 Strut braced monoplanes were tried but none of them were successful, not least due to the drag from the number of struts used.The structural forces acting on the spars of a biplane wing tend to be lower as they are divided between four spars rather than two, so the wing can use less material to obtain the same overall strength and is therefore lighter.A given area of wing also tends to be shorter, reducing bending moments on the spars, which then allow them to be more lightly built as well.