Laced Columns

by Don Friedman on February 28, 2017

The last time I talked at length about laced columns, I was talking about their failure from rusting. That seems unfair, so I want to talk about their remarkable success: engineers were convinced for decades that they were a good idea.

Built-up columns are made by fastening (riveting, bolting, welding) relatively small pieces of steel together to create a big cross-section. This was done in the past and is done today when people need columns bigger than those that are rolled in raw steel. In other words, the big columns at the bottom of tall steel-frame buildings are still usually built up.

In an ordinary built-up column, the overall cross-section is the same everywhere. For example, if a wide-flange column is strengthened by welding plates between the flange tips to make it into a box, that box is the same at any point along the column’s length. Laced columns, on the other hand, are a specific form of built-up column where some of the small pieces vary in position along the column length. The usual layout, as seen in the vertical members above, is to create a series of double-diagonal connectors between two main pieces, usually channels. The overall effect looks a bit like the lacing up a tall boot, and hence the name.

When people involved with steel construction today look at laced columns, we see an enormous amount of labor needed to fabricate and assemble all of those short connector pieces for seemingly little gain: a single large plate could be used tie the two main pieces together. That single plate uses more steel than the lacing plates but requires far less cutting. The amount of hole drilling is about the same, since the single plate would have to be fastened at about the same frequency as the lacing plates.

In purely economic terms, it’s a pretty simple decision. If the cost of cutting the lacing plates exceeds the cost of the additional steel for the single plate, than it makes sense to use a single plate. If the cost of cutting is less than the cost of the material, then it makes sense to use lacing. In the nineteenth century, steel was relatively expensive and skilled labor was relatively cheap. Today the situation has reversed. So lacing was the cheaper option then and the more expensive one now.

There are other factors, of course. Laced columns are weaker than solidly built-up columns, so they could only be used in relatively lightly-loaded areas. We often find buildings that have the same size channels used in built-up columns for the top four floors, with lacing at the top two floors and single plates at the third and fourth floors down. Laced columns expose the interior space of the built-up column box section, which can be good or bad depending on your faith in your ability to paint inside. And the geometry of laced columns is more difficult to figure out – the fastest way to get the dimensions involves trigonometry – and may have been beyond the capability of small iron works.

A final note: the picture above obviously shows laced compression struts in a bridge truss, but the design is nearly identical to laced columns in a building.

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