New Envelope for Industrial Monument

Two Liebherr cranes replace Radom membrane

In 1964, what was then the German Federal Post Office took the first parabolic antenna into operation at the earth station at Raisting (Bavaria). In those days this groundbreaking new technology still needed to be protected against wind and weather by a spherical air-supported cover, referred to as a Radom, the name was derived from the English words Radar Dome. Almost 50 years later, the cover of the installation, which has in the meantime acquired the status of an industrial monument, had become porous, and that posed the threat of the envelope collapsing. Seams were no longer sealed tight, and the membrane urgently needed replacing, because the air pressure in the interior which ensures the stability of the dome is only slightly above normal. And so, to replace the skin of the Radom, crane service providers BKL dispatched two Liebherr mobile cranes, an LTM 1400-7.1 and an LTM 1100-4.2, to the scene.

It was a tough two-day job that Rainer Speich, head of the mobile crane division at BKL, and his crane drivers had taken on. Research by the customers, the District Authority of Weilheim, showed that there had never before been a situation in which an air-supported envelope of similar size — the Radom has a diameter of 48 metres — had been replaced with the aid of a crane. Liebherr was included in the planning at an early stage, and had to calculate the maximum wind strength at which the 400-tonner mobile crane could still safely lift the cover, with its 1,000 square metres of surface area likely to catch the wind and a weight of close on 16 tonnes. The result was a maximum wind speed of three metres per second. So when the big day for dismantling came, the people involved, and the large number of the general public who had gathered to watch, had to wait patiently for about another five hours for the wind to drop.

Even the fitters carrying out the work were walking into unknown territory when it came to replacing the almost spherical cover. "Walking" can be taken quite literally in this case, because to start with three workers were lowered onto the crest of the spherical membrane so as to assemble a retaining device for the attachments. Safely tethered to the personnel carrying basket, the men bolted together a ten-cornered metal structure, for which holes were drilled into the membrane in order to tension the skin between the metal angle elements which were then introduced and the retaining device which had been placed on the outer skin.

It took a good hour to complete the work, and the clamping ring was secured. Down in the crane cab the readings from the anemometer, the wind speed gauge at the tip of the boom, were showing very low wind speeds, and at last the time had come.

Once the LTM 1400-7.1 had taken up the load, the membrane was cut away along the concrete base. It took less than two minutes for the overpressure to fall off, and some 5,200 square metres of protective skin was hanging slack on the crane hook, held clear of the sensitive parabolic mirror only by a mast framework. Like a theatre curtain in slow motion, the casing was then raised, and for the first time the huge antenna structure in the interior was exposed. When it was erected in 1963, the Radom had initially been inflated in an empty state, and it was only afterwards that the individual elements of the antenna and two mobile cranes for the assembly work had been conveyed into the air-support envelope through giant air-pressure locks.

Next day had a disappointment in store for anyone who was expecting that the new membrane could be installed more swiftly than the dismantling on the day before. There was no problem in securing the attachment fittings, because a circular fold had already been prepared on the crest of the spherical cover. As the new protective skin flowed over the parabolic antenna, a dozen retaining cables were let down from the personnel basket and the lifting stage to the men on the ground. Deflected through ground anchors, the cables were secured to the trailer couplings of a number of vehicles and then drawn taut, after an unsuccessful attempt had been made using about 30 men to exert enough tractive force to widen the opening.

But the biggest obstacle in lowering the cover was the protective structure around the radio relay antenna. The membrane material simply did not want to slide smoothly over the tips of the masts. Steffen Kaden in the larger of the Liebherr-mobile cranes had his work cut out. Only very careful and cautious raising and lowering of the load, while at the same time carrying out precise slewing movements strictly following the instructions from the assembly team, and combined with plenty of manpower on the cables, allowed the new skin to be drawn completely downwards. Soon after that, 320 bolts secured the fabric air-tight to the concrete base. And it took only 20 minutes for the support air fans to fill the Radom with enough air and re-establish the overpressure needed to guarantee the stability of the new air-support dome.

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