Engineering innovation in arch design

Most modern arches are either true arches or tied arches. The modified tied arch in Michigan USA combined the two concepts for aesthetic and safety reasons. Tied arches, where the tie is exposed, might be hit by trucks and are not desirable for grade separation structures. True arches where the thrust must be taken by the foundation elements are exposed to risk when the soil conditions are poor. The Michigan arch is modified to look like a true arch with a tied foundation. The thrust blocks are connected below the roadway by a rectangular concrete tie beam. The arch ribs are unequal to keep the two bridges closer together. The stiffness of the ribs is varied to keep the same deflection. To allow for inspections without disturbing the heavy traffic, the ribs were sealed and pressurized and pressure gauges were added to detect any leaks.