The Details of a Technological System

by Don Friedman on February 20, 2018

This is an underside view of the heel connection of a heavy timber truss. The piece of wood at the top of the picture is the bottom chord, roughly 12 inches by 12 inches in section, and you can’t see the top chord above it. The bolt ties the two chords together, but the real connection between them is the birds-mouth seat for the top chord in the top surface of the bottom chord.

The part that really grabs my eye is the tapered washer that allows the diagonally-placed bolt to bear against the flat underside of the lower chord. The washer has the form of a cylinder of wrought iron with one end cut perpendicular to the cylinder’s main axis and the other cut at an angle. Assuming the bolt has been installed at the correct angle – and with the use of these washers, there is only one correct angle – the washer will only fit one way and then the nut goes on the end. It’s worth noting that this truss was built in the 1880s, at which time the standardization of screw and bolt threads was still new.

The use of those washers eliminated a triangular cut into the bottom chord to create a diagonal face of wood for the nut to bear on. This cut was difficult to make, as the angle had to be exact and making relatively delicate cuts with a saw big enough to cut the heavy timber is a pain. That cut also reduced the section of the bottom chord, weakening it in an area of high stress, but that’s not the reason that these washers were created.

The history of developments in structural technology is, to a large degree, a history of replacing skilled labor on site with more complex fabrication off-site, usually in factories. Those tapered washers used factory fabrication in wrought iron to replace the skill of a carpenter making the no-longer needed cut. If the washers were used as templates to drill the bolt hole at the right angle, as they may well have been, they also replaced some of the skill required for drilling the bolt holes.

Point and Counterpoint

December 3, 2017

In the recent discussion of wood skyscrapers (full disclosure: I think they’re a terrible idea) I have noticed any discussion of the extent of exterior maintenance necessary to prevent wood from acting like the biological material it is. A masonry curtain wall can go thirty years with no maintenance and not fail: a lot of […]

Read the full article →

This Is Ungood

October 18, 2017

From a few years ago, some rotting wood beams and a failing brick pier. In 1989, I performed a long and difficult site visit to check on a structure I won’t name (confidentiality is still in place, even after all these years) that had to be reviewed once per year. The fellow who had been […]

Read the full article →

Steel Trusses and Wood Purlins

May 19, 2017

No deep thoughts today, just appreciation for a well-designed roof. It’s an industrial building so finishes were kept to a minimum, and there was no requirement for fire-rating. The trusses are steel because they span about 50 feet; the purlins span 18. The end bays have diagonal rod bracing in the plane of the truss upper […]

Read the full article →

Worse Than Termites

January 26, 2017

That’s a nice early-twentieth-century partition: plaster on expanded metal lath (on the far side of the studs) full 2×4 studs, diagonal fire-blocking between the studs. And, yeah, a big chunk is missing from a stud where an electrician or plumber needed some room to work. We see more damage from guys with saws than we […]

Read the full article →

A Subtle Hint

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 […]

Read the full article →

Failure Portrait: Dramatic

October 6, 2016

No, that photo isn’t rotated 90 degrees. The floor above has failed and the bathtub has fallen through. You can’t see it from this angle, but the only thing holding the tub in place is the drain pipe. One of the difficult judgement calls that we face during field investigation is when a building is […]

Read the full article →


September 28, 2016

The white stripes show the gaps between the now-removed lath: Structural engineering is about structure, right? A while ago, while looking at a historic house upstate, we ran into a problem. The building had wood-stud bearing walls sheathed with clapboard and we ran into an obvious problem: the bearing walls were not performing properly. The studs were buckling, […]

Read the full article →

Failure Portrait: A Little Off The Top

September 21, 2016

There’s a lot going on in this picture, and much of it is not good. The center of the picture is a girder* with joists on both sides framing in. (The joists on the left are mostly hidden below the plywood.) The fact that we see the tops of the joists inset past the edge […]

Read the full article →

Historic Structural Detail: The Edge

August 23, 2016

I’ve mentioned heavy-timber construction before. Here’s an interesting detail from an old warehouse: The planks above are four inches thick and splined to one another, which is typical in heavy-timber construction to prevent hot gas from a fire burning through to the next floor. (The thick wood in this kind of construction will char but […]

Read the full article →