The experimental load-displacement curves for the tested specimens and the pictures of the critical section at 1% and 2.5% drift, corresponding, respectively, to the drift under the 50% in 50yr and 10% in 50yr probability earthquakes, are illustrated in Fig. 4. Based on the results, the following observations may be made:
• all the columns had almost the same maximum force capacity, equal to 75kN, as expected;
• the cast in situ column (CS) collapsed during the second cycle at 5% drift (160mm), due to the tensile failure of one of the reinforcing bars. At the time of collapse, the residual column strength was 30% smaller than the maximum column strength. Considerable pinching appears in the cycles after the cycles at 2.5%. Up to the cycles at 2.5% drift, representative of the maximum drift under the design earthquake, the column behaviour was stable, and little damage could be observed in the column, as demonstrated by the pictures at 1% and 2.5% drift;
Fig. 1 – Tested specimens (Dimensions expressed in cm): (a) CS, cast in situ column; (b) GS4, column
with four grouted sleeves; (c) GS4B, column with four grouted sleeves and 90° hooks in the anchored
bars; (d) PF, pocket foundation; (e) GS8, column with eight grouted sleeves.
Fig. 2 – Experimental setup: (a) reaction frame; (b) loading system.
Fig. 3 – Loading history.
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Fig. 5 – Pictures at 5% drift.
from the base, leaving the role to resist compressive forces due to cyclic bending to the grout confined by the aluminium sleeves and to the vertical rebars only;
• A slightly more pronounced pinching is observed in the cycles of the GS specimens. This is due to the aforementioned progressive damage of the base grout layer, leading to a larger strain localization at the column base;
• for all of the GS specimens, the test could be carried out up to the 6.5% drift cycle. The failure of one rebar during the cycle at 6.5% drift was observed only in specimen GS4B. The higher displacement capacity of grouted sleeves compared to cast-in-situ and pocket foundation specimens is due to the heavy confinement induced by the aluminium sleeves on the grout;
• it is observed that, due to an imperfect clamping of specimen GS8 to the reaction frame, a rigid rotation occurred after each loading reversal during the test of this specimen. The observed rigid rotation did not, however, affect the overall response of specimen GS8.
Fig. 5 shows the damage at the base column section corresponding to the 5% drift cycle (maximum displacement equal to 160mm). The following observations may be made;
• specimen CS shows a large localized crack at the base, some minor cracks along the column, and some spalling. The high strain localization observed in this specimen justifies the premature bar failure observed;
• specimen PF is affected by several large cracks spreading along a length approximately equal to the column base dimension. This behaviour is typical of reinforced concrete columns;
• all of the grouted sleeve connections show concrete spalling at the corners, next to the aluminium sleeves, and a considerable crushing of the grout layer at the column base. On the other end, grouted sleeve specimens showed no other noticeable sign of damage;
• although the strain localization at the base of GS specimens should in principle lead to an anticipated collapse of the columns, the existence of heavily confined grout columns within the sleeves effectively prevented an early failure of the connections. Furthermore, the sleeve, and the confined grout within, prevented buckling of the vertical rebars, anchored in the foundation;
• the observed damage patterns allow to conclude that, being the damage limited to the grout layer existing at the base, the remaining part of the column being mostly undamaged, grouted sleeve connections are more easily repairable after a seismic event.
Finally, Fig. 6 illustrates the comparison of the energy dissipated during each cycle by all of the specimens tested.
It is observed that no significant differences exist between specimens CS, PF, GS4, and GS4B up to a 2% drift (64mm). Starting from the 2.5% cycles, specimen PF shows a larger energy dissipation, due to a smaller strength degradation and to a smaller strain localization next to the column base section (Figs. 4 and 5). Negligible differences exist between the remaining specimens up to the collapse of specimen CS, occurring during the first cycle at 5% drift.
Specimen GS8 systematically showed a considerably smaller energy dissipation than the remaining specimens. This is due to the top drift resulting from the sum of two terms, the first related
Fig. 6 – Dissipated energy.
Fig. 7 – Moment-base rotation diagrams (rotation taken over 215mm gauge length).
to the column deformation, and the second due to the aforementioned rigid rotation at the base, the latter implying no energy dissipation.
Fig. 7 shows the moment-rotation diagrams at the column base sections, where the rotation is measured over a 215mm gauge length. The results presented allow the following observations:
• the base rotation is consistently much higher for the grouted sleeve specimens than for the cast – in-situ and pocket foundation specimens. This effect once more demonstrates that a much larger strain localization occurs in the former than in the latter;
• the maximum rotation in the GS specimens is only 20% smaller then the value it would exhibit considering a rigid rotation of the column around its base. This result demonstrates that the behaviour of the column outside the base section is mostly linear elastic, and that very little cracking and damage occurs outside the base section;
• deformations in the CS specimen show a greater localization in the base section with respect to those in the PF specimen. This effect lead to the early rebar failure observed.
The experimental results presented allow to conclude that grouted sleeves ensure a ductility similar to the one of cast in situ column-foundation connections and of pocket foundations, although a slightly smaller dissipation capacity is observed.
The high ductility of the grouted sleeve solutions is related to the high confining effect of the corrugated aluminium sleeves on the grout columns contained within. Furthermore, the presence of a highly confined grout prevents longitudinal reinforcing buckling.
It was shown that, in grouted sleeves connections, the damage is localized at the column base, in the 20 mm grout layer existing between the prefabricated column and the foundation. As a result, very little damage may be observed in the column outside of the base section.
The damage of the base grout layer results in a higher strength degradation for the grouted sleeve connections with respect to more traditional cast-in-situ and pocket foundations solutions.
Due to the damage localization observed, and to the consequent small damage existing along the column, an easier post-seismic column repair has to be expected for the grouted sleeve column – foundation connections, with respect to cast-in-situ or pocket foundation solutions.
The experimental tests were carried out within a research program on precast column – foundation connections financed by Moretti SpA, Erbusco (BS), Italy.
The cooperation of ing. Cristian Ratti, ing. Andrea Zini, and ing. Andrea Belleri, in setting up the reaction frame, carrying out the experimental tests, and post-processing all the experimental test results, is gratefully acknowledged.
EC8 (2003), “Eurocode 8: Design of structures for earthquake resistance – Part 1: General rules, seismic actions and rules for buildings,” PrEn 1998-1, European Committee for Standardization, December 2003.
OPCM 3274 (2003), “First elements concerning general criteria for the seismic classification of the Italian territory, and seismic code provisions (in Italian),” march, 2003.
CEB (1996), “Frame Members in bending with or without axial force,” CEB Bullettin N. 231, May 1996.