Investigation of steel quality in the free-flight aviary at Copenhagen Zoo

After a winter night with heavy snow, Copenhagen Zoo discovered that several of the poles in the free-flight aviary (FFV) were bent out of shape. The pressure from the heavy snow on the aviary net had caused the 20-metre-high guy wires to give way.

Emil Nielsen's Smithy in Gentofte was tasked with straightening the guy wires. The poles are made of stainless steel and welded together. Mikkel Max Andersen from Emil Nielsen's Smithy explains: 

“Before we can begin the restoration, we need to know what type of stainless steel we are dealing with. It is essential to ensure that the steel quality is not subsequently impaired during heat straightening.” 

Stainless steel has different alloys and qualities depending on the amount of ferrite, chromium, nickel, or molybdenum it contains. The alloy of the steel determines its properties, and heating can alter these properties, increasing the risk of corrosion and other damages. Therefore, the restoration must take the steel's alloy into consideration. The smiths contacted FORCE Technology for a steel examination and guidance on the restoration process.

Mikkel Max Andersen says: 

“We contacted FORCE Technology because they have the equipment to perform these material tests and the expertise to advise us on the steel treatment.” 

Inspection of welds

To ensure that the welds were not damaged by the twisting of the steel guy wires, our NDT inspectors first performed a so-called penetrant inspection of the welds at the "feet" of the guy rods. This is done by applying colored liquids that reveal any cracks in the welds. If there are damages, it is essential that the welds are also repaired to ensure both quality and safety. Fortunately, the inspection showed that the welds had not been damaged.

Determination of the steel by PMI

To identify the type of steel and how it can be repaired, our technicians examined the steel on site using mobile PMI equipment. PMI stands for Positive Material Identification and encompasses various methods to assess material quality. 

The OES (Optical Emission Spectrometry) method is used for iron and steel where it is necessary to know the exact carbon content and control other elements. OES is a quick and reliable method for determining alloy elements and impurities in metal alloys. The test is performed via an electric spark on the metal surface, resulting in a small burn mark, which can be removed if necessary. 

In this case, mobile OES equipment was used to verify the carbon content in the steel, as it is crucial for the rods’ continued corrosion resistance after welding. After the analysis, our technicians produce a report, which forms the basis for a technical assessment by our metallurgists. 

The mobile equipment has the significant advantage that the structure does not need to be dismantled or samples taken to the laboratory. This makes the process faster, more flexible, and more cost-efficient for the customer, allowing measurements to be done on-site. 

Consultancy on steel straightening

After determining the steel quality, our metallurgists prepared a report with descriptions and guidance for heat treatment of the steel, which the smiths could use during restoration. This ensures that the steel quality is not reduced, thereby avoiding later corrosion.   

The rods have subsequently been straightened so the aviary can once again withstand pressure from, for example, heavy snowfall. Lars Rossum, Division Manager at Copenhagen Zoo, says: 

“It is important for us that the quality is in order and that our enclosures keep the animals inside. We always try, during repairs, to disturb the animals as little as possible and, if possible, allow our guests to continue enjoying the animals in their environments.”