Seismic assessment of precast beam-to-column connections and retrofitting solutions

The inadequacy of beam -to-column connections in RC precast structures can cause catastrophic collapses due, for instance, to loss of s upport phenomena in the case of absence of mechanical devices. In order to achieve an adequate global seismic capacity, hence, the retrofitting of these connections is a necessary step for existing precast structures. An experimental campaign is performed in order to investigate the seismic behavior of typical beam -to-column connections for precast one- story buildings. The reference connection is a dowel beam -to-column connection, tested under cyclic shear loads up to collapse. It showed a very brittle fai lure with concrete cover splitting in the column and a non- symmetric behavior for the two loading directions. In particular, for pulling loads (i.e. horizontal loads against the column frontal cover) a significant strength degradation occurs after the attainment of the maximum shear strength, corresponding to the occurrence of the first crack in the column concrete cover. After the cyclic shear test, the described beam -to-column connection was retrofitted. The proposed retrofitting solution is based on two main mechanical principles: the three hinged arch and the dowel effect so that the total seismic force applied to the beam is transferred to the other two hinges on the column by two steel profiles. The cyclic global behavior in terms of force -displacement curve was investigated and the damage patterns after the test were described by means of both visual inspections and instrumentation records. The experimental results demonstrate a good behavior under seismic actions, since no significant damage in the concrete was recorded due to the large concrete covers in the retrofitted system which prevented brittle failures in the connected elements. The horizontal shear force was mostly sustained by the three hinges mechanism, ensured by the two steel profiles unde r axial loads. Moreover, the comparison between the retrofitted and the dowel beam -to-column connection also allow to define the main critical features of this connection system, as the concrete cover of steel bars in the connected structural elements. T he retrofitted connections showed larger shear strength as well as larger dissipated energy values than the standard dowel connection.