An　experimental　and　numerical　program　investigating　the　in-plane　flexural　behaviour　of　Q1100　ultra　high　strength　steel　(UHSS)　welded　I-sections　is　presented　in　the　current　paper.　The　testing　program　comprised　of　material　coupon　tests,　measurements　of　residual　stresses,　and　in-plane　3-point　bending　tests.　Subsequently,　a　numerical　program　was　executed,　involving　the　development　of　finite　element　(FE)　models.　The　FE　models　validated　against　the　experiments　were　subsequently　employed　for　parametric　studies,　aiming　at　assessing　the　flexural　behaviour　of　Q1100　welded　I-sections　across　a　broader　spectrum　of　section　geometries.　The　data　obtained　from　the　experiments　and　numerical　simulations　were　used　to　assess　the　applicability　of　prevailing　design　provisions　outlined　in　the　European,　Chinese　and　Australian　codes　for　high-strength　steel　structures.　These　design　provisions　encompass　classification　limits　for　cross-sections　and　predicted　methods　for　cross-sectional　resistance　in　bending.　Via　reliability　analyses,　adjustments　were　made　to　the　cross-section　classification　limits.　Finally,　a　modified　direct　strength　method　was　proposed　to　predict　the　in-plane　bending　resistance　for　Q1100　welded　Isections.