Decorating Structure

The picture above is an oblique view of the Pearl Street bridge in the Brooklyn Bridge approach, showing the original (1880s) truss above and the newer (probably 1950s) arch below. As previously mentioned, one of the notable differences between the two eras is the type of steel member used: eyebar chains for the bottom chord of the original trusses versus wide-flange sections for the newer arches.

There’s a detail of the newer construction that bothers me and is worth looking at for the last part of the discussion of this bridge. Here’s a view looking straight up into the underside of the bridge:

Here’s what I want to talk about, highlighted in purple:

That’s fake lacing. There’s no other word but “fake” to describe it. Holes have been cut in the web of a wide-flange beam (a brace running sideways, arch to arch) to make it look more like the old laced members visible higher up in the truss (the top chord and the web members). There is no engineering purpose served by those cut-outs, as the reduction in weight is negligible compared to the load carried overall, there was an increase in fabrication effort (and therefore time and cost) compared to not cutting the holes, and the amount of maintenance required is increased by having more corners and edges to paint.

Here’s another example:

There has been a discussion about the potential for beauty in engineering structures going back at least as far as the mid-nineteenth century, and probably farther, but this effort is not in that spirit. One of the basic principles of structural art is that the appearance should reflect the engineering principles at work – similar to the idea of architectural honesty, but with somewhat less philosophical baggage – without attempts at masking or ornamentation. These cut-outs are almost exactly the opposite: they are structurally unnecessary and are an attempt to make twentieth-century wide-flange beams look like nineteenth-century built-up beams. Even if they are aesthetically a success, this is a questionable practice that ignores 70 years of improvements in steel technology; and the success of the aesthetics is, at best, subjective.

In work on historic buildings we are often faced with the question of substitute materials. I have no problem with designing a fake if it’s in the service of replicating historic appearance, but that’s not what we’ve got here. The beams with the cut outs could have been made by bolting together plates and angles, creating new built-up sections that look and function nearly identically to the old ones. That would not have served structural efficiency very well but it would have made sense from the viewpoint of historical appearance. Or ordinary wide flange beams could have been used, sacrificing historic appearance for engineering efficiency. This effort is somewhere in the middle, and carries the downside of both approaches with the benefits of neither.