Innovative Thinker | Why lean design creates floor vibration challenge
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July 2, 2021
Lean design techniques are creating new challenges for buildings and calling for alternative approaches, Alex Pavic tells NCE.
Dealing with floor vibrations is a complex challenge and one which current mitigation approaches struggle to meet.
Possible impacts of vertical floor vibrations on humans include impaired cognitive task performance, tiredness and poor concentration. Research into this is ongoing, with a team at the University of Exeter having developed the £5.4M VSimulator facility to reproduce vibration conditions and examine their effects.
According to University of Exeter professor of vibration engineering Alex Pavic, the problem started with the tendency for design approaches to focus on strength at the expense of structural rigidity or “stiffness”.
He explains: “We design structures for strength and to make sure they don’t collapse but stiffness is becoming equally important – so they don’t move much.”
This problem has been compounded by the move towards leaner and lighter structures, which has further decreased stiffness. Timber, for example, is lighter than traditional materials like concrete and has become increasingly popular because of its low carbon footprint.
In essence the lighter and more flexible things are, the more dynamically responsive they are as well
“By structures becoming lightweight, we were losing mass so we were losing stiffness. It was a natural consequence
of lean design,” Pavic says. “The lighter and more flexible things are, the more dynamically responsive they are as well.”
So what can be done to effectively mitigate vibration?
Along with strength, other criteria such as deflections, concrete cracking, thermal comfort, fire resistance and sound proofing all influence the design of floor structures.
To date the approach has been to optimise structures for these factors and then undertake a vibration serviceability evaluation to check the vibration level is acceptable. Modern long span floors regularly fail this evaluation, says Pavic.
“Then people start adding materials to increase the mass and increase the stiffness,” he says.
According to Pavic, this approach is inefficient and wasteful. He believes increasing damping would be better.
New technologies are moving forward, in particular the active mass damper (AMD), an autonomous 70kg electro-mechanical device capable of adding large amounts of damping to hotspot areas of a floor which requires vibration control.
Other solutions are passive and include strategic use of vertical floor partitioning to break up modes of vibration, or using a tuned mass damper (TMD). TMDs have no power supply and provide damping through limited mechanical amplification of the very small floor movement whereas the AMD has a motor and provides much larger damping force.
Overall, civil engineers have to be willing to take the risk of fixing the vibration levels towards the end of construction with the correct mass damper solution.
“We are asking engineers to consciously design sub-optimal structures which satisfy everything apart from vibration serviceability and then use these devices to bring the structure to a satisfactory performance,” Pavic says. “That’s a huge leap of faith.”
Department of Civil Engineering https://www.ibu.edu.ba/department-of-civil-engineering/
