A Subtle Hint

by Don Friedman on January 24, 2017

That picture was taken in a mid-to-late-nineteenth-century industrial building, on the top floor with only the attic above. The exposed wood beam is about 8 inches wide and 12 inches deep but appears to span some fifty feet, which is obviously ridiculous. The building has a gable roof so the most likely bet is that there are gable roof trusses, and the visible beam is the bottom chord of a truss, but there was no access to the attic to confirm the presence of those trusses.

There are two tapered washers in the photo, one just to the right of the sprinkler pipe and one just to the right of the left end of the beam. The ends of the bolts are sticking out at an angle perpendicular to the taper, which pretty much tells us all we need to know. If you look here, the left page (p150) shows a truss with wood compression members and wrought-iron or steel tension members. At the left end of the blow-up details of the connections, you see three diagonal bolts connecting the top and bottom chords. If you picture those bolts extending through the bottom chord, they’d have to end with tapered washers to bear on the horizontal¬†bottom surface of the bottom chord.

You can make deductions about unseen structure from small details, as long as you take the time to find out what the details mean.



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