Failure Portrait: The Weak Link

by Don Friedman on April 4, 2016

I have found that looking at abandoned buildings tells me a lot about exactly how various materials and systems fail. Without human interference – in the form of maintenance and repair – each material and system will fail at its own pace and its own way. The picture below is an abandoned factory. Like a lot of late-19th and early-20th century industrial buildings, it has a sawtooth roof to maximize daylight. The main structure of the roof consists of steel trusses resting on steel columns, with wood rafters for the infill spanning between steel purlins, ridge beams, and valley beams.

The wood rafters and sheathing look pretty good…except for the part that has completely rotted away. Obviously, with the roof drains no longer functioning, rain and snow have been sitting in the valleys so the wood there has failed long before the rest. The steel looks pretty good, despite the failure of its protective paint.

Unfortunately, the failure of the roof sheathing at the valley bottom will speed up the destruction of the remainder of the wood, by creating an edge that experiences strong uplift during winds. Eaves are the most vulnerable part of a pitched roof, and the ragged edge of the remaining wood is effectively an eave.

The bricks walls will probably fail slowest, despite the biological growth and missing mortar, because they are stressed the least relative to their capacity. The steel will continue to slowly rust, until the capacity of the remaining unsuited sections is less than that required; but that failure will probably be delayed by the complete failure of the wood. Once the roof sheathing is gone, the stresses on the steel will be greatly reduced, allowing the trusses, purlins, and columns to rust without immediately failing.

Previous post:

Next post: