Crane runways cause noise
Depending on the situation, the operation of the crane system throughout the building may cause disturbances due to ceiling vibrations and noise pollution caused by structure-borne noise. The type and intensity of the disturbances depend primarily on the crane runway itself. This can mean that offices or residential units can no longer be used or can only be used to a limited extent, which is associated with a corresponding loss of rent or other costs. Similar to disruptions caused by railway traffic, the majority of vibrations are generated when the crane bridges move on the rails attached to the edge beams. This is caused by slight unevenness in the rails or worn or dirty wheels. The spectrum of excitation frequencies can vary greatly; Vibraplast has measured relevant vibrations on various crane runways starting at a frequency of 20 Hz.
Attenuation by buildings is not enough
Whether the vibrations generated by the crane runway are actually perceived as disturbing in neighbouring rooms depends on the construction of the building. Depending on the rigidity and material, certain frequency ranges are damped better or worse. In practice, it is not possible to predict the transfer function of the building. Even with favourable constructions, the damping properties of the building itself are not sufficient to prevent disturbances caused by structure-borne noise and vibrations.
Sound-insulating mounting is necessary
In practice, consistent avoidance of interference can only be achieved through elastic decoupling of the crane runway. As the vibrations generated by crane runways are generally dominant in the vertical direction, the vertical decoupling must be designed to be correspondingly soft (see Figure 1) in order to reduce the transmission of vibrations to the building. It should be noted that a vertical support with low dynamic stiffness also results in correspondingly low static stiffness and therefore higher static deformations. The correct dimensioning therefore depends on an optimised design of the elastic bearings.
Solution
VIBRAX®CRANE is a system solution specially designed for decoupling crane runways. It consists of a reinforced vibration isolation mount as a vertical mount and an unreinforced vibration isolation mount as a horizontal mount. The reinforced mounts are made of natural rubber (NR), 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 dimensioning
- Detailed drawings
- Installation instructions
Dimensioning
The single-mass vibration model is usually used to dimension elastic vibration mounts. For example, when dimensioning the decoupling of a building against the vibrations of a neighbouring railway line, it can usually be assumed that the building acts as a rigid mass. This mass M is supported on the decoupling elements of stiffness Cdyn, see equation (1). A transmission factor V3 can be determined for each frequency range from the ratio β of the excitation frequencies to the first natural frequency of the supported structure (β = f/f0): The greater the mass and the lower the stiffness, the lower the natural frequency and the lower the vibration transmission V3. Equation (1) of the natural frequency with M = dynamically effective mass and Cdyn = dynamic stiffness Equation (2) of the amplification function if damping is neglected. When dimensioning crane runways, the case is much more complicated, as the mass to be used for calculating the natural frequency of the structure cannot be clearly determined. The individual masses of the supported structure are not rigidly connected, as in many buildings, but by resilient steel elements, such as the cables between the payload and the trolley, the elastic platform between the trolley and the crane runway girder and the elastic crane runway girder itself.
Measurement on supported crane runway
With an optimally designed elastic decoupling of the crane runway with VIBRAX®CRANE, excellent attenuation values of 20 to 30 dB can be achieved, depending on the frequency. This can be impressively proven by measurements. The vibrations were measured directly on the crane runway girder (green lines) and on the bracket of the precast concrete supports (blue lines on steel component; red lines on concrete). The damping performance of the bearing can be determined from the difference in vibration levels.
