By Lou Colarusso and Samuel Barker
Throughout the past decade, committee members and key industry players attending meetings held by leading organizations [Concrete Reinforcing Steel Institute (CRSI), American Concrete Institute (ACI) and others] frequently engaged in unofficial discussions centered around what changes needed to be made to advance the reinforced concrete construction industry and to improve constructability and job site efficiencies.
It must be understood that very little change had occurred within reinforcing bar standards for decades, but with improvements in metallurgy and production capabilities, rebar was poised to take center stage in the evolution of design. Change to ACI 318, Building Code Requirements for Structural Concrete was imminent as the document needed to be updated to be aligned with new technologies and design opportunities. But bringing those needed changes forward and solidifying them under one regulatory banner would be a challenge that the industry would have to work through to overcome.
The unofficial discussions transformed into months and years of committee meetings and research involving experts from across the country, including Lou Colarusso, Michael Ugalde and Craig Guy from nVent LENTON (formerly ERICO LENTON). The partnership between suppliers, builders and research institutions toiled tirelessly to marry the needs of the overall industry with the technology available. What resulted, after the hard work on the part of industry volunteers across the country, were critical changes to ACI 318-19, which catapulted practitioners toward making the building and design process more efficient. In this article, a few of the important changes to the ACI 318 code will be examined with regard to how they improve designing reinforced concrete structures.
High-Strength Rebar Comes of Age
One of the most significant additions to ACI 318-19 was the broadening of the approved uses for high-strength rebar (HSRB), commonly understood to be reinforcing steel with a specified yield strength of 80,000 psi or greater (e.g., Grade 80 and Grade 100). When working with prior versions of ACI 318, designers and builders had the option of using HSRB, but the uses were more restricted as the material standards did not provide the right framework around which such reinforcement could be specified—resulting in an inefficient process for designing and building with HSRB.
As engineers find new ways to use HSRB, design, supply and construction become more efficient. HSRB means less rebar to cut, bend, couple, anchor, tie and place. Labor cost is reduced and construction accelerated, allowing owners and end users to more quickly generate revenue. Most importantly, HSRB can improve the structural integrity of the building and increase floor space as columns and other concrete elements can be reduced in size.
Mechanical Splicing Eliminates Large HSRB Lap Splice Development Lengths (ACI 318-19, Section 25.4.2)
The traditional and most common method of transferring loads between reinforcing bars has always been the lap splice. One of the difficulties with lap splicing HSRB is that calculated development lengths per ACI 318-19 Section 18.104.22.168 increase rapidly with the new ѱg rebar grade modification factor. As the strength of the rebar enters the Grade 80 and Grade 100 realm (Section 22.214.171.124), the modification factor jumps to 1.15 and 1.3, respectively.
Structural engineers, architects and specifiers continue to discover that mechanical splicing rebar has many advantages over the conventional method. Not only do mechanical splices solve the development length challenge, but mechanically splicing also improves the structural integrity of the building by making the strength of the mechanically spliced rebar equivalent to one continuous bar.
Use of Headed HSRB Facilitated (ACI 318-19, Section 25.4.4)
Designing for anchorage with larger-diameter HSRB prior to the release of ACI 318-19 was challenging due to the difficulty in bending HSRB and the inconveniently long hook lengths required for development. Replacing a hook with a mechanical head presented its own obstacles when designing to prior versions of ACI 318, as Section 126.96.36.199 prohibited the use of the simplified development length formula in 188.8.131.52 with any rebar strength above Grade 60. Granted, designing for mechanically headed HSRB was a possibility by using Chapter 17 of ACI 318-14 (formerly Appendix D), but the process was viewed as complex and inefficient for design engineers.
Fortunately, research conducted between the years of 2014 and 2019 at respected institutions [University of Kansas; Purdue University; University of California, Berkeley; University of California, Los Angeles; and University of Texas] and sponsored by organizations such as CRSI, Pankow Foundation and ACI has provided the fundamental testing to sanction the use of mechanically headed HSRB in ACI 318-19. This research confirmed that headed bars and hooks are influenced by similar parameters, like cover, confinement, yield and concrete compressive strength. However, heads are more efficient in developing the rebar. Engineers designing for headed rebar with ACI 318-19 can now easily calculate development length for headed HSRB by using the modified development length formula in Section 184.108.40.206.
Headed Rebar Spacing Requirements Relaxed (ACI 318-19, Section 25.4.4)
Dr. David Darwin at the University of Kansas, in conjunction with colleagues across the country, spearheaded research and code provisions that enabled a significant change to ACI 318-19 regarding the spacing requirement for headed rebar of all grades (Section 220.127.116.11). ACI 318-14 (and prior) restricted clear spacing between headed bars to 4 bar diameters (db) and greater. ACI 318-19 is updated and now allows center-to-center spacing between headed rebar of 3 db. In terms of clear spacing, the change is from 4 db to 2 db, representing a 50 percent reduction. In terms of center-to-center spacing, the change is 5 db to 3 db, representing a 40 percent reduction.
What This Means For You
The changes to ACI 318-19 discussed in this article were realized through an industrywide collaboration and were the culmination of more than a decade of discussions and committee meetings. Designers now have more options when working with HSRB, headed HSRB and headed rebar in general—options that streamline design, supply and placement; reduce construction cycle times; and, provide overall cost savings. Though seemingly small on the surface, the code changes are an enormous step forward, opening a gate to new paths for the industry to blaze. —Lou Colarusso is Sales Director and Samuel Barker an Engineering Technologist with Solon, Ohio-based nVent LENTON, a specialist in concrete reinforcing steel connections.
Typical installations using mechanical versus lap splicing. The additional space required for lap splicing along with overall installation complexity exceed what is needed for mechanical splicing.