Concerts nowadays, particularly when they involve pop music, are becoming more and more extravagant. Accordingly, the equipment used by the artists for light and sound is large and heavy. As there is a limit to the maximum load that can be attached to a roof or lighting gantry structure, this must be effectively monitored. Load monitoring in the unique roof design of the Olympiahalle arena in Munich presented a major challenge to the engineers. This has been resolved with measurement technology from HBM.
The architecture of the Olympiapark, that was built in Munich in 1972, on the occasion of the XXth Summer Olympics, was intended, in contrast to the 1936 Olympic Games in Berlin, to symbolize freedom and democracy.
Designed by the Behnisch & Partner Architectural Office Olympiapark Architecture Group, together with Frei Otto, the world-famous roof structure spans the stadium, the indoor Olympic pool and the Olympiahalle arena, as well as the space between the sports facilities. The sports facilities are incorporated in a 850,000 m2 park, with made-up hills and an artificially constructed lake. As some of the sports facilities are below ground level, the large, 80,000 m2 curved roof is an impressive focal point of the Olympiapark. The roof is a steel cable structure, suspended on pylons that are up to 80 m high, and is covered with acrylic glass plates.
The roof structure of the Olympiahalle arena with two main girders and four lighting gantries, is suspended at four points.
During the Olympic Games in 1972, the Olympiahalle arena was home to the gymnastics and handball competitions, but now, it mainly stages concerts and other major events. Lighting is a great extravagance for promoters, particularly with major concerts.
The Olympiahalle arena has four lighting gantries available, running from east to west under the tented roof, from which promoters can suspend their technology. These four lighting gantries are statically determinate and are attached to the two main girders that run from north to south. These are also statically determinate, and are suspended by steel cables in the stanchions, that also support the tented roof.
The transition between the main girders and the suspension cable is resolved by a threaded spindle, 80 mm or 110 mm in diameter, that runs through the supporting beam. There is a needle bearing between the spindle and the supporting beam, which prevents the spindle being subjected to bending stress in the north-south direction. Similar mobility from west to east was not planned for its earlier use and so was not provided.
The entire structure of main girders, lighting gantries and fixed lighting, sound and video technology, weighs in total around 350 tons and because of the cable suspension, it behaves similarly to a swing. Unevenly distributed loads in the west-east direction, which is the rule in concerts for the majority of the technology, because the stages are arranged on one side, lead to a shift in the entire gantry structure, and the threaded spindles are thus subjected to unscheduled bending stress.
So while the Olympiahalle arena was being renovated, Stadtwerke München, the city of Munich’s public utility company and owners of the building, decided, together with Olympiapark GmbH, the operator, to install a monitoring system to check both the bending stress of the spindles in the building and to permanently monitor the weight of the technology suspended for an event, as well as its distribution over the gantry.
Before the assembly work starts, the promoter of a concert has to inform Olympiapark GmbH in so-called rigging plans, which loads are going to be applied where on the lighting gantries. Previously, it had only been possible to monitor the actual loads by taking time-consuming individual measurements, something that it was difficult for the promoter to implement, with tight time schedules.
As the concert technology gets ever more extravagant and the overall structure approaches the limits of its load-bearing capacity, it is necessary to have a monitoring system to check the imposed loads and their distribution over the gantries. Stadtwerke München commissioned Dr. Linse Ingenieure GmbH to check the feasibility of a relevant system, as well as plan and install it. The company was founded in 1997 as an engineering office, and has been trading in its current corporate form as an independent engineering company since 2006, acting in a planning, testing and advisory capacity in many fields of civil engineering.
Load monitoring in the Olympiahalle arena was ultra-demanding on measurement technology. The original approach of the public utility company, of fitting strain gauges on the suspension mountings of the lighting gantries and on the spindles, in order to determine the deformation, and thus the suspended weight, was quickly rejected after a computerized analysis, as the intended measurement accuracy of 500 kg would not have been sufficient.
“The vast temperature fluctuations in the hall are a further major problem”, explains Dr techn. Robert Schmiedmayer, the managing partner at Dr. Linse Ingenieure who is responsible for the project. In the winter months, the temperature inside the empty hall is around 15°C. But as spotlights produce a tremendous amount of waste heat, the prevailing temperature in the lighting gantry area during an event can be as much as 45°C.
To ensure reliable and precise measurement, the experts at Dr. Linse Ingenieure designed a comparably extravagant structural solution. In the region of the lighting gantry suspension, they welded two metal holders for the strain transducers on the underside of the main girders (compression zone), at each of two points.
The variations in the load on the lighting gantry cause the supporting beam to deform, with the deformation being transferred to the transducers via the metal holders. TypeSLB700A strain transducers from HBM are used here. The transducers work with strain gauge full bridges made from stainless steel and feature a very low zero offset and good temperature compensation. To increase the precision of the measurement, the transducers were fitted with an insulated housing. “This made it possible to minimize what in some cases were extreme temperature fluctuations, as well as the associated temperature gradients”, explains Dr Schmiedmayer. “We also attached temperature transducers in the housings, to perform additional temperature compensation.”
The structure is even more complex for the four spindles of the main girders. Here the engineers designed quadripartite nuts that fitted exactly onto the threaded spindles and which can be held in place by a bolted connection. Two of these nuts on each of the threaded spindles act as holders for four strain transducers. In contrast to the situation on the lighting gantries, these are not connected in parallel, they are evaluated individually. This allows any bending of the spindles that occurs with a load and which cannot be compensated by the needle bearings, to be detected.
The engineers also had to get the effects of electromagnetic interference under control. “In the first preliminary tests, we noticed that when the large spotlights right next to the measurement technology were switched on and off, there could be major interference”, remembers Dr Schmiedmayer. Shielded measuring leads then had to be used, that were also run through continuously enclosed metal pipes, to double the shielding effect.
At the same time, care was taken to ensure that none of the measuring leads were longer than 20 m. MX840A modules from the HBM QuantumX data acquisition system are used as the amplifiers. These compact amplifiers each have eight channels and are easily installed on the gantry structure, making it possible to keep measuring leads short. A total of four amplifiers transmit the measurement data with an interposed fiber-optic link to a type CX27 gateway module, acting as a central measurement node.
The CX27QuantumX gateway module is housed in a control cabinet in the Olympiahalle lighting rostrum, together with the power supply, a UPS and the network components. From there, the measurement data is transmitted via TCP/IP to the computer center of Stadtwerke München, where there is a relevant server.
The measurement node also triggers a direct display of the load monitoring. A big set of traffic lights, clearly visible inside the Olympiahalle, indicates the loading that is reached. If the traffic lights jump to amber, this means that either the planned event load, or 98% of the maximum gantry load has been reached. If the maximum load for the structure is reached, the traffic light shows red. This is a sign to the hall inspector that stage workers must stop work immediately and that some of the load must be removed from the lighting gantries.
HBM catman®AP software runs on the server in the computer center, and is used to configure and perform load monitoring. The permissible loads for
- each measuring point,
- lighting gantry
- and main girder,
- as well as the total load,
can be specified for an event. The advance warning range, when the traffic light in the Olympiahalle changes from green to amber, can be configured here. The measurement frequency and thus also the traffic light control, is 1 Hz. For data archival, the average and the maximum value in a minute are stored in each case.
If there is an overload situation or a system failure, the system administrator is informed by e-mail and SMS. The measured data are stored and can, at the same time, be seen from any number of places, via internet.
After all the components and systems were installed in spring 2010, the load-monitoring system was calibrated. To do this, different loads of up to three tons were attached at different points on the gantries. It was then possible to achieve the planned measurement accuracy of 2% of the load or 100 kg for smaller loads.
Dr Schmiedmayer is confident about the system he has designed and installed. “The resolution is so good, that I can see from the measurement data when and where I was working on the lighting gantries. It only took thermal deformation of the access gantries and associated deformations in the system to bring about system-dependent variations in the measured values, with these only affecting the immediately adjoining gantries and making up less than 1% of the entire measuring span.”
The precision and reliability of the measurement technology components from HBM play a major part in this. The engineer had come to know and appreciate this from time spent at the Munich University of Technology. “The good advice that I get from HBM, and the metrological know-how of their employees, also make me feel that I am in good hands”, is how he sums up his experience.
As far as the Olympiahalle arena is concerned, the load monitoring system has now become an advantage for the location. Some concert promoters are tending to start major concert tours in Munich. If the lighting technology has complied with the strict system in Munich, other hall operators can obviously rely on the information provided by the concert promoter.