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Design of Concrete Structures Using High-Strength Steel Reinforcement (2011)

Chapter: Chapter 3 - Recommendations, Conclusions, and Suggested Research

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Suggested Citation:"Chapter 3 - Recommendations, Conclusions, and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2011. Design of Concrete Structures Using High-Strength Steel Reinforcement. Washington, DC: The National Academies Press. doi: 10.17226/14496.
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Suggested Citation:"Chapter 3 - Recommendations, Conclusions, and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2011. Design of Concrete Structures Using High-Strength Steel Reinforcement. Washington, DC: The National Academies Press. doi: 10.17226/14496.
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Page 60
Page 61
Suggested Citation:"Chapter 3 - Recommendations, Conclusions, and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2011. Design of Concrete Structures Using High-Strength Steel Reinforcement. Washington, DC: The National Academies Press. doi: 10.17226/14496.
×
Page 61
Page 62
Suggested Citation:"Chapter 3 - Recommendations, Conclusions, and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2011. Design of Concrete Structures Using High-Strength Steel Reinforcement. Washington, DC: The National Academies Press. doi: 10.17226/14496.
×
Page 62
Page 63
Suggested Citation:"Chapter 3 - Recommendations, Conclusions, and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2011. Design of Concrete Structures Using High-Strength Steel Reinforcement. Washington, DC: The National Academies Press. doi: 10.17226/14496.
×
Page 63

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59 3.1 Summary of AASHTO LRFD Clauses Having Recommended Changes The objective of this work was to evaluate existing AASHTO LRFD Bridge Design Specifications relevant to the use of high- strength reinforcing steel and grades of reinforcing steel having no discernable yield plateau. The primary deliverable is recom- mended changes to the AASHTO specifications. The recom- mended changes were submitted to the project panel in the form of a redline copy of the specifications; this document is not included here. The following provides a summary of the nature of the proposed changes. Specific language is not provided in this report, as this may conflict with eventual AASHTO-adopted language. In all cases, language was proposed that specifically permits the use of high-strength reinforcing steel with specified yield strengths not greater than 100 ksi when the specific article per- mits it. This methodology is consistent with the manner by which the AASHTO specifications handle high-strength con- crete, allowing its use only when a specific article permits it. LRFD specifications Sections 3, 5, and 9 were identified as hav- ing clauses potentially requiring changes. Although considered in its entirety, no potential changes were identified in the AASHTO LRFD Bridge Construction Specifications. It is noted that 2009 revisions to §9.2 of the construction specifications permit the use of A1035 reinforcing steel. 3.1.1 Proposed Changes to Section 3 of the LRFD Specifications Appendix B3 of the specifications was identified as possibly requiring changes. This appendix deals with plastic hinging of columns and references §3.10.9.4.3a, which deals with earth- quake forces and design procedures in Seismic Zones 3 and 4. In Section 5, the proposed use of reinforcing steels with speci- fied strengths up to 100 ksi is restricted to Seismic Zone 1 due to lack of research (NCHRP 12-77 did not conduct seismic testing). For this reason, no changes are proposed to the 75-ksi limit for Seismic Zones 3 and 4, and, therefore, no changes are required in Section 3. 3.1.2 Proposed Changes to Section 5 of the LRFD Specifications Section 5 has the most proposed changes; these are summa- rized in Table 29. Corresponding changes also are proposed for the commentaries. 3.1.3 Proposed Changes to Section 9 of the LRFD Specifications Article 9.5.3, Fatigue and Fracture Limit State, states that the fatigue limit state does not need to be investigated for bridge decks in multi-girder applications. Although reinforcing steel having yield not exceeding 100 ksi is proposed to be permitted in bridge decks, no changes to this requirement are recom- mended for the following reasons: (1) Although the stress levels in higher strength reinforcing bars will be higher, data indicate that the fatigue limit is also higher (proposed changes to §5.5.3.2 address this); (2) in multi-girder applications, research shows that the concrete decks carry load primarily through arching action rather than flexure (see C9.7.2.1, Empirical Design); and (3) bridge deck design tends to be driven by stiffness concerns and, therefore, the increase in re- inforcing bar stress associated with the use of high-strength bars will be marginal. Article 9.5.2, Empirical Design, specifies bar area and maxi- mum spacing, independent of yield strength. Using higher strength reinforcing steel results in a one-for-one bar substitu- tion, which is permitted now regardless of steel strength. Thus, no changes are needed. The use of higher strength reinforcing steel affects §9.7.3.2, Distribution Reinforcement, as far as the use of such reinforce- ment in the primary direction will result in a lower required C H A P T E R 3 Recommendations, Conclusions, and Suggested Research

60 Table 29. Summary of proposed changes to Section 5 of AASHTO LRFD specifications. Article Brief Summary of Changes 5.2 DEFINITIONS Modified the definition of tension-controlled section by changing “0.005” to “tension- controlled strain limit.” Added definition of tension-controlled strain limit. 5.3 NOTATION Modified the definition of f y to allow higher yield strengths. Added definitions of cl and tl ; compression- and tension-controlled strain limits, respectively. 5.4.3.1 and C5.4.3.1 Reinforcing Steel, General Permits the use of reinforcing steel with specified yield strengths up to 100.0 ksi when allowed by specific articles. 5.4.3.2 Reinforcing Steel, Modulus of Elasticity E s =29,000 may be used for specified yield strengths up to 100.0 ksi. 5.4.3.3 and C5.4.3.3 Reinforcing Steel, Special Applications Permits the use of reinforcing steel with specified yield strengths up to 100.0 ksi in Seismic Zone 1. 5.5.3.2 and C5.5.3.2 Fatigue Limit State, Reinforcing Bars Modifies the fatigue equation for reinforcing bars to allow the equation to be used for specified yield strengths up to 100.0 ksi. 5.5.4.2.1 and C5.5.4.2.1 Resistance Factors, Conventional Construction Allows the use of reinforcing steel with specified yield strengths up to 100.0 ksi in Seismic Zone 1. Modifies the equation, figure, and commentary. These now use cl and tl, (compression- and tension-controlled strain limits) in place of 0.002 and 0.005. 5.7 and adds C5.7 DESIGN FOR FLEXURAL AND AXIAL FORCE EFFECTS Allows the use of reinforcing steel with specified yield strengths up to 100.0 ksi in Seismic Zone 1. 5.7.2.1 and C5.7.2.1 Assumptions for Strength and Extreme Event Limit States Keeps compression- and tension-controlled strain limits of 0.002 and 0.005 for reinforcing steels with specified yield strengths up to 60.0 and 75.0 ksi, respectively. Provides compression- and tension-controlled strain limits of 0.004 and 0.008 for reinforcing steel with a specified yield strength equal to 100.0 ksi. Linear interpolation is used for reinforcing steels with specified yield strengths between 60.0 or 75.0 ksi and 100.0 ksi. Equations are provided for when f y may replace f s or f s ’ in 5.7.3.1 and 5.7.3.2. 5.7.3.2.5 Strain Compatibility Approach Limits the steel stress in a strain compatibility calculation to the specified yield strength. C5.7.3.3.1 Maximum Reinforcement Replaces 0.0 05 with “tension-controlled strain limit.” 5.7.3.5 and C5.7.3.5 Moment Redistribution Adjusts strain limit to allow moment redistribution in structures using reinforcing steel with specified yield strengths up to 100.0 ksi. C5.7.4.2 and C5.7.4.4. Limits for Reinforcement Warns that designs should consider that columns using higher strength reinforcing steel may be sm aller and have lower axial stiffness. 5.7.4.6 Spirals and Ties Permits spirals and ties made of reinforcing steel with specified yield strengths up to 100.0 ksi in Seismic Zone 1. 5.8.2.4 and C5.8.2.4 Regions Requiring Transverse Reinforcement 5.8.2.5 and C5.8.2.5 Minimum Transverse Reinforcement Permits transverse reinforcement with specified yield strengths up to 100.0 ksi in applications with flexural shear without torsion. C5.8.2.7 Maximum Spacing of Transverse Reinforcement Indicates that spacing requirements have been verified for transverse reinforcement with specified yield strengths up to 100.0 ksi in applications of shear without torsion. 5.8.2.8 and C5.8.2.8 Design and Detailing Requirements. Permits transverse reinforcement with specified yield strengths up to 100.0 ksi in applications with flexural shear without torsion.

area of reinforcement in the secondary direction. However, spacing requirements of §5.7.3.4 will limit how much the area of the primary reinforcement can be reduced, and thus, also limits the permitted reduction in the secondary reinforcement. No change is proposed. 3.2 Conclusions Based on the presented experimental and analytical studies, the following conclusions are drawn. The conclusions are grouped based on the main tasks of this work. 3.2.1 Yield Strength A critical objective of the present work was to identify an appropriate steel strength and/or behavior model to adequately capture the behavior of high-strength reinforcing steel while respecting the tenets of design and the needs of the designer. A value of yield strength, fy, not exceeding 100 ksi was found to be permissible without requiring significant changes to the specifications. 3.2.2 Flexure The current specifications design methodology for flexure, that is, a simple plane sections analysis using stress block factors to model concrete behavior and an elastic-perfectly plastic steel behavior (having Es = 29,000 ksi), is shown to be appropriate for values of fy ≤ 100 ksi. To ensure ductility, steel strains correspon- ding to tension- and compression-controlled limits (defined in §5.7.2.1 of specifications) are recommended as follows: Current Recommended §5.7.2.1; No Limits for Recommended High-Strength Changes Reinforcement fy ≤ 60 ksi fy = 100 ksi Tension-Controlled εt ≥ 0.005 εt ≥ 0.008 Section Compression-Controlled εt ≤ 0.002 εt ≤ 0.004 Section Values may be interpolated between limits. These strain limits were developed through a rigorous ana- lytical study of 286 cases, which included seven different grades of reinforcing steel, three concrete strengths, and multiple sec- tion geometries. Six large-scale beam specimens reinforced with A1035 reinforcing steel confirmed the appropriateness of the proposed tension- and compression-controlled limits. All beam specimens met and exceeded their designed-for strength and ductility criteria and exhibited predictable behavior and performance similar to beams having conventional reinforcing steel. 61 C5.8.3.3 Nominal Shear Resistance Identifies that transverse reinforcement with specified yield strengths up to 100.0 ksi may be used in applications with flexural shear without torsion. 5.8.3.5 Longitudinal Reinforcement Permits longitudinal reinforcing steel with specified yield strengths up to 100.0 ksi. 5.8.4.1 Interface Shear Transfer, General Clarifies that fy is limited to 60.0 ksi in Equation 5.8.4.1.3. 5.10.2 and C5.10.2 Hooks and Bends Permits hooks with specified yield strengths up to 100.0 ksi with transverse confining steel in Seismic Zone 1. 5.10.6.1 and C5.10.6.1 Transverse Reinforcement for Compression Members, General Permits spirals with specified yield strengths up to 100.0 ksi in Seismic Zone 1. 5.10.11.1 Provisions for Seismic Design, General Permits the use of reinforcing steel with specified yield strengths up to 100.0 ksi in Seismic Zone 1. 5.11.1.1 and C5.11.1.1 DEVELOPMENT AND SPLICES OF REINFORCEMENT, Basic Requirements 5.11.2 and C5.11.2 Development of Reinforcement Permits the development length equations to be used for reinforcing steel with specified yield strengths up to 100.0 ksi. 5.11.2.1 Deformed Bar and Wire in Tension Requires transverse confining steel for development of reinforcing steel with specified yield strengths greater than 75.0 ksi. 5.11.5 and adds C5.11.5 Splices of Bar Reinforcement 5.11.5.3 and C5.11.5.3 Splices of Reinforcement in Tension Permits splices in reinforcing steel with specified yield strengths up to 100.0 ksi and requires transverse confining steel. Table 5.11.5.3.1-1 Classes of Tension Lap Splices Requires transverse confining steel in splices of reinforcing steel with specified yield strengths exceeding 75.0 ksi. Article Brief Summary of Changes Table 29. (Continued).

3.2.3 Fatigue Two large-scale proof tests conducted as part of this study and a review of available published data demonstrate that presently accepted values for the fatigue or “endurance” limit for reinforcing steel are applicable, and likely conservative, when applied to higher strength bars. Additionally, it is shown that fatigue considerations will rarely affect the design of typi- cal reinforced-concrete members having fy ≤ 100 ksi. 3.2.4 Shear Five large–scale, reinforced-concrete beams and four AASHTO Type I prestressed girders were tested to evaluate the performance of high-strength A1035 steel as shear re- inforcement in comparison to that of the commonly used A615 steel. Test specimens were designed using the specifica- tions’ approach of summing concrete and steel contributions to shear resistance (i.e., Vc + Vs). All beams exhibited good per- formance with little difference noted between the behavior of spans reinforced with A1035 or A615 transverse steel. The use of current specifications procedures for calculating shear capacity were found to be acceptable for values of shear rein- forcement yield fy ≤ 100 ksi. 3.2.5 Shear Friction A series of eight direct push-off (shear proof) tests of “cold construction joint” interfaces reinforced with either A1035 or A615 bars demonstrated that current specifications require- ments for such joints are adequate. Significantly, the restric- tion that fy be limited to 60 ksi when calculating shear friction capacity must be maintained regardless of the reinforcing steel used. This limit is, in fact, calibrated to limit strain (and, there- fore, interface crack opening) to ensure adequate aggregate interlock capacity across the interface and is, hence, a function of steel modulus rather than strength. As noted, steel modu- lus does not vary with reinforcing bar grade. 3.2.6 Compression Analytical parametric studies were performed to examine behavior of columns reinforced with A1035 longitudinal and transverse reinforcement. Results indicate the current specifi- cations requirements for both longitudinal and transverse reinforcement design in compression members are applicable for fy ≤ 100 ksi. 3.2.7 Bond and Development The applicability of current specification requirements for straight bar and hooked bar development lengths was con- firmed through a series of spliced-bar beam tests and pull-out tests, respectively. “Proof test” spliced-bar beam specimens, having development lengths that were shorter than those required by the present specifications equations (with all appropriate reduction factors applied), were tested. All devel- oped bar stresses exceeding fy and approaching the ultimate bar capacity, fu, prior to the splice slipping and in one case bar frac- ture. Tests of hooked bar anchorage resulted in bar rupture outside of the anchorage region with very little slip clearly indi- cating the efficacy of the hooked bar development require- ments in the specifications. Significantly, it is recommended that development, splice, and anchorage regions be provided with cover and confining reinforcement—based on current design requirements—when high-strength bars are used. Existing equations for development where no confinement is present are demonstrated to be unconservative. The presence of confining reinforcement effectively mitigates potential split- ting failures and results in suitably conservative development, splice, and anchorage capacities. 3.2.8 Serviceability—Deflections and Crack Widths A fundamental issue in using A1035 or any other high- strength reinforcing steel is that the stress at service load ( fs; assumed to be on the order of 0.6fy) is expected to be greater than when conventional Grade 60 steel is used. Conse- quently, the service-load reinforcing strains (i.e., εs = fs/Es) are greater than those for conventional Grade 60 steel. The large strains affect deflection and crack widths at service loads. Based on the results of the flexural tests conducted in this study, deflections and crack widths at service load levels were evalu- ated. Both metrics of serviceability were found to be within presently accepted limits and were predictable using current specifications provisions. A limitation on service-level stresses of fs ≤ 60 ksi is recommended; this is consistent with the recommendation that fy ≤ 100 ksi. 3.3 Recommended Research The following topics associated with the adoption of high-strength reinforcing steel and steel grades having no discernable yield plateau have been identified as requiring further study. 3.3.1 Application in Seismic Zones 2, 3, and 4 The present study did not address seismic applications and is, therefore, limited in its application to Seismic Zone 1. In bridge structures, the seismic effects on single- and multiple- column piers are most significant. The design of these elements would potentially benefit from the use of higher strength re- 62

inforcing by both reducing element size and congestion of reinforcement. 3.3.2 Fatigue Limited available data indicate that the fatigue limit of higher strength, and particularly micro-composite alloy steel, may be markedly improved over that of conventional black steel. A study to establish reliable S-N relationships for differ- ent grades of reinforcing steel is recommended. Such a study must consider full-section bars (not coupons) and include a range of bar sizes. 3.3.3 Shear Friction As discussed in Section 2.6 and in Zeno (2009), the basis for current shear friction design methodology is entirely empirical and does not represent the actual observed behavior. While the current design approach is calibrated for the use of steel hav- ing yield strength less than 60 ksi, it is shown to be inadequate for other cases (both higher and lower yield strengths). It is rec- ommended that an extensive study be undertaken to establish a more rational design basis for establishing shear friction capacity. Such a study will also support the understanding of shear capacity in general. 3.3.4 Moment Redistribution Analytical formulations were used to establish the strain limit for which negative moments at the internal supports of continuous beams can be redistributed. This strain limit needs to be verified experimentally by testing continuous beams. 3.3.5 Control of Flexural Cracking and Corrosion The current provisions in the AASHTO specifications for maximum spacing of reinforcement for Class 1 exposure are based on an assumed crack width of 0.017 in. A Class 2 expo- sure corresponds with a crack width of 0.013 in. At the same time, there appears to be little or no correlation between crack width and corrosion. The current equation for maxi- mum spacing requires that the tensile stress in steel rein- forcement at the service limit state be calculated. For a beam, this is relatively simple. However, for a bridge deck, it is more complicated because of arching action and two-dimensional load distribution. Research is needed to address the issue of control of cracking by distribution of reinforcement and its impact on corrosion of reinforcement. The research should include all types and grades of corrosion-resistant reinforcement. 63

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 679: Design of Concrete Structures Using High-Strength Steel Reinforcement evaulates the existing American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) Bridge Design Specifications relevant to the use of high-strength reinforcing steel and other grades of reinforcing steel having no discernible yield plateau.

The report also includes recommended language to the AASHTO LRFD Bridge Design Specifications that will permit the use of high-strength reinforcing steel with specified yield strengths not greater than 100 ksi.

The Appendixes to NCHRP Report 679 were published online and include the following:

APPENDIX A—Material Properties

APPENDIX B—Flexural Resistance of Members with Reinforcing Bars Lacking Well- Defined Yield Plateau

APPENDIX C—Strain Limits for Tension-Controlled/Compression-Controlled and Strains to Allow Negative Moment Redistribution

APPENDIX D—Flexure Beam Tests

APPENDIX E—Fatigue of High-Strength Reinforcing Steel

APPENDIX F—Shear Beam Tests

APPENDIX G—Analytical Studies of Columns

APPENDIX H—Beam Splice Tests

APPENDIX I—Crack Control

APPENDIX J—Survey Results

APPENDIX K—Design Examples

APPENDIX L—Proposed Changes to Section 5 of the AASHTO LRFD Specification

APPENDIX M—2010 AASHTO Bridge Committee Agenda Item

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