By Andrew Runnoe, PE, SE – Structural Engineer | Orange County Office
Floor vibration is a structural design consideration gaining attention in the southern California area, particularly with the increased presence of biomedical research facilities in the region. Building tenants are frequently adding floor vibration requirements to their leases to accommodate vibration-sensitive equipment. Whether you have an existing building with future tenants in mind, new construction that should meet the needs of future tenants, or an existing tenant raising concerns, there are a few factors for owners to consider in meeting the floor vibration needs.
While there is no code requirement for floor vibration design, there are guidelines that engineers utilize to help determine the anticipated vibration a floor will produce. The American Institute of Steel Construction (AISC) Steel Design Guide 11 was originally published in 1997 and updated in 2016 to assist engineers in determining how a steel-framed concrete floor over a metal deck will vibrate under walking excitation (e.g., a person walking down a hallway) or rhythmic (e.g., a workout class jumping up and down to music), and how the vibration will then progress around the building. This design guide is extremely beneficial to engineers designing offices, laboratories, and steel stairs. While research is underway for vibrations in concrete and wood-framed floors, these design guidelines currently only exist for steel structures.
There are many variables that determine floor vibrations, including human or equipment movement, and natural frequency of the floor framing system. Vibrations caused by human movement vary due to weight, walking pace, and impact on the floor. One person sitting at a desk may be more sensitive to vibrations than another person sitting in the same spot. Vibrations move from the source (i.e., a human walking or equipment moving) through the floor slab and beams into the supporting walls and columns. However, one of the biggest factors affecting floor vibrations is the natural frequency of the floor framing system, which determines the amount of vertical vibration that can be achieved. Because frequency is directly related to the stiffness, the structural engineer will look to stiffen the framing system to change the frequency of the floor framing. It is also important to note that the frequency of floor framing will vary within a single bay of framing. Because the floor is stiffer near columns and walls, it is harder to detect vibrations in these locations, which provides an easy solution for vibration-sensitive equipment. This is something that often gets overlooked and doesn’t involve much in the way of construction or engineering.
Another large contributor to vibration is floor framing mass. The heavier your floor framing system is, the more difficult it is to move, further reducing vibration. This can be improved during the design process either by adding concrete to the topping or by using normal concrete instead of light weight concrete. However, adding mass could impose consequences elsewhere in the structural system and should be thoroughly vetted before utilizing this solution to meet floor vibration requirements. Adding mass also increases construction cost, which is a major consideration in the design process.
The good news is that there are several options for vibration remediation, some as simple as moving the affected person or equipment closer to a wall or column where the floor vibrates less. More sophisticated options include stiffening the existing framing from below to reduce vibrations or adding mass for a damping effect. Our structural design team will work with the building owner, architect and/or tenant to provide an efficient and cost-effective solution.