Skip links

Woolworth: Bracing

I might as well continue the parallelism between the Singer and Woolworth Buildings that I started yesterday. The Woolworth building has a fairly complicated wind-bracing system, which is in part a reflection of the very different conditions in the top 30 stores (the “tower”) and the bottom 30 stories (“the base.”) The tower is slender and symmetrical; the base is stocky and a deep U-shape in plan, leading to very different overall stiffnesses in the north-south and east-west directions. We can just see the top of the base in the picture above, getting its terra-cotta skin, while the tower steel is still being erected.

The tower bracing is simpler and part of it is visible above. The spandrel beams along the perimeter are built-up plate girders much deeper than they need to be for their gravity loads. If you look closely you can see small diagonal knee braces above and below the girder at the interior columns, creating moment connections. Full-bay cross-bracing is difficult at the exterior wall, as it tends to interfere with windows, which is probably one reason why Singer used two-story high cross-bracing, to reduce interference.

The most interesting detail in the photo is that there are K-braces at the corner columns instead of the smaller knee braces at the interior columns. There’s a reasonably obvious structural justification for that, using the assumptions of the 1910s. Today, in an era where seismic design is common, there’s a mantra of using “strong columns and weak beams”, which simply means that if a piece of the frame were to fail from overload we want it to be a beam, causing local damage, rather than a column, causing large-scale and possibly catastrophic damage. The Woolworth frame, in an era of wind-load design only and greater belief that the maximum loads were known, has strong beams. But if you look at the moment design, beams that are stiffer than the columns creates a problem in the end bays, where the beam part of the juncture is only half as stiff because there’s only a beam on one side. Using portal-frame analysis (probably the method used by Gunvald Aus), the end-bay beams won’t carry their intended share of the total floor moment (the total wind force on the portion of the building above the floor being designed times the vertical distance from the centroid of the wind force to the floor being designed) because of those less-stiff end connections. By putting K braces at the ends, the end-bay girders do their full job.

I’m not going to say that those K braces are advanced steel design, because engineers had been using similar ideas in truss bridges for the previous thirty years, but this is technology transfer. Rather than simply using full-bay bracing or identical moment connections, the bracing was tailored to the location in the frame; this type of design was still (circa 1910) moving from bridge to building design.

Finally, what we see in the picture is not the whole story: there is also bracing around the central elevator core, in a different patter.