I always read the Confidential Reporting on Structural Safety newsletters, because the types of errors reported appear everywhere. The specifics don’t really matter as much as the reasons why the errors have occurred. The latest newsletter had an item that got me thinking about a category error that we all make at times. Report number 764 “Hidden defects in railway masonry arch viaducts” discusses changes in ownership and occupancy for the space below the arches of railroad viaducts. This is a type of structure that barely exists in the US, so the specifics are not important in my practice, but the overall issue certainly is:
- When long-term leases are contracted for those spaces, inspection and maintenance will be reduced.
- The masonry structure of the vaults moves under load.
- The masonry structure may already be partly hidden from view by liner arches meant to reinforce the viaduct.
- Mortar may be crumbling loose in the arches.
Those four points, added together, mean that a viaduct that is safe today may become safe without anyone knowing, and one that may be unsafe today may become worse to the point of imminent failure without anyone knowing. Those conclusions apply to many other types of structure, including some that I see every day.
The category error I mentioned is simple: treating safety as an inherent property of a structure rather than as the sum of a number of changeable factors. A structure can be safe without meeting the provisions of current codes (the New York City Department of Buildings, for example, is housed in a building with unreinforced-masonry structural bearing walls that could not be built in their extant form under current code) but cannot be safe with complete load paths. We generally assume that existing and occupied structures are safe unless we have a reason to believe otherwise, but the conditions that make them safe change over time. The reason that New York’s facade inspection law has a five-year cycle is that safe conditions can become unsafe that fast if there’s an underlying problem (a blocked drain forcing water into the back of a parapet, for example).
An honest assessment should say that a building is safe today for a given set of loads, and assuming no major changes will be safe for a while longer. A lack of maintenance means major changes may occur. A lack of visibility means there’s no way to tell ion major changes are occurring. Loading that is outside of known historical norms (like that in the picture above) is a major change.
