Laminated veneer lumber gives new life to Kaifu-Bad swimming pool complex.
For 115 years, the indoor pool at the Kaifu-Bad swimming pool complex served as an oasis for water lovers amid the hustle and bustle of Germany’s second largest city, Hamburg. But by 2009, deteriorating conditions and ailing roof trusses in the complex’s indoor pool building forced staff to close the building. Deconstruction and renovation started in November 2011. Only this time around, as part of the renovation project, one of the pools was converted from an indoor freshwater pool into a salt-water bath.
This posed a unique challenge for the architect selected to lead the renovation, Manfred Voss from the architecture firm MRLV, Hamburg. Voss and structural engineer Stefan Heidrich from WTM Engineers, also of Hamburg, had to come up with a solution that could withstand the salty and humid conditions of the new saltwater pool, hold up under significant weight loads and over long spans, preserve the barrel-shaped vaulted ceilings, and stay within budget.
Humidity, corrosion challenges
“What sounds simple actually presented unusual challenges,” said Voss. “The salt water has a salt content of 6%, which is good for humans but bad for steel. The salt will consequently cause an enormous exposure to corrosion. We knew from the beginning that a standard solution of steel was not an option,” he added.
Having ruled steel out, Voss and Heidrich turned to Kerto LVL (laminated veneer lumber) from Finnish lumber company Metsä Wood, Espoo, Finland.
Wood is an old building material that has been getting new life as advancements in engineered products have made it a more attractive building solution than concrete and steel for some applications. Its renewable and sustainable characteristics are a big draw for builders and architects concerned about the carbon footprint of other common building materials.
Voss and Heidrich selected prefabricated engineered Kerto LVL products for their ability to provide solutions to the challenges of the Kaifu-Bad renovation. The LVL product handles both significant changes in temperature and the highly corrosive effect of the high humidity, salt water, and chlorinated water.
In addition, the ability to have all building components fabricated to precise specifications in the factory resulted in significant time reductions and budget savings. What is unique about this build is that even the metal joints were replaced with wood solutions. Any metal joints that are still visible only serve as an assembly aid, and they remain there without static importance. “Using industrially manufactured, dimensionally stable wooden trusses helped us to achieve an efficient solution. We were able to develop an object-specific design that meets both the static and the financial requirements,” Voss said.
Improving load strength
Because the engineered wood was fabricated to customized specifications, it allowed construction solutions that conserve the unique historical architectural elements of the building while meeting the structural requirements. The laminated veneer lumber panels are cross-bonded, meaning that the veneers are glued crosswise. This criss-cross composure dramatically improves the lateral bending strength and stiffness of the panel, increasing the material’s shear strength and allowing beams to be thinner. This provides extreme flexibility in the architectural elements they can be used to create, according to the manufacturer.
For example, the main trusses of the Kaifu-Bad renovation span nearly 46 ft., yet they were manufactured with a thickness of only 5.2 in. Also, the 2.7 in.-thick Kerto-Q LVL used for the roof shell paneling not only acted as panels, but also stiffened the roof structures and stabilized the original mason-bricked walls against wind loads and structural asymmetries. Full-length panels were the material of choice in floor and wall panels for their ability to stabilize the whole structure while highlighting the natural beauty of the wood.
The prefabricated wooden trusses were constructed in two halves, which are a mirror image of each other. The halves were joined by a round metal bar, which works as a pin in the structure. The entire roof truss is fully accessible to ensure easy monitoring of the roof’s condition and the replacement of parts. Two rows of wooden purlins, one for the roof and one for the ceiling, connect the main trusses with special wooden joints. The purlins were made of slender, high-strength Kerto-S LVL beams. The main trusses were drilled for service access in a way that is not visible from the swimming pool.
The forged drawstrings that are based on historical models, give the wood trusses the necessary horizontal bracing. The metal parts add a touch of historical continuity, as they are fully visible above the swimming pools.
Thanks to the strength of the engineered wood, architects were able to make arched cuts in the bottom of the trusses. These cuts, together with the forged metal drawstrings that echo the bygone steel trusses, preserve the building’s grandiose, historical barrel-vaulted ceilings and gracefully frame the many windows and skylights.
Saves time, money
Economics played a significant role in the selection of Kerto-Q engineered wood by the project’s architects. All wooden parts were manufactured at a factory and ready to install on site. This meant that the whole project was finished much faster than a conventional reconstruction, saving time and money. Moreover, the savings from the quicker and more efficient installation covered the extra costs of this extremely challenging salt-water structure, including other moisture-proofing requirements.
After nearly five years of preparations and renovations, Hamburg’s first saltwater pool opened Jan. 1, 2016. Although the building renovation posed multiple challenges that neither traditional construction methods nor steel structures could address, the use of prefabricated engineered-wood elements made the project a success. Use of prefabricated wooden elements on this scale is still a new process for renovation projects, but as the Kaifu-Bad renovation demonstrates, it can provide significant time and budget savings.