Crane runways cause noise
Depending on the situation, the operation of the crane system can cause disturbances throughout the building due to ceiling vibrations and noise nuisance caused by structure-borne sound. The type and intensity of the disturbances depend mainly on the crane runway itself. This can lead to offices or residential units no longer being usable or only usable to a limited extent, which is associated with corresponding loss of rent or other costs. Similar to disturbances caused by rail traffic, a large proportion of the vibrations generated when the crane bridges are in motion are produced on the rails fixed to the edge girders. The cause is slight unevenness in the rails or worn or contaminated wheels. The range of excitation frequencies can vary greatly. Relevant vibrations have already been measured by Vibraplast on various crane runways at a frequency of 20 Hz.
Damping provided by the building is insufficient
Whether the vibrations produced by the crane runway are actually perceived as disturbing in neighbouring rooms depends on the construction of the building. Depending on stiffness and material, certain frequency ranges are damped better or worse. It is not possible in practice to predict the transmission function of the building. Even with favourable constructions, the damping properties of the building are not sufficient to avoid disturbances caused by structure-borne sound and vibrations.
Sound-insulating mounting is necessary
Consistent avoidance of disturbances in practice can only be achieved by elastically decoupling the crane runway. As the vibrations produced by crane runways are generally dominant in the vertical direction, the vertical decoupling must be made correspondingly soft (see Figure 1) to reduce the transmission of vibrations to the building. It should be noted that vertical mounting with low dynamic stiffness also results in correspondingly low static stiffness and thus higher static deformations. Correct dimensioning therefore depends on an optimal design of the elastic bearings.
Solution
VIBRAX®CRANE is a system solution specifically for decoupling crane runways. It consists of a reinforced vibration damping bearing as a vertical bearing and an unreinforced vibration damping bearing as a horizontal bearing. The reinforced bearings are made of natural rubber (NR – Natural Rubber), which is known for its excellent damping properties and low deflection even under high loads. With the VIBRAX®CRANE system, improvements of 20 to 30 dB can be achieved depending on the frequency. The VIBRAX®CRANE system is available for all crane runways and includes:
- Project-specific design
- Detail drawings
- Installation instruction
Design
The single-mass oscillator model is usually used for dimensioning elastic vibration mounts. For example, when dimensioning the decoupling of a building against vibrations from an adjacent railway line, it can usually be assumed that the building acts as a rigid mass. This mass M is mounted on the decoupling elements with stiffness Cdyn, see equation (1). From the ratio β of the excitation frequencies to the first natural frequency of the mounted structure (β = f/f0), a transmission factor V3 can be determined for each frequency range: The greater the mass and the lower the stiffness, the lower the natural frequency and the lower the vibration transmission V3. Equation (1) for the natural frequency with M = dynamically effective mass and Cdyn = dynamic stiffness Equation (2) for the amplification function neglecting damping. In the case of crane runway dimensioning, the situation is much more complicated, as the mass to be used to calculate the natural frequency of the structure cannot be clearly determined. The individual masses of the mounted structure are not, as in many buildings, rigidly connected, but via resilient steel elements, such as the cables between the load and the trolley, the elastic platform between the trolley and the crane runway girder and the elastic crane runway girder itself.
Measurement on a mounted crane runway
With optimally designed elastic decoupling of the crane runway using VIBRAX®CRANE, excellent insulation values of 20 to 30 dB can be achieved, depending on the frequency. This can be impressively demonstrated by measurements. The vibrations were measured directly on the crane runway girder (green lines) and on the console of the precast concrete columns (blue lines on the steel component; red lines on the concrete). The insulation performance of the mounting can be determined from the difference in vibration levels.
