codes, ACI 318-95 and ACI 318M-95, lists what the Commentary calls the more important items of information that must be included on design drawings or in project specifications. Items required-those related to reinforcing bars-are anchorage or embedment length, the location and length of lap splices, and the type and location of mechanical splices and welded splices (Sections 1.2.1-h and 1.2.1-l in either Code).

Let us consider a scenario in which a structure has been designed with inch-pound Grade 60 rebars in accordance with ACI 318-95. The Architect/Engineer^† has specified the rebar embedment and lap splice requirements on the design drawings or in the project specifications as mandated by ACI 318-95. And let us suppose that the Rebar Fabricator intends to furnish soft metric Grade 420 bars for the inch-pound project. The scenario leads to the practical question: Will the embedment and lap splice lengths specified for the Grade 60 bars be suitable for the metric Grade 420 bars?

Development lengths and lap splice lengths of reinforcing bars, whether tension or compression, are directly proportional to the specified yield strength, f_y , of the bar. The ratio of the minimum yield strengths, metric vs. inch-pound, is (60.9/60) = 1.015 or a variance of 1.5 percent. Because of the nearly identical minimum yield strengths, the anchorage or embedment lengths and lap splice lengths which the Architect/Engineer has specified for the Grade 60 rebars will be suitable for the substitute Grade 420 rebars.

Adoption of the soft metric bar sizes in the ASTM Specifications also benefits mechanical splices. Since the soft metric bars are completely interchangeable with corresponding sizes and the strength levels of the inch-pound bars, the same proprietary mechanical splices can be used for either set of bar sizes.

^† In ACI 301-96, the party "Architect/Engineer" or "Engineer/Architect" is defined as: "The architect, engineer, architectural firm, engineering firm, or architectural and engineering firm issuing project drawings and project specifications, or administering work under the Contract Documents".