A vehicle door assembly comprising: an inner panel including a substantially rectangular frame portion having a continuous perimeter said frame portion integrally formed by upper and lower horizontal beams (30; 32) interconnected by front and rear upright beams (34; 36), and comprising opposite front and rear U-shaped side frame members (28 1; 28 2) non-detachably attached to each other by upper and lower intermediate frame members (28 3; 28 4) so as to form the continuous perimeter; and an outer panel attached to said inner panel; characterized in that a single intermediate upright beam extends between said upper and lower horizontal beams (30; 32) of said frame portion and is non-detachably attached to said intermediate frame members (28 3; 28 4) of said frame portion at distal ends thereof; and that a single gusset beam is non-detachably secured to said frame portion and slanted so as to extend between said front U-shaped side frame member (28 1) and said upper intermediate frame member (28 3); wherein said side frame members (28 1; 28 2) are made of a first steel material and said intermediate frame members (28 3; 28 4) made of a second steel material; said first steel material of said side frame members (28 1; 28 2) being different from said second steel material of said intermediate frame members (28 3; 28 4); said intermediate upright beam is made of a third steel material that is different from said second steel material of said intermediate frame members (28 3; 28 4) and different from said first steel material of said side frame members; and said gusset beam is made of a fourth steel material that is different from said second steel material of said intermediate frame members (28 3; 28 4). an inner panel including a substantially rectangular frame portion having a continuous perimeter said frame portion integrally formed by upper and lower horizontal beams (30; 32) interconnected by front and rear upright beams (34; 36), and comprising opposite front and rear U-shaped side frame members (28 1; 28 2) non-detachably attached to each other by upper and lower intermediate frame members (28 3; 28 4) so as to form the continuous perimeter; and an outer panel attached to said inner panel; characterized in that a single intermediate upright beam extends between said upper and lower horizontal beams (30; 32) of said frame portion and is non-detachably attached to said intermediate frame members (28 3; 28 4) of said frame portion at distal ends thereof; and that a single gusset beam is non-detachably secured to said frame portion and slanted so as to extend between said front U-shaped side frame member (28 1) and said upper intermediate frame member (28 3); wherein said side frame members (28 1; 28 2) are made of a first steel material and said intermediate frame members (28 3; 28 4) made of a second steel material; said first steel material of said side frame members (28 1; 28 2) being different from said second steel material of said intermediate frame members (28 3; 28 4); said intermediate upright beam is made of a third steel material that is different from said second steel material of said intermediate frame members (28 3; 28 4) and different from said first steel material of said side frame members; and said gusset beam is made of a fourth steel material that is different from said second steel material of said intermediate frame members (28 3; 28 4).