PRELIMINARY EUROPEAN RECOMMENDATIONS FOR THE DESIGN OF SANDWICH PANELS WITH OPENINGS A STATE OF THE ART REPORT European Conventionfor Constructional Steelwork N° 134, 1^ edizione 2014, EUROPEAN RECOMMENDATIONS ON THE STABILIZATION OF STEEL STRUCTURES BY SANDWICH PANELS – European Convention for Constructional Steelwork N° 135, 1^ edizione 2014 (CM 5/2014)