Experimental Evaluation of a Device Prototype Based on Shape Memory Alloys for the Seismic Rehabilitation of Historical Buildings

Arches, vaults and roof trusses are typical elements of historical buildings. In many cases, however, suitable structural details (concrete curbs, tie beams, steel tie rods) for absorbing the out-of-plane horizontal thrusts exerted by such structural elements are missing, inadequate or deteriorated. Furthermore, many historical buildings were built to a much lower seismic intensity compared to similar modern structures. Seismic retrofit measures, integrated into the building without altering its character and appearance, are then needed.

In this paper, a post-tensioning system based on the superelastic properties of Ni-Ti SMA is proposed, as an alternative to currently used metallic tie rods.

The main advantages of the proposed SMA system, compared to traditional metallic ties, are: (i) calibrating the stress during the post-tensioning process, (ii) avoiding buckling under negative seismic displacements, (iii) controlling structural deformations, if a given target displacement is exceeded, under unexpected earthquakes, (v) dissipating energy during the earthquake. An additional feature is the possibility of limiting force changes in the steel tie due to temperature variations.

The mechanical behaviour of the proposed SMA system has been evaluated through cyclic tests on a full-size prototype, realized with 20 1mm diameter Ni-Ti SMA wire loops post-tensioned at about 2% strain. The tests have been conducted at 0.5 Hz and strain amplitude increasing from 2% to 8%.

The seismic effectiveness of the proposed SMA system has been verified through shaking table tests on a 1:4-scale timber roof truss model. One artificial accelerogram and one natural record have been used as input signals of the earthquake platform. The seismic intensity has been progressively increased during the tests up to the operative limits of the SMA system. Tests have been then repeated after removing the SMA system. The comparison between the structural response with and w/o SMA system confirms the achievement of the design objectives.