infty = 1000;
eps = 0.01;

gap = 25;       // the air gap of the bridge
width = 10;      // width of the bridge
thickness = 1;  // thickness of the bridge
height = 2;     // height of the column under the bridge
top_len = 4;    // length of the top of the bridge column
base_add = 2;   // make the base of the column this wider and longer

// two columns
for (x = [-1,1]) {
	scale([1, x, 1])
	translate([0, gap/2+top_len/2, 0])
	hull() { // base column
		// bottom rectangle
		translate([-(width+base_add)/2, -(top_len+base_add)/2, 0])
			cube([width+base_add, top_len+base_add, eps]);
		// top rectangle
		translate([-width/2, -top_len/2, height])
			cube([width, top_len, eps]);
	}
}

// top rectangle
translate([-width/2, -(gap+2*top_len)/2, height])
	cube([width, gap+2*top_len, thickness]);