On 15 September 2021, the time had come at last: after a four-year renovation project, the new Tonhalle Zürich – one of Europe’s pre-eminent concert halls – reopened its doors. The listed complex that houses the Tonhalle and Convention Center has been restored to its former glory, thanks in part to a sophisticated fire engineering concept.
Although the podium and audience seating in the great hall of the Tonhalle Zurich are empty, there is a palpable sense of excitement in the air on an otherwise ordinary working day in March 2021. Today, the attention of those present is firmly fixed on a unique “instrument” by the name of Izar, which takes to the stage whenever there is a need for hot-smoke fire testing. The fire engineering team from Basler & Hofmann have been working on the project at Zurich’s Convention Center and Tonhalle for over ten years, charged with providing comprehensive fire engineering consultation from project planning through to final acceptance. The venerable events location on the shore of Lake Zurich was in desperate need of renovation. The Tonhalle – with its great hall and small hall – was built in 1895, while the Convention Center was constructed for the Swiss National Exhibition in the 1930s. Funds for the renovation were approved by the people of Zurich in 2016.
Conflicting objectives of fire engineering, conservation and intended use
This unique building complex is subject to a preservation order, which presented a particular challenge for the planning companies engaged to perform the renovations. For the appointed architects, Elisabeth Boesch and Roger Diener, and the cantonal monument preservation office, it was important to interfere with the fabric of the building as little as possible. At the same time, the two operating companies for the Convention Center and Tonhalle imposed clear requirements with regard to the future use of the buildings. For the venues to be operated economically, it is important for as many people as possible to attend the events. One thing was soon clear to the fire engineers at Basler & Hofmann: using the standard solutions envisaged by current regulations to implement fire engineering measures would require a disproportionate amount of effort.
Flames flicker, smoke rises
Back to Izar, today’s star of the stage. Juan Blond and Christoph von Stauffenberg carefully manoeuvre the hot-smoke testing device into the Tonhalle’s great hall. Once inside, the two experts begin setting up the system, which was developed in-house at Basler & Hofmann. “With Izar, we can test whether our computer-based smoke extraction simulations were correct and whether the fire engineering measures are working as intended,” explains Christoph von Stauffenberg. The results serve as proof of safety for the fire prevention authorities.
White smoke is already starting to rise from Izar’s silver pipes. “At the heart of the system lies a gas burner the size of a table. We use this to produce a hot heat flow, into which we feed a heat-resistant and precipitate-free fog fluid. The controls on the unit enable us to simulate various fire scenarios. Sensors record the temperature distribution throughout the hall and allow us to run comparisons with the simulations once testing is complete,” says Juan Blond. In contrast to a real fire, there is no risk to the building or its contents. For example, Izar’s heat output can be regulated precisely to prevent sprinklers from being triggered. After around 15 minutes, Christoph von Stauffenberg shuts off the gas valve, and the team continue at the next location.
When standard solutions are not enough
To accommodate the unique architectural features of the buildings and the usage requirements of the operating companies, the fire engineers at Basler & Hofmann have devised a number of special solutions. However, if the plans drawn up deviate from standard solutions, a comprehensive risk assessment must be provided as evidence of guaranteed safety. To meet this requirement, the team simulated smoke extraction and evacuation routes, and used these simulations as a basis for devising the corresponding measures. “The smoke extraction simulation, for example, revealed that the smoke extraction system in the great hall did not need to be as powerful as specified. The high ceiling means that a stable smoke layer develops high up in the hall, which gives even guests on the balcony sufficient time to escape,” explains Christoph von Stauffenberg.
Successful integration of architecture and fire engineering
The protection concept centres around smoke extraction. “To preserve the magnificent decorative ceiling in Tonhalle’s great hall, we used existing openings in the ceiling for smoke extraction,” explains Juan Blond, pointing to one such hole in the ceiling above. Planning how to route the smoke extraction ducts around the existing steel joists presented a particular challenge. In addition to there being no tested fire engineering solution for such a situation, the ceiling also has to continue performing its acoustic function as a sound absorber. “After developing a number of detailed designs, we decided to equip the steel joists with sound-absorbent elements,” says Christoph von Stauffenberg. A further example of the successful integration of architecture and fire engineering is the terrace with steps leading towards the lake. “Early in the project, we realised that additional escape routes were required for the Convention Center and proposed the idea of a terrace with steps. Today, this is a key architectural element that had already existed in the 1930s,” explains Christoph von Stauffenberg.
Reopening with former glory
Autumn has since arrived in Zurich. The tests with Izar were successful. The fire engineering measures are working as planned and have been signed off by the authorities. The inaugural concert was a resounding success, and critics praised the acoustics of the reopened Tonhalle as being better than ever before. And the cantonal monument preservation office is impressed by the architectural revival of this unique events location. “We hope that the measures remain hidden from visitors and that they are never needed in a real-life situation,” says Juan Blond.