Can several metal beams, attached to a wall, be used support the schach?
The schach should not be directly supported by metal beams, even if they are attached to a wall. The reason we don’t do this is because if one sees the schach placed directly on the item which could be makabel tuma, even if right now it isn’t, he might make a mistake and use such materials for schach. For example, a person who has a closed in porch, with aluminum walls, should not place the schach directly on those walls.
The correct way to place the schach would be to place wooden beams from one beam to another and then place the schach on top of that.
M:B 629-22, Minchas Yitzchok 4-45 (3), Piskei Teshuvos 629-7.