Vibration performance assessment
Repurposing floors for laboratory use often requires an assessment by a vibration engineer to assess the performance of the existing structure and to determine the feasibility of achieving the vibration goals of the project. An assessment should include measurements at various locations on the floor(s) and must consider all relevant site vibration sources. With this information, it is possible to produce a map of vibration classes for various areas of the building. The base performance provides the owner and design team with insights into the divergence of the current performance from the target criteria, indicating whether remedial measures are necessary.
It is standard practice to measure floor responses to various pedestrian pacing rates (e.g. slow, moderate, fast), as well as to perform dynamic modal tests to identify vibration frequencies and damping ratios. Where environmental sources are a concern (e.g. railway traffic), the worst-case impacts to performance should be quantified and feasibility of mitigation verified. Any major building services to remain following renovations should also be considered as these sources may be particularly problematic for many laboratory tools (e.g. MRI, high-resolution microscopes, NMR, etc.).
Changes to the existing vibration performance of the building should be anticipated when there will be significant changes to massing and partition layouts. Floor areas that previously supported heavy office furniture or filing cabinets and were highly partitioned may be repurposed as laboratory modules with little-to-no partitioning and lower superimposed structural loads. In such cases, the floor may become livelier due to reduced mass and damping. Conversely, the introduction of partitions spanning slab-to-slab through mid-bay will generally result in lower vibration levels following reuse. These changes can be significant and may warrant additional/supplemental vibration control measures to achieve performance objectives.
Numerical finite element modeling (FEM) is a useful tool for estimating vibration performance, considering the effects of both structural and non-structural elements associated with planned reuse. The model is developed and calibrated using data which is collected during the measurement survey. The dynamic parameters that are outputs from FEM are used to generate response maps (heat maps) that illustrate the expected distribution of vibration levels on a floor. They can be an effective tool for evaluating layouts and control strategies and for identifying zones of high or low vibration response.
Vibration control strategies
Controlling unwanted floor vibration has become a growing challenge for structural engineers, building owners, and laboratory operators. Traditional measures involve increasing section sizes, modifications to existing sections to improve stiffness, and increasing concrete thickness. These strategies can result in substantial material costs and added embodied carbon.
