Indoor swimming pool at Sport Centre Cents

The roof structure for the new indoor swimming pool at the Sport Centre Cents in Luxembourg, was designed by the German architectural office Auer Weber. The ceiling was built using 23.75 metres long wooden elements, which enabled the long spans of the roof. The use of lightweight Kerto® LVL (laminated veneer lumber) made it possible to build the roof structure fast and sustainably.

Attractive roof executed with precision

The wooden ceiling was structured by slim ribs and an above average web height. The Knippers Helbig engineers worked out the structural design together with the Auer Weber’s architects. The design was implemented in collaboration with the wood construction company Holzbau Amann from Bannholz, Germany, which was in charge of the implementation. Use of Metsä Wood’s Kerto LVL Q-panels made the desired roof appearance and the structure possible.

The dimensions of the roof are impressive. The structure was produced using the Kerto LVL web with a height of 1,1 metres and a thickness of only 75 millimetres. With this slim design, the structure spans an unbelievable range of 23.75 metres. There is also an unusual use of bearing surfaces and the web in the bearing area has a very low carrying height. This was possible only because of the specific material properties of Kerto LVL, the unparalleled laminated veneer lumber product.

23,75 metres long wooden roof elements

A range of over 23 metres could be achieved using a variety of structures and wood materials. But the challenge was in the engineering design of the roof structure. “Slim ribs only 75 millimetres thick which had to be made with a height of 1,100 millimetres were attached to form a π element with roof panels made of 69 mm Kerto LVL Q-panel,” explains Hendrik Pfeffinger, the project manager at Holzbau Amann, and continues that, “in addition, we went beyond standards, such as the ratio of thickness to the height of the beams, so special solutions had to be established which would on the one hand meet the requirements of Knippers Helbig, while at the same time conforming to the structural analyses and approvals in Luxembourg.”

For the implementation of these slim dimensions with a range of over 23 metres, the Q-panels were virtually predestined for the job thanks to their high load-bearing capacity. The calculations were very close to the performance limitations and a series of concepts and calculation methods were needed to reach the final solution.

Stability with LVL panels

The goal of the structural design was to create a wooden rib ceiling with deep cassettes which would match the large spatial volume. The dimensional stability of the Kerto LVL web was a great advantage here. As a result, the engineers were able to develop the roof surface with precise torsion-resistant webs. To achieve stability from the outset and take advantage of a high degree of prefabrication, Amann joined the Q-panel webs in pairs or even three at a time to the elements.

The extremely long wooden roof elements were prefabricated at Holzbau Amann’s factory and delivered by extra-long trucks to the construction site. Although the elements are 23.75 metres in length, their installation was quick and required only two carpenters on site. The end faces were finished off with end panels, to finalise the appearance of the roof. Quite noteworthy are the bearing notches, which are very large when compared to the size of the elements (structural components). They are on concrete ring beams on one side and on precisely positioned steel beams on the other. This required the highest degree of precision from all those involved in the construction.

Suspended floating rib roof

Due to the freely suspended character of the ribs, the webs had to be deeply notched in the bearing area. And here the focus was to achieve a strong link which would rule out any shearing of the Kerto LVL beam. “To this end, we came up with a project-specific solution,” reports Pfeffinger, adding that “with our know-how and different product and material components, we achieved enough capacity in the bearing area.” In addition, the Kerto LVL roof panels with Kerto LVL beams were joined to form π elements.

The chloride-containing air of the indoor swimming pools creates a corrosion challenge for the connections. To prevent damage, the connecting screws are hot-dip galvanised and hidden in the wooden π elements.

Lightweight and accurate Kerto LVL

The suspended floating wood rib roof had pushed things to the limits in a number of ways, be it in the dimensioning, in bearing capacity or even in the joining method. Thanks to the high durability and unbelievable form stability of the Kerto LVL web, the planned dimensions could be implemented in a slim structure. In this project Kerto LVL once again proved its capability as an efficient and millimetre precise construction material.