Nowadays, structural vibrations, rather than strength govern the design of modern lightweight floors. Human-structure interaction is currently not taken into account but our recent research on full-scale structures found that it is crucial for determining accurately the level of vibration in lightweight floors. This vibration level in turn determines the floor’s vibration serviceability, and the amount of its embodied carbon, which is of rapidly increasing importance for the UK’s net zero carbon goal in 2050.
Realistic walking paths, walking frequencies that include the real probability of somebody causing resonance, the existence of partitions, and human-structure interactions are factors which, if considered, can dramatically reduce the calculated (and measured) floor vertical vibration responses due to walking. Taking them into account can ‘pass’ a lightweight floor which would otherwise ‘fail’ when checked assuming worst case scenarios and neglecting the above listed factors which can dramatically improve the floor’s vibration performance. Worst case scenarios are good design for ultimate but not for vibration serviceability limit states.
Dr Ahmed Mohammed and Prof. Aleksandar Pavic of FSD Ltd and University of Exeter have just published in top quality journal Mechanical Systems and Signal Processing as a peer-reviewed technical paper entitled Human-structure dynamic interaction between building floors and walking occupants in vertical direction which pioneers quantification of the effects of human-structure interaction for walking (as opposed to stationary which are well known) pedestrians specifically on lightweight building floors (as opposed to footbridges, where this effect has been well established). The figure below shows the dramatic effects (reduction of 45% or more) which the presence of 2, 4 and 6 people simply walking normally across a floor can have on acceleration levels across a range of natural frequencies of a lightweight floor.
If similar reductions were sought via the traditional method of increasing the mass of the floor, which is a typical wasteful way of controlling floor vibrations, a floor approximately twice as heavy would be needed with a much bigger embodied carbon footprint. Considering that 60% of the building mass is in floors, this feature has a tremendous effect on the overall building’s embodied energy. An alternative which we propose is to embrace lean design and acknowledge that walking-induced vibrations are caused by people whose presence on the floor both causes and reduces the floor vibrations and that the two always go together in lightweight floors where the effect of human-structure interaction is particularly pronounced.
A novel comprehensive methodology that takes this important effect into account is also published in the above MSSP paper and is already available for commercial application. It is part of our consulting services in Full Scale Dynamics Ltd where the first author Ahmed Mohammed now works as a consulting engineer after being awarded a PhD degree related to floor vibration serviceability earlier this year.
Managing Director, Full Scale Dynamics Ltd