In confining spiral reduction for SRC columns in the Taiwan SRC Code[21], steel shares part of the SRC column axial force, leading to relative decline of the axial force that concrete has to bear, hence a reduction in SRC column spirals. However, the Code does not consider the role of steel flanges in confining the core concrete in SRC column.
Weng’s Formula design method[22] further considers that the “highly confined concrete” in the SRC column may be confined by the steel flanges, while the concrete to be confined with spirals in the SRC column is largely the “ordinary confining concrete”, hence relaxed confining spirals in the SRC column. What is characteristic about Weng’s Formula is the simultaneous considera-tion of the “steel consumpconsidera-tion” and the “confinement of steel flanges to the core concrete” in the SRC column, and the adoption of a new reduction factor (as shown in Section 3.2) to relax the confining spirals required for the SRC column.
As shown in Table 1, the two foregoing methods were employed in designing confining spirals of the SRC column speciments, with results as follows:
Figure 20 Hysteresis loops of the specimen C-SRC4 subjected to cyclic loading. (a) Force-displacement relationship; (b) moment-rotation relationship.
Figure 21 Envelop of the hysteresis loops of the specimen C-SRC1.
Figure 22 Envelop of the hysteresis loops of the specimen C-SRC2.
Figure 23 Envelop of the hysteresis loops of the specimen C-SRC3.
Figure 24 Envelop of the hysteresis loops of the specimen C-SRC4.
Figure 25 Variations of the strains recorded from specimen C-SRC1. (Note: Strain readings were recorded from strain gauges on longitudinal rebar, steel flange and large spiral, located 300 mm from the SRC column base).
Figure 26 Variations of the strains recorded from specimen C-SRC2. (Note: Strain readings were recorded from strain gauges on longitudinal rebar, steel flange and large spiral, located 300 mm from the SRC column base).
(1) Taiwan SRC Code was applied to design the specimens C-SRC1 and C-SRC3. The spiral spacing was 95 mm; the spiral consumption per unit length was 283 N/m; and the spiral reduction factor was 0.79. The spiral reduction factor represents the ratio of the usage of confining steel calculated in accordance with the Taiwan SRC Code to that of the ACI-318 Code. The smaller the reduction factor, the more saving of the spirals.
(2) Weng’s Formula was adopted in designing specimens C-SRC2 and C-SRC4, with fewer spirals required for the two specimens and the spiral spacing was 115 and 110 mm. The spiral consumption per unit length was 235 and 245 N/m, and the spiral reduction factor was 0.65 and 0.68, respectively. The spiral reduction factor represents the ratio of the usage of confining steel calculated in accordance with Weng’s Formula to that of the ACI-318 Code.
The test results suggested that although fewer spirals were used in specimens C-SRC2 and
C-SRC4 where Weng’s Formula was applied, observations of the hysteresis loops of the four SRC columns in Figures 17 to 20 and the envelops of Figures 21 to 24 indicate that the speci-mens to which Weng’s Formula was applied are satisfactory in strength, toughness and seismic resistance, as good as the SRC column specimens (C-SRC1 and C-SRC3) to which Taiwan SRC Code was applied.
Table 4 makes analysis and comparison of the flexural strengths of the four SRC columns. The comparison shows that the test flexural strengths, (Mn)test, of specimens C-SRC2 and C-SRC4 (to which Weng’s Formula was applied and fewer spirals were used) are not necessarily weaker than the strengths of specimens C-SRC1 and C-SRC3 (to which Taiwan SRC Code was applied and more spirals were used); where the flexural strength of the specimen C-SRC1 is 1791 kN-m, slightly smaller than 1875 kN-m of the specimen C-SRC2; and the flexural strength of the speci-men C-SRC3 is 1915 kN-m, slightly larger than 1853 kN-m of the specispeci-men C-SRC4. In addition, the table also shows the ratios of test flexural strength over nominal flexural strength of the four SRC columns, (Mn)test/(Mn)SRC, which comes to 1.27, 1.33, 1.29 and 1.25, respectively, where the values of (Mn)SRC were calculated using the measured material strengths given in Table 2. The ratios indicate that the strengths of the SRC columns confined with 5-spirals are excellent, with over-strength factors ranging from 125% to 133%.
Table 4 Flexural Strength of the SRC columns tested in this study SRC column
cross-section Specimen designation Pa(kN) (1) col-umn calculated according to Taiwan SRC Code. (2) PP'is the lateral load caused by the P' effect, as given in eq.(18). (3) Ph is he recorded maximum lateral load applied to the column from the MTS actuator. (4) (Ph)testis the total lateral load applied to the column including the P' effect; (4) = (2) + (3). (5) (Mn)SRC is the bending moment which can be resisted by the column while subjected to the axial load Pa, calculated according to Taiwan SRC Code. (6) (Mn)testis the total bending moment applied to the column including the P' effect. The value of (Mn)testis equal to the product of (Ph)test and the distance between the load-ing point of the lateral force and the center of the plastic hload-inge of SRC column near the column base.
The test results showed that although Weng’s Formula relaxes confining spiral spacing of the SRC column, the strength and ductility of the SRC columns designed with Weng’s Formula are as good as the specimens to which Taiwan SRC Code design is applied. The application of rectan-gular SRC columns confined with 5-spirals plus spiral spacing designed with Weng’s Formula may bring about satisfactory seismic resistance and economic benefits for the SRC column.
Weng’s Formula design method will further rationalize the confining spiral arrangement of the
SRC column, may generate better design results and simplify the construction process of the SRC column hoops. In general, the test results have suggested that with significant saving of the con-finement steel, the newly innovated 5-spirals can be successfully applied to the rectangular SRC columns.
Finally, observations of the final conditions of the four SRC columns after the test ended, as shown in Figures 12 to 16, suggested that when the drift angle reached 6% radians, although concrete cover in the plastic zone of the SRC column tangibly flakes off, concrete within the con-finement of the 5-spirals remained sound, the longitudinal reinforcements did not buckle, nor did the spirals break. More significantly, the strengths of the four SRC columns were found to be able to sustain and stay at high level without tangible decline trend throughout the entire cyclic load-ing process. These observations indicated that the 5-spirals provided excellent concrete confine-ment and prevented the buckling of longitudinal reinforceconfine-ments.
5 Conclusions
Seismic cyclic loading tests of four full-scale rectangular SRC columns confined with the newly innovated 5-spirals were carried out successfully. The test results showed that the SRC columns enjoy excellent strength, toughness and seismic resistance. As the 5-spirals can be manufactured with automation, they have significant economic benefits and are very suitable for precast con-struction. Based on the test results, the following conclusions can be drawn within the scope of this study:
(1) For the four SRC columns which underwent seismic cyclic loading tests, their hysteresis loops were very rich, the drift angles of the four SRC columns reached 6.0% radians, indicating rectangular SRC columns confined with 5-spirals have excellent capability in energy dissipation and seismic resistance.
(2) Observations from the tests indicated that the strengths of the four SRC columns were able to sustain and stay high without tangible decline trend throughout the entire cyclic loading proc-ess. This phenomenon suggests that the SRC columns confined with 5-spirals are capable of ef-fectively resisting horizontal shear caused by strong earthquake without rapid collapse. This fea-ture is of obvious significance to the safeguarding of lives and properties at the outbreak of major earthquakes.
(3) When the tests ended, observations of the final conditions of the SRC columns found tan-gible flake-off of concrete cover in the plastic zone near the SRC column base, but concrete within the confinement of the 5-spirals remained sound, the longitudinal reinforcements had no buckling, nor did the spirals break. These observations indicate that the 5-spirals can provide ex-cellent concrete confinement and help to prevent the buckling of longitudinal reinforcements.
(4) With significant saving of the confinement steel, the test results showed that although the Weng’s Formula relaxes the confining spiral spacing, the strength and ductility of the SRC col-umns designed with the Weng’s Formula are as good as those specimens designed in accordance with the Taiwan SRC Code. Thus the application of rectangular SRC columns confined with the 5-spirals plus the spiral spacing designed with the Weng’s Formula may generate satisfactory seismic resistance and economic benefits for the SRC columns.
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