One rather unique treatment periodically seen in stone arch bridges is a tapered arch. The arch ring starts thick at the abutment, and gradually thins out towards the crown of the arch. This interesting design is often not merely a cosmetic treatment; rather, it actually can be used to make the bridge stronger.
Keeping the Thrust Within the Arch
The primary reason for a tapered arch ring is to aid in keeping the thrust lines well within the arch. There are several situations where this may be necessary. A common example is seen in the basket-handle arches. A freestanding basket-handle arch tends to be relatively unstable. The haunches of the arch have decided tendency to “blow out,” resulting from the fact that the natural line of thrust of a freestanding arch is far from being a basket-handle shape and the weight of the crown of the arch is working to shove out the haunches. To compensate, basket-handle arches are often built much thicker towards the ends than in the middle.
By making the arch much thicker towards the haunches, a basket-handle arch is considerably strengthened, as this tapered shape puts more material in the arch where it is needed the most. The final result is that the arch is lighter at the crown than the haunches. Thus, there is less force on the haunches (after all, there is less material on the crown) and the haunches now have a larger mass to resist any tendency to blow out. Best of all, since the arch is tapered and therefore thinnest at the very top, this design can be implemented without increasing the “hump” of the bridge.
Applications of Tapered Arches
Tapered arch rings are ideal for arches that have an inherent tendency for the crown to sag down and the haunches to bulge. These arches tend to be weak when freestanding unless very thick. To be sure, the fill loading a stone arch bridge will tend to load the arch into stability, but it is often best not to rely on the fill over the bridge to hold everything together, and the strongest results are often had when the arch is inherently stable. Roman arches and basket-handle arches often benefit the most from a tapered arch ring, due to their inherent weaknesses. In effect, the tapered arch ring serves to put more arch thickness where it is needed the most.
Occasionally, unique loading conditions may make a tapered arch ring the ideal choice. On railroad stone arch bridges, a tapered arch ring was occasionally recommended to help absorb the heavy thrust of a loaded locomotive braking on the structure. Of course, for the DIY enthusiast, this is hardly a likely scenario that will need to be accounted for. However, overall, for many arches a tapered arch ring is an outstanding way to increase the strength of the bridge without raising the “hump” over the top, and can be used to great advantage in DIY bridges.