modules for beams (just long cubes, really, but axis-centered and parametric, therefore easy to use)

beams.scad

// beam with length in x direction
// it is centered along its central axis at the x axis
module beam_x(
        wy = 10,  //width
        wz = 10,  //height
        l = 100  //length
        )
{
    translate([0, -wy/2, -wz/2])
    cube([l, wy, wz]);
}

// beam with length in y direction
// it is centered along its central axis at the y axis
module beam_y(
        wx = 10,  //width
        wz = 10,  //height
        l = 100  //length
        )
{
    translate([-wx/2, 0, -wz/2])
    cube([wx, l, wz]);
}

// beam with length in z direction
// it is centered along its central axis at the z axis
module beam_z(
        wx = 10,  //width
        wy = 10,  //height
        l = 100  //length
        )
{
    translate([-wx/2, -wy/2, 0])
    cube([wx, wy, l]);
}