I don't know if there is a bus, but you can take the pole stone bridge at Hongjialou and go straight south along the viaduct. You can see it in about ten minutes, on the west side of the road.

You can take bus 1 at Hongjialou to quancheng square, and then take bus 102, 49 and 19 at the south of the square to get off at Gongshiqiao.