This　paper　analytically　formulates　the　ultimate　capacities　for　various　failure　modes　of　composite　beams　consisting　of　a　modular　fiber　reinforced　polymer　(FRP)　web-flange　sandwich　deck　supported　by　a　steel　girder.　Pultruded　FRP　box　sections　are　incorporated　between　two　flat　panels　to　form　the　web-flange　sandwich　deck,　in　which　the　pultrusion　direction　of　box　sections　can　be　either　parallel　or　perpendicular　to　the　span　direction.　Such　a　deck　can　then　be　assembled　via　adhesive　bonding　or　blind　bolt　connections　to　a　supporting　steel　girder　to　form　a　composite　beam　system.　On　the　basis　of　theoretical　analysis　of　critical　sections　for　such　composite　beams　at　ultimate　states,　potential　failure　modes　are　identified,　including　flexural　failure　of　the　steel　girder　yielding　in　tension　and　GFRP　deck　failure　in　compression,　vertical　shear　failure,　and　horizontal　shear　failure　at　adhesive　bonding　at　bolted　connections　and　at　webs　of　FRP　box　sections.　For　each　failure　mode,　the　corresponding　ultimate　capacity　is　analytically　formulated;　thereby,　the　critical　failure　mode　and　ultimate　load　capacity　can　be　determined.　The　developed　ultimate　limit　design　approaches　are　validated　by　experimental　results　from　the　literature　for　such　FRP-steel　composite　beams.　©　2018　Elsevier　Ltd