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14 csengineermag.com august 2019 ments on seismic hazard, selection of earthquake ground motions, load combinations and independent peer review. High Strength Reinforcement Current U.S. building codes limit rebar strength based on decades-old research, with most reinforcement used in concrete construction in the United States being Grade 60. Producers are now able to produce rebar, however, that is almost twice as strong as it was several decades ago. ACI 318-19 permits the use of Grade 100 reinforcement to resist mo- ments and axial forces from gravity and wind load combinations. The use of higher-grade reinforcement raised concerns about serviceability (cracking and deflections), which were addressed through a series of changes for slab and beam minimum reinforcement, effective moment of inertia and requirements for deflection calculations for two-way slabs. Strength and ductility concerns were addressed by introducing new requirements for mechanical properties of reinforcing bars; adjusting the method for calculating the strength reduction factor for moment, combined moment and axial load; revising development length provi- sions; and limiting the value of fy that can be used for calculating the maximum axial compressive strength, Pn, max, of columns. Grade 100 reinforcement is likely to be used mostly for vertical bars of shear walls and columns, though it might also be used for heavily loaded floor systems. Substantial new research funded by Pankow Foundation, ACI Founda- tion and others has demonstrated acceptable performance of members of special seismic systems reinforced with ASTM A706 Grade 80 re- inforcement and A706-equivalent Grade 100 reinforcement. Recogniz- ing this, ACI 318-19 now permits special moment frames with A706 Grade 80 reinforcement and special structural walls with A706 Grade 80 and A706-equivalent Grade 100 reinforcement. The provisions al- low the use of the higher grades to resist moments, axial forces and shear. To accommodate these higher grades, additional restrictions on hoop spacing, beam-column joint dimensions and lap splice locations have been added that will contribute to more reliable performance of special structural systems. Thick Slabs and Deep Beams As more large structures are designed to include thick slabs and other large members that support upper floors, shear provisions have been updated. ACI 318-19 sections on one-way shear and two-way shear (i.e., punching shear) consolidate what were previously a wide range of equations. They also provide a method to include size effect in shear design to avoid issues wherein increasing a member's size can reduce the unit shear strength of a section. The new shear equations also al- low the design engineer to take the effect of reinforcement ratio into consideration. Materials and Technology Shotcrete, a method of placing concrete by projecting it at high veloc- ity, was not explicitly discussed in previous versions of 318, but is now specifically included. The unification is expected to clarify both the design process and construction requirements for the use of shotcrete. Post-tensioning updates included clarifications of the construction requirements regarding loss of prestress, use of a new reference docu- ment for determining prestress losses, deformed and bonded reinforce- ment spacing limitations and several clarifications on requirements for anchorage zone reinforcement. Post-installed concrete screw anchors are increasingly used and this anchor type is recognized in ACI 318-19. The document also intro- duces provisions for shear lugs comprising a steel element welded to a base plate. Shear lugs are usually used at the base of columns to transfer large shear forces through bearing to a foundation element. Numerous changes were made to the durability of concrete sections including additional requirements for sulfate exposure classes and con- crete exposed to water. Lightweight concrete provisions throughout the code received numerous changes and clarifications based on the new method for determining λ, the lightweight modification factor. The new method calculates λ based on concrete mix proportions. ACI 318-19 is available in both printed and digital formats, and includes full-color images throughout, and is available with either inch-pound or SI units.

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