Failure Portrait: Crushing

by Don Friedman on March 24, 2016

Our daily work covers a number of structural materials – starting with steel, concrete, brick, and wood – and there’s a tendency to put structures in little mental boxes based on material. “Wood, therefore the loads are small” is bad logic, but it represents a lot of ordinary work. We most often see wood as floor joists and studs, carrying small amounts of load for relatively short distances.

Not always:

The three dark-gray pieces of wood are the end of a long-span roof truss, spanning away from us, out of the plane of the screen. (The center piece is the sloped top chord, the two side pieces are the flat bottom chord.) The brown piece of wood running across the bottom half of the picture is a sill, resting on top of a masonry wall and providing anchorage for the roof trusses and the common rafters between the trusses. (The rafters span between the sill, the ridge, and sets of purlins, and the job of the trusses is to carry the ridge and the purlins.) The red thing is an iron shoe for the truss-end connection.

This is a picture of failure, but of a kind that hasn’t yet become dangerous. The sill wood is partially rotten, and the weight of the truss end – 30,000 pounds under full code load, and at least 15,000 pounds during, say, the big storm we had this winter – has crushed the sill. The truss has dropped more than an inch at this end, which is not enough to be dangerous, but if the condition were left to decay, the truss might well slip off its bearing as the sill continued to crush down. That could lead to an uncontrolled failure of the truss, which is something we call “bad.”

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