ThinKing April 2019: The “inflated” lightweight column



Take two thinly cut superposable sheets of stainless steel, weld the edges of both sheets together and pump the space between them full with water, and there you have it:

The form of the hybrid column “PERFECTO”. In reality, it’s a lot more difficult, of course. However, scientists working with numerical simulations at the KIT Steel and Lightweight Structures Research Center for Steel, Timber and Masonry have come up with the perfect basic sheet form which doesn’t wrinkle or develop bumps when welded together and blown up. This process can be used to make up to 16-metre long construction elements without the need for expensive form tools. Instead the resulting stainless steel body is used in the “PERFECTO” hybrid column, which is not only optically impressive but also has extraordinary load bearing capacity despite the minimum use of material.

The Development Agency for Lightweighting Baden-Württemberg presents this innovation in the April 2019 edition of the ThinKing. Leichtbau BW GmbH uses this label to showcase excellent lightweight technology products or services from Baden-Württemberg every month.

What do a plastic ice cube bag and the “PERFECTO” hybrid column have in common? Not much at first glance, you might think. But if you fill them with water, they both billow out into a cushionlike shape. “Things really did first get started a few years ago with an ice cube bag. I found myself staring at one and asking myself whether it would also be possible to inflate two metal sheets joined together along the edges,” is how Professor Thomas Ummenhofer explains the idea behind the project. The problem is that both sheets really must be the perfect shape because otherwise they pucker and crinkle when water is used to change their shape. The weld seam must also be of high quality as otherwise strong weld distortion will lead to more crinkling or the structure will burst during the inflation.
“We worked our way slowly towards the optimum initial form using numerical simulations,” continues Ummenhofer. It is no coincidence that the project bears the name “PERFECTO”: the Karlsruhe-based scientists have succeeded in finding the “perfect” form for the metal sheet, which can then be blown up without it crinkling. “We can manufacture the column in customised and reproducible lengths and diameters right down to the last centimetre,” says Ummenhofer with pride. But is there no other way of making such a doubly bent stainless steel skin? "Yes, sure. Theoretically it would be possible to form the sheet metal by deep drawing. The problem is that you would need an extremely expensive forming machine to work a component as large as a 16-metre-long column,” says Ummenhofer.

The column load is borne by an internal rod
Its unusual form and reflective surface have a fascinating effect if you walk around the stainless steel body. This is why the Karlsruhe-based scientists thought primarily of using the stainless steel form in architecture: the outcome is the “PERFECTO” hybrid column. It’s hybrid because the “inflated” sheet metal forms a decorative outside skin while a high-strength steel rod inside the column links both ends of the column along a straight line. The space between the outer shell and the rod is filled with self-compacting concrete. “The outer form is almost ideal from a structural perspective and we can transfer the same loads using less material than is the case with conventional columns. We achieve a maximum of function and load bearing capacity with a minimum of materials,” says Ummenhofer. So the column not only looks good, it can also transfer the highest loads. “That's what makes the column an unbeatable building component and not just a design element,” according to Ummenhofer. The next development step will be to get the column through fire resistance tests. “Stainless steel is so expensive that there are very few rod system constructions made of it. We therefore believe that the market opportunities for hybrid columns, which thanks to their thin shell only need very little stainless steel, are good as they make highly efficient use of stainless steel in their rod system structure,” says Ummenhofer.

Presence at trade shows in Munich, Karlsruhe and Dubai
The “PERFECTO hybrid column” was first exhibited at the BAU München in January 2019 at the joint stand run by the “Informationsstelle Edelstahl rostfrei”. It will also be on show at the Karlsruhe Institute of Technology’s NEULAND Innovation Day on 10th July 2019. “The Big 5” Expo in Dubai is the leading and biggest construction trade show in the Middle East. A six-metre version of the hybrid column will be exhibited there in November 2019 at the State of Baden-Württemberg’s joint stand.

ZIM Cooperation Project
The KIT has developed the “PERFECTO” hybrid column through to a ready-for-market prototype as part of a ZIM cooperation project with the Calw-based company Seyfried Metallbau GmbH.

The hybrid column in figures
Dimensions: individual lengths (up to 16 m) and corresponding mid and end diameter
Outer shell (stainless) steel: Wall thickness of 1 mm to 5 mm
Surface finishes: e.g. polish finish, satin finish, electrochemical colouring or galvanic noble metal deposition
Examples: Load bearing capacity of 3-metre-long column with a mid-diameter 21 cm and end diameter of 6 cm: 1,000 kN
Application: Single hinged column or as element in rod system constructions (e.g. space frames)