![]() ![]() The maximum longitudinal reinforcement ratio in special moment frame beams is lowered to 0.02 for Grade 80 reinforcement (18.6.3.1). For special structural walls, the minimum reinforcement area follows the same pattern, except the steel yield strength is not limited in this calculation (18.10.2.4). However, 80 ksi is the maximum yield strength permitted to be used in equations in 9.6.1.2, equating minimum reinforcement areas for Grade 80 and Grade 100. The minimum amount of longitudinal reinforcement for flexural members is inversely proportional to reinforcement yield strength and hence is lower for HSR than for CR. **Best case refers to the upper limit where (c b + K tr)/d b = 2.5, in conjunction with Eq. *Use of equation in Table 25.4.2.3 (traditionally used by structural engineers for most typical conditions without epoxy coating) Also note that, for lap splices of HSR, the code now requires a minimum amount of splice confinement provided by transverse reinforcement along the splice. Those two equations remain largely the same except for an added reinforcement grade multiplier (ψ g) that is equal to 1.0 for Grade 60, 1.15 for Grade 80, and 1.3 for Grade 100 Example 1 illustrates splice length calculation according to ACI 318-19 with f´ c = 6 ksi. In past versions of the code, engineers could use two equations to calculate development and lap lengths. Perhaps the most significant changes to designing with HSR relate to detailing requirements. These new provisions apply to ASTM A706 Grade 60 reinforcing as well. For ASTM A706, the requirement on deformation profiles calls for “the radius at the base of each deformation… be at least 1.5 times the height of the deformation.” This requirement is intended to avoid low-cycle fatigue cracks at these locations along the bar and improve the number of half-cycles to fracture. The ACI 318 Committee chose to address these refinements directly in the code, in Chapter 20, by setting requirements for smoother bar deformation profiles, various minimum strength ratios, and minimum elongations before fracture. Despite this, the adoption of higher grades was not independent of new refinements to rebar manufacturing. These revisions occurred without the introduction of new ASTM specifications for HSR. Changes in use of reinforcement grades between ACI 318-19 and ACI 318-14. Refer to Table 1 for a summary of major reinforcement grade changes from ACI 318-14 to ACI 318-19. Additionally, various gravity elements, which were previously limited to Grade 80, are now extended to Grade 100. Reinforcement in special lateral force resisting systems, which were previously limited to Grade 60 for flexural, axial, and shear reinforcement, can now use up to Grade 80 or Grade 100 depending on the application. ACI 318-19 was released in July 2019 and will likely be referenced in the 2021 IBC. In response to the research, ACI 318-19 introduces significant changes allowing more applications of HSR in concrete buildings. This article introduces changes in ACI 318-19 related to the use of HSR and presents considerations engineers should be cautious of before specifying HSR. Later, extensive research answered many of the identified gaps (the online version of this article includes a summary of this research). ![]() In 2014, two reports identified experimental tests required and provisions of ACI 318 that would need to be updated to allow the use of HSR in seismic applications (ATC 2014 NIST 2014). The main expected advantage of HSR over conventional reinforcement (CR) is a lower volume of reinforcement material in construction, resulting in lower construction time and costs (Price et al. This restriction remained in the building code until recently due to a lack of data on cyclically loaded members with HSR. However, the maximum yield strength of reinforcement in elements resisting seismic loads was limited to 60 ksi. The outcome of this research first appeared in ACI 318-71, Building Code Requirements for Structural Concrete, which allowed limited use of reinforcement with a higher grade than 60 ksi. Research on the use of high-strength reinforcement (HSR) began in the late 1950s. Using higher strength reinforcement is a natural solution to this problem. Even if theoretical sizes can be calculated, it may be impossible to construct tightly spaced rebar cages or congested joint connections. As buildings get taller, bigger, and are required to resist higher seismic forces, the amount of reinforcement needed becomes impractical. ![]()
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