Recently,　structural　optimization　has　become　an　important　tool　for　structural　designers,　because　it　allows　a　better　exploitation　of　material,　thus　decreasing　a　structure＇s　self-weight　and　saving　material　costs.　Moreover,　structural　optimization　helps　the　designer　to　find　innovative　design　solutions　and　structural　forms　that　not　only　better　exploit　material　but　also　give　the　structure　greater　aesthetic　value　from　an　architectural　point　of　view.　In　this　article,　the　seismic　retrofitting　of　a　bridge　originally　designed　in　reinforced　concrete　is　illustrated,　showing　how　lightening　the　bridge　superstructure,　rather　than　reinforcing　the　already　completed　foundations　and　abutments,　allowed　these　latter　features　to　resist　greater　seismic　actions　as　required　in　the　recent　update　of　the　Italian　seismic　code.　Therefore,　besides　using　the　steel-concrete　composite　typology,　the　bridge　superstructure　was　lightened　through　structural　optimization.　After　having　optimized　the　thickness　of　webs　and　flanges,　it　was　necessary　to　further　lighten　the　bridge　superstructure　by　removing　unexploited　material　from　the　bottom　flange　through　the　insertion　of　large　cavities.　For　this　purpose,　topology　optimization　is　shown　to　be　a　powerful　tool　that　allowed　the　designer　to　find　that　the　hole　shape　was　basically　elliptic,　thus　suggesting　their　regularization　as　ellipses.　Comparisons　were　made　between　several　design　solutions,　each　characterized　by　a　specific　volume　reduction　of　the　bottom　flange.　Identification　of　the　highest-performing　solutions　through　computer-aided　procedures　led　to　a　weight　reduction　of　40%　with　respect　to　the　design　solution　in　reinforced　concrete.　Retrofitting　the　already　existing　foundations　and　abutments　to　satisfy　the　updated　provisions　of　the　new　seismic　code　was　thus　avoided　by　defining　an　innovative　layout　of　arch　bridges　with　holes　in　the　bottom　flange,　which　has　never　been　used　before.　©　2013　American　Society　of　Civil　Engineers.