An Extreme Rarity

That picture was taken in a theater attic. The diagonals on the upper right are portions of the timber-framed roof; the wall separates the auditorium area from the lobby. It’s the thing embedded in the wall that is rare and odd. In short, that’s the end of a long-span steel truss (over the auditorium, on the other side of the wall) supported on a wood post. The truss end connection is an actual pin (the large round item) and there’s a strap coming down vertically that is hooked into a notch cut in the post.

There is a wide variety of possible structural elements, but unlike LEGOs, they don’t all work well with another. In 1900, you’d use steel for sheer strength (for long spans and heavy loads) and because it could be reasonably fireproofed; you’d use masonry for ordinary compressive loads and as fire and weather separation, and you’d use wood for lightly-loaded members because it was cheap. These are three very different rationales for using three very different materials.

Following that logic, why would you sit a steel truss on a wooden post embedded in a wall? If the loads are large enough to need the steel, then the post will be stressed to or past its limit; embedding the post in the brick strengthens it a bit, but in that case why not just sit the truss on the wall? This was all done very deliberately, so it doesn’t appear to be a contractor’s field improvisation. There’s no sign of damage (I was there looking at something else entirely) so it’s functioning as is. If the loads are smaller than I think they are, who go to the trouble of fabricating the truss and the (fancier than usual) pinned connections?

Of course, I’ll never know the answers. However, the fact that this exists here means it may exist elsewhere, so I’ll keep it in the back of my head as a possibility when looking at other theaters.

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