Hidden

I’m a structural engineer working primarily in buildings and our office is a group of structural engineers who work primarily on buildings. Out of some 4000 projects in our records, there are about six bridges and they’re all quite small. (There’s also one dam, but that’s a long story in itself.) This is not a lament – I’m proud of the building work we do – but simply a statement of fact. That fact raises a question: why have I spent so much time discussing bridges in blog posts if we don’t work on bridges? The answer is so simple it’s bordering on ludicrous: because the structure of bridges is visible.

The. building above, once described as a “brick barn of a church” is Plymouth Church in Brooklyn Heights. It is one of the oldest buildings in New York built and continuously operated as a church, but it is most famous for its first pastor, Henry Ward Beecher, who was there from when this building opened in 1849 until his death in 1887. Beecher was an abolition activist before the Civil War and a champion of other social causes after. He is today less well known than his sister, Harriet Beecher Stowe, but he was as famous as it was possible to be for several decades, long ago. None of that history really helps in understanding the building, although a construction date of 1849 gives a few clues.

The first real treatise on truss bridge construction published in the US was Squire Whipple’s 1847 Work on Bridge-Building: Consisting of Two Essays, the One Elementary and General, the Other Giving Original Plans, and Practical Details, for Iron and Wooden Bridges. It was, unfortunately, badly organized and so imported European treatises, meant for engineers with more formal training than most Americans had, remained the best source of analysis techniques. Herman Haupt‘s 1850 General Theory of Bridge Construction was much better, but too late to help at Plymouth Church. Why am I talking about bridge texts in discussing a church? Because the sanctuary is a large open space that required some form of roof truss to be feasible.

Pretty, but no information on the roof structure. Because of the church’s fame, it was one of the first group of buildings documented by HABS, in 1934. The transverse section of the building shows the hidden information.

The left half is a section looking to the front of the sanctuary/back of the building; the right half looks to the back of the sanctuary/front of the building. And there are the long-span roof trusses, above the plaster ceiling. The use of wrought-iron rods as the quarter-point verticals pushes me towards calling it a gable Warren truss rather than a king-post truss, but the line between those two descriptions is blurry at best. Note that the lower chord is in tension only, so the use of a scarf joint at midspan to splice the timbers is a workable detail. (On another topic, the “tunnel” connects the two areas of cellar, front and back, past the large center area that is a crawl-space over backfilled earth.)

We have plenty of pictures of the trusses in various church attics, but they are rarely understandable by themselves, and so are not much a basis for a 500-word blog post. Expect to see more here on bridges, where it’s all hanging out.