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This paper compiles the experimental and finite-element parametric study on member distortional buckling behaviour of new built-up metal hybrid double-I-box beams (HDIBBs). The cross-section of this built-up beam is unique and looks similar to the shape of a double-I-box fabricated using four channel sections. The flange plates were provided with an intermediate stiffener. In these built-up beams there is more material in the flange portions far away from the horizontal centroidal axis of their cross-section. Hence, there is an increase in the flexural rigidity that enhances the moment capacity of the beam, under major axis bending. The geometry consists of torsionally rigid closed-box web portion that provides high resistance to minor axis lateral-buckling. The varying parameters considered were the ratio of yield stresses of the flange to the web steel plates, the ratio of breadth to the depth of the section, and the flange plate thickness. In the experimental programme, all the HDIBB members failed due to kinds of distortional buckling which was identified by web buckling and flange twist along edges. The results revealed that when flange plate slenderness increases there is a drop in the moment resistance capacity of the beams. The numerical study was performed using ABAQUS software. In comparison, there was good agreement between experimental and numerical results. The validated finite element models were further extended to perform parametric studies on ideal HDIBB models. Both the experimental and parametric study results were compared with the predicted strengths using effective width method equations specified in the Euro code standards EN 3-1-3. It was found that the current Euro code design rules slightly over-estimate the distortional buckling resistance capacity of closed form built-up cold-formed steel members. A new design equation was formulated and recommended for estimating the reduction in distortional buckling moment resistance capacity for HDIBBs. Key words: distortional buckling behaviour, built-up, cold-formed steel, metal hybrid, flange plate slenderness, effective width method. Cet article constitue la compilation d'une etude experimentale et parametrique des elements finis au sujet du comportement en flambage distorsionnel des membrures d'une nouvelle poutre-caisson double en I hybride (PCDIH) composee metallique. La coupe transversale de cette poutre composee est unique et ressemble a la forme d'un caisson double en I fabrique avec quatre sections en U. Les semelles de renforcement sont munies d'un raidisseur intermediaire. Dans ces poutres composees il y a plus de materiau dans les parties des semelles loin de l'axe centroide horizontale de leur coupe transversale. Par consequent, il y a une augmentation de la rigidite a la flexion qui ameliore la capacite de resistance au moment de la poutre, a la flexion du grand axe. La geometrie est constituee d'une partie tubulaire fermee rigide en torsion qui fournit une haute resistance au flambage lateral de l'axe mineur. Les differents parametres consideres etaient le rapport des limites d'elasticite de la membrure aux plaques d'acier de l'ame, le rapport de la largeur a la profondeur de la section et l'epaisseur de la semelle de renforcement. Au niveau du programme experimental, toutes les membrures PCDHI ont echoue en raison du genre de flambage distorsionnel determine comme etant le voilement de l'ame et la torsion de membrure le long des bords. Les resultats ont revele que lorsque l'effilement de la semelle de renforcement augmente il y a une baisse de la capacite de resistance au moment des poutres. L'etude numerique a ete realisee a l'aide du logiciel ABAQUS. En comparaison, il y avait une bonne concordance entre les resultats experimentaux et numeriques. Les modeles d'elements finis valides ont ete etendus afin de faire des etudes parametriques sur des modeles ideaux de PCDHI. Les resultats de l'etude tant experimentale que parametrique ont ete compares aux resistances prevues utilisant des equations de methode de largeur efficace indiquees dans les normes du code Euro EN 3-1-3. Il s'avere que les regles de conception actuelles du code Euro surestiment legerement la capacite de resistance au flambage de distorsion des membrures fermees composees en acier plie a froid. Une nouvelle equation de conception a ete formulee et recommandee pour evaluer la reduction de la capacite de resistance au moment de flambage de distorsion pour les PCDHI. [Traduit par la Redaction] Mots-cles: comportement de flambage de distorsion, constitue d'elements assembles, acier plie a froid, metal hybride, effilement de semelle de renforcement, methode de largeur efficace., Introduction The applications of thin-walled built-up cold-formed steel (CFS) metal hybrid products are growing predominantly for residential, commercial, and industrial buildings, in Canada, Europe, and North America and also in [...]