Figure 14 shows a load-displacement hysteretic cyclic diagram. It shows a decrease of the peak load during the cycles, mainly due to both a damage of the steel reinforcement and geometrical effects,
i. e. the instability of compressed bars which deform in a flexural mode instead of a pure axial mode. The slope of the curves is a measure of the stiffness of the column and it presents two different situations: (i) an elastic phase, i. e. a phase where the steel bars load or unload in the elastic regime;
(ii) an elastic plastic stage, where the bars are loaded in tension or in compression/bending in the plastic range; these slopes are continuously decreasing because of the continuous damage of the concrete material during the cycles.
The bar chart of Figures 15 and 16 shows, for each cycle, the maximum horizontal force and the cumulative dissipated energy, expressed as a function of the cycle number.
It is easy to verify that the maximum load carried out by the column reinforced by stainless steel is more than 20% the corresponding reinforced with carbon steel and, more important, the resistance stays constant during the cycles more than the columns reinforced with “carbon” steel. The total energy dissipated is almost twice in favour of the stainless steel.
We can conclude that the same column, reinforced with stainless steel, will present after the same seismic action, probably, very less severe damages than the analogous column reinforced with carbon steel. Figure 17 shows pictures of the damaged “plastic hinge” at the final cycle of the loading history. It may be interesting to remark that some specimen present a plastic tube which prevent bond between the longitudinal bar and the concrete in the plastic hinge zone. The question was: does the bond influence the behaviour of the plastic hinge? The answer is: no. Interesting to observe in the right picture on the top of Figure 17 that the deformation of the “plastic hinge” is mainly due to shear; this happens when the concrete trust has collapsed. If the steel bar does not fail, then shear may be considered as the collapse mode of the column. The stainless steel greater strain hardening behaviour imposes a revision, in Eurocodes 2 and 8 (EN1992, 2004, EN1998, 2004), of the maximum shear
force the reinforced concrete member has to sustain if maximum advantage of the stainless steel resistance has to be exploit. Otherwise, like shown in the left picture of the second row, a failure of a stirrup is very likely to happen.
AISI304 stainless steel rebars have been tested under quasi-static tensile load as well as under tension-compression cyclic loading. The class of resistance is a 500 MPa characteristic yield limit. The rebars have demonstrated unusual ductility level, both in the monotonic and the cyclic loading, as compared to traditional carbon steel rebars. Tests on column prototypes have shown an analogous ductile structural behaviour but at a higher horizontal force. This fact suggests the idea of using stainless steel in seismic areas for a more limited behaviour factor (larger horizontal forces) but using, at the same time, a limited steel area (because of the higher resistance of the stainless steel) and therefore resulting in expected limited damages to the structure after the earthquake.