This invention relates to an elongated hollow non-deformable structure that can be used as beams and pillars of modular building structures. The structure includes first and second elongated C-shaped channels (20) with inwardly bent flanges (25) and means for holding the two channels (20) together so that the flanges (25) contact each other. The first and second C-shaped channels (20) may be formed without flanges (25). L-shaped (42) and T-shaped (40) and H-shaped flanges of appropriate design are provided with holes in the flanges at predetermined locations to correspond to the holes in the end portions of the beams and pillars. There are plugable holes (21) provided in the end portions of the beams and pillars so that workers can easily access inside the hollow space to use nuts (72), washers, and bolts (70) to couple the elongated hollow structures together. Instead of the C-shaped channel (20) structure two identical L-shaped channels (20b) may be used.