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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 §5.5.3.2 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 §3.10.9.4.3a, 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 §9.7.3.2,
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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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.
5.4.3.1 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.
5.4.3.2 Reinforcing Steel, Modulus of Es=29,000 may be used for specified yield
Elasticity strengths up to 100.0 ksi.
5.4.3.3 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.
5.5.3.2 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.
5.5.4.2.1 and C5.5.4.2.1 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.
5.7.2.1 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
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 0.005 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 Warns that designs should consider that
Reinforcement columns using higher strength reinforcing steel
may be smaller 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 Permits transverse reinforcement with
Transverse Reinforcement specified yield strengths up to 100.0 ksi in
5.8.2.5 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.
5.8.2.8 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.
