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

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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 fy to allow higher yield strengths. Added definitions of cl and tl; compression- and tension-controlled strain limits, respectively. and C5.4.3.1 Reinforcing Steel, Permits the use of reinforcing steel with General specified yield strengths up to 100.0 ksi when allowed by specific articles. Reinforcing Steel, Modulus of Es=29,000 may be used for specified yield Elasticity strengths up to 100.0 ksi. and C5.4.3.3 Reinforcing Steel, Special Permits the use of reinforcing steel with Applications specified yield strengths up to 100.0 ksi in Seismic Zone 1. and C5.5.3.2 Fatigue Limit State, Modifies the fatigue equation for reinforcing Reinforcing Bars bars to allow the equation to be used for specified yield strengths up to 100.0 ksi. and C5. Resistance Factors, Allows the use of reinforcing steel with Conventional Construction 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 Allows the use of reinforcing steel with AND AXIAL FORCE EFFECTS specified yield strengths up to 100.0 ksi in Seismic Zone 1. and C5.7.2.1 Assumptions for Strength Keeps compression- and tension-controlled and Extreme Event Limit States 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 fy may replace fs or fs' in and Strain Compatibility Approach Limits the steel stress in a strain compatibility calculation to the specified yield strength. C5. Maximum Reinforcement Replaces 0.005 with "tension-controlled strain limit." 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 Warns that designs should consider that Reinforcement columns using higher strength reinforcing steel may be smaller and have lower axial stiffness. Spirals and Ties Permits spirals and ties made of reinforcing steel with specified yield strengths up to 100.0 ksi in Seismic Zone 1. and C5.8.2.4 Regions Requiring Permits transverse reinforcement with Transverse Reinforcement specified yield strengths up to 100.0 ksi in and C5.8.2.5 Minimum Transverse applications with flexural shear without Reinforcement torsion. C5.8.2.7 Maximum Spacing of Transverse Indicates that spacing requirements have been Reinforcement verified for transverse reinforcement with specified yield strengths up to 100.0 ksi in applications of shear without torsion. and C5.8.2.8 Design and Detailing Permits transverse reinforcement with Requirements. specified yield strengths up to 100.0 ksi in applications with flexural shear without torsion.