On Cruciform Columns

Why put up with torsional strut buckling?

The photograph above shows some columns that I came across at an old industrial site that was to be converted into a modern mixed use development. It was interesting because the columns, which were clearly cast iron, had a cruciform shape rather than the conventional circular hollow form. This is relatively unusual and made them older than their circular cousins.

Today cruciform columns are perhaps even less common than they were in the past. The form is certainly not included within modern codes of practise. This is relevant, because the cruciform shape has an unusual buckling mode that does not apply to other shapes. Subjected to excessive compressive load it will exhibit torsional strut buckling. The modern engineer is familiar with conventional strut buckling and lateral-torsional buckling, which afflict more common shapes, but much less so with torsional buckling. 

This isn’t intended to be a post about torsional strut buckling per se, except to say that torsional buckling reduces the capacity of a column and causes it to fail before other modes of failure. This is why its important to know. The question for this post is why designers in the past would choose a cross section that has a reduced capacity? Did they not know what they were doing?

It is certainly true that modern methods of analysis were not available at the time, though there were column sizing formulae that were used. Typically however, cast iron columns were proof tested and therefore manufacturers and engineers had seen and did understand the failure of columns, at least in a practical or empirical sense.

If we are to understand the existence of cruciform columns we must therefore look elsewhere. We must first understand the material from which they are made and then the way in which they are made. 

Cast-iron has many useful properties. It is strong in compression, it is mouldable, it is resistant to corrosion and crucially it is non-combustible. This last property was considered vital in the context of its early use in the construction of mills. It is well documented that there had been many catastrophic fires.

The challenges of cast iron were its brittleness, its low tensile strength and the tendency for flaws and blowholes to appear. These last two issues are primarily the result of manufacturing.

As the name would suggest cast-iron sections are not rolled or extruded, like other metals. It is like concrete in the sense that it is cast in a mould. Of course unlike concrete it sets hard by cooling rather than by chemical reaction. Also, like concrete it will shrink in the mould, though by a much smaller amount.

Since hardening is by cooling the rate of cooling is vitally important. If one part of the structure cools faster than another then the internal structure of the iron will be different. The faster it cools the better the tensile strength. Similarly, if one part has cooled, and therefore shrunk, while an adjacent part has still to cool and shrink then internal restraint will cause stresses to be built into the column before it has been loaded. In some cases such restraint might even cause fracture during the casting process. The shape of the casting is therefore important.

Another important factor is section thickness. The thicker the member the more likely the surface of the member is to cool before the interior. This would again cause internal restraint and internal stresses to develop.

It follows that a shape had to be developed that was straightforward to mould and allowed iron to flow inside quickly and easily. It had to be symmetrical to promote balanced cooling and the section could not be too thick to stop the interior from cooling too slowly.

The earliest attempts were a crude star shape with a solid centre, however it is not difficult to see that the next logical step would be to extend the points of the star to create ribs, thus forming a cruciform shape. It fulfilled all of the criteria for casting.

Interestingly the ribs were often cast with classical proportions, being wider at the center of the column than at the ends. From this we might conclude that the designers new full well that columns were prone to buckle in the centre and it was an advantage to have more material at that point.

While some modern engineers may look back at early columns and dismiss their designers for having a primitive understanding of buckling behaviour there is of course a deeper truth. The designers from that era understood that columns can buckle, but they also knew about the perils of casting iron and at that point in time it was a bigger factor in the safety of columns than buckling was.

A supplementary question might be why circular columns were not manufactured from the beginning, after all they have a good resistance to buckling and a shape that encourages rapid, even cooling.

I think the answer is probably rather prosaic. Molten iron is extremely hot and is therefore cast in moulds of sand. I imagine nobody had yet worked out how to produce a mould from sand with a void in the middle.

Something else that is perhaps worthy of comment is the relatively large projecting tables at the column heads on which the timber beams are supported. Given what we know about the low strength of cast iron in tension, and by extension flexure, these cantilever projections would appear to be a significant weakness.

In fact there is little evidence of table failure and it has therefore been conjectured that the relatively thin sections cool rapidly after casting and develop a higher tensile strength than is found in thicker castings. 

So there we have it, counter to what you might think, based on a modern mindset, cruciform columns were actually, at the time, a really good idea.