Designing the civil structures for the serviceability limit state is having equal importance to designing for the ultimate limit state. As a serviceability concern, vibration is a most important factor to be considered as it is most annoying to the people in the building.
Vibration is an oscillatory movement which makes the quality of the occupants in the building questionable. The vibration on a building can be induced by several sources. Most of the engineers are accounting for the building vibration but they don’t pay much attention on the floor vibration which might cause simply by walking around and across the floor.
The sway of London Millennium bridge which was opened for use in June 2000 is a good lesson for this human induced vibration. London Millennium footbridge which is known as London’s “wobbly bridge” was opened in 2000. In 2000, when the people started walk across the bridge, the bridge was swayed unexpectedly with an amplitude of about 50 mm to 75 mm approximately. This phenomenon was not well-known at that time. Later on, from the investigations it was found that, the bridge was aerodynamically stable, and the resonance occurred due to the forces excited by footfalls. Then the bridge was retrofit with the dampers so that the induced vibration energy can dissipate quickly.
With the advanced technical solutions, structures become more lighter with the aid of lightweight materials and prone to resonate even for lower frequency values of footfalls excitation which falls in the range of 1.5 Hz to 2.8 Hz. In addition, the excitation frequencies are accumulated with the several footsteps simultaneously. So, the overall response should be taken into account when evaluating for the footfall vibration.
The response of the floor to the footfall excitation can be discussed into two forms
- Resonant Response: The resonant response of the structure should be evaluated when the natural frequency of the structure is lesser than 4.2 times walking frequency
- Transient Response: The transient response of the structure should be evaluated when the natural frequency of the structure is greater than 4.2 times walking frequency
Even a lower level of vibration but which can be perceptible by the human being is a problem to the human comfort and their activity. So limiting the level of vibration arising is an important design criterion for the engineers to ensure the occupancy comfort.
For the vibration evaluation, ISO (ISO 2631.1-1997), British (BS 6472-1992) and Australian (AISC) guidelines are the guidelines used in general to limit the acceptable vibration level with the intended use of the building and the possibility of the occurrence of the vibration. The floors should be designed considering the annoyance causing by the footfall vibration based on these guidelines or a proper mechanism should be adopted on the structure to mitigate the vibration from the floor.
