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29 AASHTO LRFD 9.5.3 excludes concrete deck slabs from 2.4.4.3 Results of Fatigue Test 1 being investigated for fatigue. AASHTO justifies this exclusion Fatigue Test 1 was conducted between 3/10/2009 and based on results reported by de V Batchelor et al. (1978). It has been shown that slabs resist applied loads primarily 3/31/2009. The applied load at midspan was cycled between through internal arch action (AASHTO 9.7.1) and that the 7 and 17 kips at a rate of 1.2 Hz for 2 million cycles. The mea- nominal steel required is primarily to resist local flexural effects sured stress range in the A1035 longitudinal steel was 31.1 ksi (punching) and to provide confinement such that the arching in the initial test cycles. Strain gages were lost during the first action may be developed (Fang 1985 and Holowka et al. 1980). 100,000 cycles (loss of gages due to fatigue loading is expected). Due to the nature of fatigue damage, however, the stress range will increase marginally throughout the test (Neville 2.4.4 Fatigue Test Specimens 1975 and Harries et al. 2006). Moreover, equipment malfunc- 2.4.4.1 Fatigue Specimen Details tion during a few initial cycles resulted in unintentional load- ing of Fatigue Test 1 beyond 30 kips. These higher stress range Specimen details were selected to correspond to the details cycles had little impact on the beam behavior beyond causing of flexural specimen F3 (see Section 2.3.4 and Appendix D). additional cracks. Two beams 16 in. deep by 12 in. wide having four #5 A1035 During fatigue cycling, no notable degradation in beam longitudinal bars and #3 A 615 stirrups spaced at 9 in. along stiffness was observed. A small drift in absolute displacements the entire length of the beam were cast with 10 ksi concrete. was observed; the drift is attributable to degradation of the The beams were 18.5 ft long and were tested in midpoint flex- neoprene pads and "shakedown" of the test frame. Nonethe- ure over a span of 16.5 ft. Four-inch-wide neoprene supports less, the differential displacement, measured between 7 and were used; therefore, the face-to-face dimension of the span 17 kips applied load, remained essentially constant. Figure 13 is 16 ft-2 in. The fatigue test beams had the same shear span shows both the deflection (left axis) and secant stiffness mea- details as flexural specimen F3 but were not provided with a sured between applied loads of 7 and 17 kips (right axis) cycle constant moment region. This difference is due to the nature histories for Fatigue Test 1. Crack width measurements both of large-scale fatigue testing and the difficulties in providing during fatigue cycling and following the fatigue test during a accurate and safe four-point bending conditions. The meas- monotonic load cycle to 46 kips (capacity of actuator used) ured material properties of the steel reinforcement are given were remarkably consistent and confirmed the measured and in Appendix A. In summary, fy = 130 ksi (based on 0.2% analytically calculated bar stresses (Soltani 2010). Fatigue offset method), and the measured concrete compressive Test 1 behaved very well. The results indicate that the A1035 strength was 9.71 ksi. Cyclic testing was carried out at a fre- bars can maintain 2 million cycles at 31 ksi with little or no quency of 1.2 Hz. At regular intervals, the frequency was apparent damage. reduced to 0.003 Hz (1 cycle in 5 minutes) and a fully instru- mented cycle was carried out. 2.4.4.4 Results of Fatigue Test 2 2.4.4.2 Fatigue Test Protocol Fatigue Test 2 was conducted between 4/14/2009 and 4/16/2009. The applied load at midspan was cycled between Details of how the fatigue test protocol was established are 7 and 25 kips at a rate of 1.2 Hz. The measured stress range provided in Appendix E. The protocol adopted involved test- in the A1035 longitudinal steel was 45.5 ksi in the initial ing the first beam at a stress range (in the primary #5 A1035 test cycles. One of the four reinforcing bars (a corner bar) reinforcing bars) of 32 ksi. The justification being that if the experienced a fatigue failure at N = 155,005. The final meas- beam withstands 2 million cycles at stress greater than the ured cycle was N = 100,000. As shown in Figure 14, the theoretical endurance limit (for N = 2,000,000) of 28 ksi (see deflections were increasing with a rising number of cycles Appendix E), it has de facto exceeded the current AASHTO although the differential displacement (between 7 and requirements and thus represents a proof test with good con- 25 kips) remained relatively constant. The secant stiffness fidence. Since the first beam successfully resisted 2 million (also measured between 7 and 25 kips) demonstrated some cycles, the second beam was tested at a greater stress range, decay in the initial 100,000 cycles. The final data points at 46 ksi, to provide a second data point along the S-N curve. All N = 155,005 in Figure 14 were obtained from a single cycle test control is based on reinforcing bar stress measured using following fatigue failure and clearly indicate the effect of the strain gages. Four strain gages were used in each specimen: loss of one of the four primary reinforcing bars. Figure 15 one mounted on each A1035 bar. Gages on bars 1 and 3 were shows the ruptured bar following testing (and removal of located 8 in. to the left of the midspan loading point and those cover concrete). The bar failed at the location near a stirrup on bars 2 and 4 were located 8 in. to the right. which is typical of such fatigue failures because of fretting