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