It sometimes happens that the arch of a stone bridge does not collapse so much as distort in shape. The obvious question is whether the bridge is safe or not. The short answer is it just depends.
Not All Distortion in an Arch is Significant
Not all distortion in an arch is serious. A common example is the fact that, when the forms are removed from a newly finished arch, the arch tends to settle slightly as the stones compress closer together. In this scenario, some slight flattening of the arch is even to be expected, though this flattening should arguably be so little as to be not even noticeable — otherwise, the mason(s) building the arch did not do a very good job, and the stones were rather loose, poor fits. Over the years, many arches on bridges become flatter in shape as mortar joints and the stones themselves compress, and if some slight shifting of the abutments occurs. A very mild flattening at the top of the arch from these causes is normal, and is hardly a problem. If, however, the arch is seriously flattened — which can occur from undermining of the bridge — the structure is quite weak, and potentially even unsafe. It is important to keep in mind, however, that arches can be surprisingly forgiving. How much flattening is too much is something of a judgment call that must be made by a qualified person. Sometimes, nothing needs to change — the bridge can still be used. Other times the weight limit will have to be significantly reduced, or, in extreme cases, the bridge must be closed as it is simply unsafe.
Different Levels of Distortion in Arches
Occasionally a stone arch bridge may be found where the arch is a notably distorted shape, actually consisting more of a series of short straight lines rather than an arch (something like a octagon in shape, for example). This is likely to be a problem with the construction of the bridge itself; the arch can only be as true as the temporary formwork it was laid on, and a common error in formwork construction is to create a series of straight lines, which is much easier to create than a true arc. Such a bridge is very weak, and will have always been weak.
Straight sections in an arch can also be a sign of some form of settling. How serious this is depends on how much of the arch has straightened out and by how much. If the straight section is flat enough, pressure may actually blow that section of the arch outwards, leading to the collapse of the bridge.
Another form of arch distortion is the uplift of the crown of the arch. If the top of the arch is primarily the only affected portion of the structure, this can be caused by pressure against the abutments, pushing the arch together. This is uncommon.
A common distortion in the shape of an arch is flattening of the crown of the arch; settling or undermining of the substructure is a common cause.
Another common scour-related distortion of an arch is a partial settling of the arch. In this case, only part of the entire width of the arch settles, resulting in a drop of a section of the arch, which is often accompanied by somewhat dramatic debonding of masonry joints and the overall loosening of the arch stones in the affected section.
A dramatic distortion of the arch is “hinging,” where the arch is no longer a fixed, stable structure; it has turned into a movable mechanism. One type of hinging failure is for the top of the arch to push up, and for both sides of the arch to buckle inward. Kansas’s H. Branson Bridge in Cowley County (at 75 feet, this was the longest single arch span in the state at the time it was built, a title it likely held until it collapsed) failed over a period of time in this dramatic fashion. A bridge with hinges like this is doomed, and will collapse sooner or later, when being dependent on how long till the arch finishes its movement out of unstable equilibrium.
Remedying Distorted Arches
So how do you deal with a distorted arch? Frankly, in many cases, the arch is still sufficiently serviceable as to not pose a serious concern. What is often much more serious than the arch’s distortion itself is the reason why the arch has become distorted. A common cause of this distortion of an arch in a bridge is undermining and resultant settling of a pier or abutment, which, of course, distorts the arch resting upon it. (Technically, the arch will not become distorted by this settling of the substructure if both foundations the arch is resting on settle evenly together. This, of course, is highly unlikely, though nevertheless really does occur, perhaps accompanied by annoying road problems.) This undermining of a pier, obviously, is very serious and would need to be dealt with immediately.
Some mild distortion is not really worth worrying about, but a major distortion of the arch merits some attention. The main result of a major distortion of the arch (though not so severe that hinges have developed) is an overall weakening of the structure. Sometimes, weaker arches are “propped” in some fashion; two common props used underneath arches are corrugated metal liners (something like a section of a standard metal culvert being fitted underneath the arch) or a reinforced concrete arch cast underneath the stone one.
One method occasionally used to prop up flattened sections of an arch is the building of a wall to hold up the flattened section of the arch. This can hardly be called an ideal solution. Arguably, the best solution for repairing a severely distorted arch (especially one so far misshapen as to have hinges) is the demolishing and rebuilding of the arch back to its proper shape. (In the case of an arch where only a section of the width is distorted, one may be able to get by with merely demolishing and rebuilding a section of the arch.) Such a method, of course, is rather expensive, and requires skill, but may be worth the effort in some cases.