There are several reasons why stone arch bridges are removed from road service. Two common reasons for this are narrow width and insufficient weight-carrying capacity. Fortunately, there are ways to remedy both of these problems while keeping the existing stone bridge on the road. Furthermore, there is some overlap between the methods used to strengthen a stone bridge and methods of widening it. For a specific focus on widening a stone arch bridge, click here. This post will focus more on strengthening a stone arch bridge. As it happens, there is actually a surprising number of ways to do this. To effectively understand and apply something, however, it is best to understand why what you are doing works. We will start, therefore, by understanding what happens when an arch overloads.
Understanding How an Arch Overloads
A somewhat counterintuitive method of increasing the weight-carrying abilities of a stone arch bridge is to increase the amount of fill over the top of the arch. As most of the methods of strengthening a stone arch bridge work similarly to this method, it is helpful to understand what is going on here.
When a vehicle crosses a stone arch bridge, it applies a shifting load to the arch. The load moves from one haunch of the arch, to the crown of the arch, then to the other haunch of the arch. Furthermore, a vehicle applies its load in relatively few concentrated points because a vehicle rests its weight on relatively few tires compared to its entire length. The upshot of this is that a relatively small portion of the arch has to carry a high amount of weight.
When an arch fails from overload, it practically is never from crushing of the stones; the material of the arch can withstand the weight. On the other hand, an overloaded slab bridge usually fails when the material can no longer sustain the forces imposed on it. When an arch bridge fails, then, it is not from material failure; rather the arch can longer contain the forces imposed on it within its structure. The upshot, then, of what happens is that the concentrated weight on a specific portion of an arch forces the stones out of place, causing collapse. Usually, the haunches of the arch in a stone bridge can carry the lowest amount of weight; this is largely related to the fact that the haunches of the arch already have to handle much more weight than the crown due to a stone arch bridge’s inherent design. (See our post on The Challenge of Long-Span Arch Bridges for more details.)
A stone arch operates in compression. Compressive forces pack the stones tightly together, wedging them such that they cannot escape the arch. For stones to be forced out of place, then, there must be a shift from compression to tension at some point within the arch. Even hinging, which results in arch distortion, but not complete arch failure, is a result of tension partway through the width of the arch. Thus, when a vehicle overloads a bridge, the heavy weight imbalance on the arch results in tension within the structure. A common way this turns out is heavy weight on the haunches of the arch shoves the haunches down (compression) while forcing the crown of the arch up (tension).
Clues to the Problem
By understanding how an arch fails, as described above, clues can be found for how a stone arch bridge might be strengthened. To sum up the important points of arch behavior at overload as related to strengthening:
- An arch works only in compression to the exclusion of tensile forces. (Please note that some authorities suggest that some very limited tensile forces may be acceptable in some cases.)
- Arches rarely fail due to material overload; rather the arch collapses when the arch system fails. This is caused by tensile forces/loss of compression within the arch.
- It follows, then, that if a way can be found to either add tensile strength to the arch or, better yet, keep it under compression, the structure can be strengthened.
- Furthermore, failure is more likely to occur at the haunches of the arch as opposed to the crown of the arch and usually results in the haunches buckling and forcing the crown up.
- Vehicles apply a heavy weight to small portions of the arch at a time.
- The upshot, then, is that when a vehicle overloads a stone arch bridge, what happens is that the vehicle causes a heavy weight imbalance in the arch bridge which in turn causes loss of compression in part of the arch.
Putting It All Together
Putting it all together, then, we can improve a stone arch bridge’s weight-carrying abilities by increasing the amount of compression in the arch in order to prevent load imbalances from causing tensile forces. On the other hand, a easier and more direct approach to the imbalanced vehicle loading problem is to distribute the weight of the vehicle more evenly over the arch. And this works, too. If you’ve ever wondered why some stone arch bridge weight limit signs reveal that longer vehicles can impose more weight on the bridge than short vehicles, it is because longer vehicles tend to distribute their weight over the arch better, and therefore can be heavier.
In the final tally of stone arch bridge strengthening, both increasing compression in the arch and distributing weight are used. Another method of strengthening includes modifying the bridge to allow for tensile forces in the arch. Other strengthening measures used involve reducing or even eliminating loading of the arch altogether. In the next post of this series, we will investigate practical ways a stone arch bridge can be strengthened.