While limiting slab span length helps to reduce the intensity of vibration, this may not be an option due to the nature and purpose of the building. In other words, there is a need for closer study of such floors in vibration.
There are two ways to set limits for restricting the intensity of vibration. While limiting the lowest natural frequency of the floor is the more straightforward and conservative method, limiting the vibration response, like acceleration, requires more detailed vibration analysis. When the frequency limit is used for CLT timber floors, a value as high as 9 Hz is often used. In practice, fulfilling such a strict criterion may not be possible without heavily increasing the slab mass and stiffness. However, if the vibration performance of the floor is found acceptable by studying and limiting the response, having the lowest frequency less than 9 Hz is not a problem.
Plain CLT slabs are light, and fulfilling the chosen vibration criterion may lead to a large CLT slab size. Instead of using just plain CLT slabs, both the modal mass and stiffness of CLT slabs can be increased cost-effectively by adding a concrete topping on CLT, therefore converting plain CLT slab into composite slab. Combining the properties of concrete and timber also works well in static design, since a concrete layer can transfer some of the compressive forces.
DELTABEAM®-CLT hybrid floor vibrations are studied with finite element models using different kinds of limitations – the lowest frequency and response limits – and performance comparisons are made between the plain CLT slabs and CLT-concrete composite slabs. As a result we find out what kind of structures are needed to fulfil the requirements of example design case, and what is their economic impact.
Results show that limiting the response instead of the lowest frequency provides a lot of flexibility for choosing the type of slab. Concrete topping improves slab mass and stiffness, and offers a clear economic benefit as lower CLT types can be used. Combining these two key aspects, limitation based on response and the benefit of concrete topping, provides a solution involving the lowest and most economical types of CLT slabs and DELTABEAM®.
Read Peikko White Paper: Performance of DELTABEAM® – CLT floors in human-induced vibration
Senior Structural Engineer
Peikko Group Corporation
Business Manager, Designers
Peikko Finland Oy