Causes
The structural degradation of arches can occur for a variety of reasons. The stones or masonry components that make up an arch are vulnerable to physical and chemical attack over long periods of time. Sources of damage include atmospheric conditions, human induced mechanical stress, improper design, and natural disturbances like earthquakes or wildlife.Ground shifting
Movement of the ground beneath an arch will eventually destroy it by changing the mechanical relationship between stones in the arch. An arch will evenly distribute forces across the surfaces of each stone when it is properly emplaced. Moving the ground below the arch will negatively affect the distribution of stress. An arch will slump in the center if the two sides move apart. If the sides are moved closer together, a hinge will form in the arch, increasing the stress on that area. Arches can also become angled away from level, inducing forces that are not normal to the faces of the individual stones. The ground can shift over long periods of time for many reasons: earthquakes, frost, swelling clays, tree roots, animal burrows, and unstable soils all have the potential to move the foundation of a building.Weathering
Vibration
Human activities causing vibration can inadvertently damage stone arches, like other structural elements. TheDesign flaws
Arches that lack adequate support on the sides will slump and fail. All arches convert some of the weight of the structure into a horizontal outwards thrust. The structure may be stable for a time, but the arch will push the walls around it outwards continuously. This is prevented by proper engineering. Arches can be supported by aPrevention
The bald arch condition can be prevented by taking precautions during the construction phase and by properly maintaining the stonework over time. Ground movement can be avoided by using a solid foundation. Removing vegetation from near the foundation and preventing animals from burrowing will help to keep the foundation stable. Frost damage can be reduced by ensuring drainage around the building and by keeping the roof in good condition. This will keep water from saturating the stone and cracking it during cold weather. Keeping water off of the stone will also prevent salt and acid rain damage. Unfortunately, it is not always possible to keep the rainwater off the walls. Paint can be applied to the stones to protect them from rain and pollution, but this is often rejected because it alters the appearance of historic buildings. Cleaning the stone regularly can help to reduce the effects of chemical weathering. Pollutants accumulate on the exterior surfaces of the stone. They react with the stone and degrade the outer portions. This will degrade the appearance of the stone and eventually weaken it to the point of failure. The most popular methods are scrubbing with fresh water or steam cleaning. This is very expensive and time-consuming for a large building. It may be impossible to treat the entire building in one season, and scaffoldings will need to be erected to reach the higher parts.Restoration
It is often desirable to preserve an arch because they are common in historic and monumental architecture. Numerous methods are employed for restoring or preserving degraded arches. The first stage in restoring an arch is to evaluate the structure to determine how serious the damage is and what technique is best suited for repair. An architect or engineer will measure the rate of deformation over some period of time, to see how fast the arch is degrading. The deformation could have occurred immediately after construction during the settling period. If that is the case, it is less urgent to repair the damage. The goal of the restoration is an important consideration. If the goal is only to prevent further deterioration, there will be less work than if the goal is to restore the original condition.Shoring
An arch may be shored up by adding new materials to the structure. The mode of failure determines how the arch should be reinforced. The most common failure for arches is an outwards movement of stones near the top. An arch will constantly apply pressure sideways, so the outwards pressure must be managed. If the arch is failing sideways, a buttress can be built to contain the lateral thrust. Steel tensioning rods can take the place of an external buttress to maintain the original appearance in some cases. Removing any stone from an arch without preparing the entire arch will cause it to fail. The arch must be supported before any work can be done. This is done with aFurther reading
* ''The Observer’s Book of Architecture''– John Penoyre and Michael Ryan * ''The Churchwarden’s Guide to Church Maintenance''– James RedcockReferences
{{reflist Medieval architecture Arches and vaults