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1、 Highway IDEA Program Active Confinement of Bridge Piers Using Shape Memory Alloys Final Report for Highway IDEA Project 135 Prepared by: Bassem Andrawes, University of Illinois at Urbana-Champaign February 2010 ii INNOVATIONS DESERVING EXPLORATORY ANALYSIS (IDEA) PROGRAMS MANAGED BY THE TRANSPORTAT
2、ION RESEARCH BOARD (TRB) This NCHRP-IDEA investigation was by Research five sheets at Zone 2 and two sheets at Zone 3. However, Zone 1 (plastic hinge zone) was retrofitted differently in the three columns. For the GFRP retrofitted column, Zone 1 was wrapped with 10 layers of GFRP; while for the SMA
3、column, 0.08 in. NiTiNb SMA wire was used as a spiral with 0.4 in. pitch. Based on the previously discussed mechanical properties of the GFRP and using a jacket efficiency factor of 0.5 (3), the predicted confinement pressure corresponding to 10 layers of GFRP was 218 psi. Using the SMA recovery str
4、ess results (see Figure 3 and 4), the pitch of the SMA spirals was determined so that the spiral would exert the same confinement pressure at Zone 1 as that of the 10-layer GFRP jacket. The amount of SMA and GFRP was cut into half for the hybrid column (i.e. 5-layer GFRP jacket + SMA spiral w/0.8 in
5、. pitch), which resulted in half of the confinement pressure being applied actively, while the other half applied passively. 13 (a) As-built(c) SMA Zone1 (b) GFRP(d) Hybrid(a) As-built(c) SMA Zone1 (b) GFRP(d) Hybrid FIGURE 17 Four column specimens before testing. TABLE 4 Confining techniques at eac
6、h column Specimen Zone 1 Zone 2 Zone 3 GFRP Column 10-layer GFRP jacket 5-layer GFRP jacket 2-layer GFRP jacket SMA Column SMA spiral w/0.4 in. pitch 5-layer GFRP jacket 2-layer GFRP jacket Hybrid Column SMA spiral w/0.8 in. pitch + 5-layer GFRP jacket 5-layer GFRP jacket 2-layer GFRP jacket 3.3.2 L
7、oading Protocol Figure 18 shows the load protocol that was used in the tests. The columns were loaded cyclically with a rate of 0.2 in./min up to 1.5 % drift and 0.6 in./min thereafter. Initially a load increment of 0.5 % drift was adopted until a drift of 6 % was reached, after which an increment o
8、f 1 % was used until 12 % drift, followed by an increment of 2 % until the end of the test. 01234 x 10 4 -10 -7.5 -5 -2.5 0 2.5 5 7.5 10 Time (sec) Drift (%) 01234 x 10 4 -10 -7.5 -5 -2.5 0 2.5 5 7.5 10 Time (sec) Drift (%) FIGURE 18 Loading protocol used in the study. 3.3.3 Test Results Figure 19 s
9、hows the lateral force versus lateral drift of the four tested columns. The columns retrofitted with SMA spirals were able to sustain larger force and drift and dissipate significantly more hysteretic energy compared to that of the as- built and GFRP column. The yielding of the longitudinal steel in
10、 the as-built column was first observed at 1.5 % drift ratio and the column reached a maximum strength of 7.76 kips at 2.8 % drift ratio. For the GFRP retrofitted column, the 14 maximum strength recorded was 7.89 kips at a drift ratio of 3.5 %. After which the column started showing signs of gradual
11、 strength degradation. At 8% drift, the strength was reduced to 34.6 % of the maximum strength. The maximum strength of the SMA column was found to be 8.27 kips at 12 % drift ratio. After steel yielding, hardening behavior was observed. The Hybrid column had the maximum strength of 8.00 kips at the
12、10 % drift ratio. For the SMA and hybrid columns, hardening behavior was observed after steel yielding. This could be attributed to the elastic behavior of the SMA spirals. The test of the SMA retrofitted columns was stopped when the load-carrying capacity degraded by approximately 20 % of the maxim
13、um strength. The strength degradation was due to the rupture of one of the longitudinal reinforcement bars. The longitudinal reinforcement bars of the SMA and Hybrid columns ruptured at 12 % drift and 10 % drift, respectively. -8-6-4-202468 -8 -6 -4 -2 0 2 4 6 8 Displacement (in) Force (Kips) -16-12
14、-8-40481216 -8-6-4-202468 -8 -6 -4 -2 0 2 4 6 8 Displacement (in) Force (Kips) -16-12-8-40481216 Drift (%) 2.8 = -8-6-4-202468 -8 -6 -4 -2 0 2 4 6 8 Displacement (in) Force (Kips) -16-12-8-40481216 -8-6-4-202468 -8 -6 -4 -2 0 2 4 6 8 Displacement (in) Force (Kips) -16-12-8-40481216 Drift (%) 2.8 = -
15、8-6-4-202468 -8 -6 -4 -2 0 2 4 6 8 Displacement (in) Force (Kips) -16-12-8-40481216 Drift (%) 3.3 = -8-6-4-202468 -8 -6 -4 -2 0 2 4 6 8 Displacement (in) Force (Kips) -16-12-8-40481216 Drift (%) 3.3 = -8-6-4-202468 -8 -6 -4 -2 0 2 4 6 8 Displacement (in) Force (Kips) -16-12-8-40481216 Drift (%) 6.7
16、= -8-6-4-202468 -8 -6 -4 -2 0 2 4 6 8 Displacement (in) Force (Kips) -16-12-8-40481216 Drift (%) 6.7 = (a) As-built(b) GFRP (c) SMA(d) Hybrid -8-6-4-202468 -8 -6 -4 -2 0 2 4 6 8 Displacement (in) Force (Kips) -16-12-8-40481216 Drift (%) 8.0 = -8-6-4-202468 -8 -6 -4 -2 0 2 4 6 8 Displacement (in) For
17、ce (Kips) -16-12-8-40481216 Drift (%) -8-6-4-202468 -8 -6 -4 -2 0 2 4 6 8 Displacement (in) Force (Kips) -16-12-8-40481216 Drift (%) 8.0 =8.0 = -8-6-4-202468 -8 -6 -4 -2 0 2 4 6 8 Displacement (in) Force (Kips) -16-12-8-40481216 -8-6-4-202468 -8 -6 -4 -2 0 2 4 6 8 Displacement (in) Force (Kips) -16-
18、12-8-40481216 Drift (%) 2.8 = -8-6-4-202468 -8 -6 -4 -2 0 2 4 6 8 Displacement (in) Force (Kips) -16-12-8-40481216 -8-6-4-202468 -8 -6 -4 -2 0 2 4 6 8 Displacement (in) Force (Kips) -16-12-8-40481216 Drift (%) 2.8 = -8-6-4-202468 -8 -6 -4 -2 0 2 4 6 8 Displacement (in) Force (Kips) -16-12-8-40481216
19、 Drift (%) 3.3 = -8-6-4-202468 -8 -6 -4 -2 0 2 4 6 8 Displacement (in) Force (Kips) -16-12-8-40481216 Drift (%) 3.3 = -8-6-4-202468 -8 -6 -4 -2 0 2 4 6 8 Displacement (in) Force (Kips) -16-12-8-40481216 Drift (%) 6.7 = -8-6-4-202468 -8 -6 -4 -2 0 2 4 6 8 Displacement (in) Force (Kips) -16-12-8-40481
20、216 Drift (%) 6.7 = (a) As-built(b) GFRP (c) SMA(d) Hybrid -8-6-4-202468 -8 -6 -4 -2 0 2 4 6 8 Displacement (in) Force (Kips) -16-12-8-40481216 Drift (%) 8.0 = -8-6-4-202468 -8 -6 -4 -2 0 2 4 6 8 Displacement (in) Force (Kips) -16-12-8-40481216 Drift (%) -8-6-4-202468 -8 -6 -4 -2 0 2 4 6 8 Displacem
21、ent (in) Force (Kips) -16-12-8-40481216 Drift (%) 8.0 =8.0 = FIGURE 19 Lateral force vs. lateral drift of the four tested columns. The displacement ductility ratios () of the four columns were computed to evaluate and compare their flexural ductility capacity. The ductility ratio is defined as the r
22、atio between the drifts at the ultimate point (measured at 80 % of the ultimate strength) and the yielding point. Based on this definition, the ductility ratios of the As-built, GFRP, SMA, and hybrid columns were 2.8, 3.3, 8.0, and 6.7, respectively. A summary of the strength and ductility ratio res
23、ults is presented in Table 5 showing the strength and ductility ratio normalized values based on the as-built column results. The conventional passive confinement technique using GFRP wraps increased the strength and the ductility by 2 % and 18 %, respectively. However, the columns retrofitted with
24、SMA spirals showed 37 % increased strength, and 139 %185 % increased ductility compared to the as-built column. Furthermore, the GFRP retrofitted column started showing significant signs of stiffness degradation and strength deterioration at a drift ratio of 5 %, while in the case of the SMA retrofi
25、tted columns, no signs of degradation were observed until the column reached a drift ratio of 12 % (SMA column) and 10 % (Hybrid column). TABLE 5 Maximum and normalized strength and ductility of the four columns As-built GFRP SMA Hybrid Max. Strength (kips) 7.76 7.84 8.27 8.00 Normalized Strength 1
26、1.01 1.07 1.03 Ductility Ratio 2.8 3.3 8.0 6.7 Normalized Ductility Ratio 1 1.18 2.85 2.39 15 3.3.4 Evaluation of Damage In addition to improving the column ductility, it is important for the retrofitting method to limit the damage sustained by bridge column during major seismic events. This will en
27、sure that the functionality of the transportation network will not be disrupted after an earthquake. The effectiveness of the new retrofitting technique using SMA spirals was evaluated after the columns were tested. Figure 20 shows the damage sustained by the four columns at a drift ratio of 8 % exc
28、ept the as-built column, which was tested until 5 % drift. For the as-built column, the column was severely damaged, having completely spalled cover concrete, heavily crushed core concrete, and ruptured and buckled reinforcement bars at the 5 % drift ratio (see Figure 20.a.). Also it could be seen f
29、rom the figure (see Figure 20.b) that at 8 % drift, the GFRP retrofitted column sustained significant damage in the form of rupture of GFRP sheets, complete spalling of the concrete cover, and significant crushing of the concrete core. However, for the case of the SMA and the hybrid columns, only mi
30、nor damage in the form of horizontal cracks in the concrete cover or GFRP jacket was observed (see Figure 20.c and d). This limited damage could be attributed to the large active confining pressure applied by the SMA spirals, which helped in delaying the crushing of the concrete underneath. a) As-bu
31、iltb) GFRPd) Hybridc) SMAa) As-builtb) GFRPd) Hybridc) SMA FIGURE 20 Damage sustained by the as-built column at 5 % drift (a) and retrofitted columns (b), (c), and (d) at 8 % drift Figure 21 shows the four columns after removing the wrappings and cleaning up the crushed concrete. As indicated earlie
32、r, the final drift ratios sustained by the columns were 5 %, 8 %, 14 % and 14 % for the as-built, GFRP, SMA and hybrid columns, respectively. As shown in the figures, the as-built and GFRP columns sustained severe and irreparable damages. Even though during testing the SMA and hybrid columns reached
33、 75 % more drift than the GFRP column, their level of damage was extremely less than that of the GFRP column. This clearly demonstrated that using SMA spirals is not only effective in improving the flexural ductility of the columns, but also in limiting their damage during earthquakes, which will ha
34、ve a significant impact on maintaining the post-earthquake bridge functionality. a) As-builtb) GFRP d) Hybrid c) SMAa) As-built b) GFRP d) Hybrid c) SMA FIGURE 21 Damage sustained by the four columns after the GFRP sheets and SMA spirals are removed. 16 4. PLANS FOR IMPLEMENTATION This project helpe
35、d in proving the concept of using thermally prestressed SMA spirals for retrofitting vulnerable RC bridge columns. The product (i.e. SMA spiral) is ready for immediate use in retrofit and emergency repair projects. Although the project has ended, work is still underway to provide the bridge engineer
36、ing community with more data and information on the behavior and longevity of these spirals when used in real bridges. Among the issues which are currently being investigated is: Understanding the behavior of the spirals and the actively confined concrete under real seismic loading (i.e. strain rate
37、 effects). Studying the durability of the spiral under harsh environmental conditions. Searching for other cost-effective SMAs with thermomechanical characteristics suitable for the application of interest. Modeling the behavior of the actively confined concrete using SMA spirals. This will help in
38、studying the impact which this new retrofitting/repair technique has on the entire bridge system. Studying the feasibility of using the same concept studied in this project in retrofitting/repairing non-circular columns. 5. CONCLUSIONS This project explored a new application of SMAs that could poten
39、tially transform how RC bridges are retrofitted and/or repaired. The results of the project clearly proved the superiority of the proposed SMAs spirals compared to the currently used FRP jackets in terms of: (1) Increasing the flexural ductility of the columns (more than 2.4 times the ductility obta
40、ined from using GFRP jacket). (2) Limiting the damage sustained by the columns even under excessive lateral drifts (14 %-drift). Furthermore, the amount of SMA used to reach such superior behavior was relatively small and the amount of time and labor required for installing the SMA spirals were mini
41、mal. Unlike using prestressed strands or FRP jackets, installing the thermally prestressed SMAs will require minimal labor and hardware. Further, in contrary with FRP jackets, the proposed SMA spirals do not require any curing time, which makes the spirals very suitable for emergency repairs followi
42、ng a major earthquake or a collision accident. This project has provided bridge engineers with an effective and easy tool for applying the concept of active confinement on-site. This very concept can be used to mitigate the effects of various man-made and natural hazards (e.g. earthquakes, impacts,
43、blasts, etc.) on bridges. 6. REFERENCES 1. Richart, F.A., Brandtzaeg, A. and Brow, R. L. A study of the failure of concrete under combined compressive stress. Bulletin 185, University of Illinois Engineering Experiment Station, Urbana IL, 1928. 2. Otsuak, K., and Wayman, C.M. Shape memory material. Cambridge University Press, New edition, 2002. 3. L.D. Lorenzis and R. Tepfers. Comparative Study of Models on Confinement of Concrete Cylinders with Fiber- Reinforced Polymer Composites. J.Composite for Construction. ASCE, Vol. 7, No. 3, 2003, pp. 219-237.