Footfall is a very common vibration source responsible for the disruption of human comfort in residential and office buildings. Other common sources of perceptible vibrations are dancing, running, jumping etc. that can be observed in ballrooms, health clubs and entertainment venues. Modern open concept architectural lay-outs with long span structural bays can also lead to more responsive floors.
The office tower consists of extensive floor plates with long span, open concept layouts replicated over ten levels. Floor framing was designed to meet footfall-induced floor vibration requirements for human comfort.
A maximum peak acceleration was selected as office floor vibration criteria following the recommendations of the American Institute for Steel Construction.
A finite element model of each typical office floor plate was built based on the most up-to-date structural and architectural drawings. Next, a footfall vibration analysis was performed at each structural bay, and maximum peak floor accelerations were calculated and compared against the criteria.
The bays that didn’t meet the vibration criteria were re-designed by increasing the stiffnesses of the beams and/or girders, while the structural bays that performed much better than the criteria were evaluated to reduce the weight of the beams and the girders. Structural drawings were updated for the optimized framing.
One of the unique challenges of Foundation Medicine’s biomedical lab/office project was the design of an interior bridge with a small conference room right in the middle. The bridge structure used a maximum of 10” deep steel built-up girder tapering down to 4” at the edges of the deck with a 4 ½” topping slab. The bridge was designed to meet office vibration criteria at the mid-span with consideration for people walking along the bridge.
A maximum peak acceleration was selected as office floor vibration criteria following the recommendations of the American Institute for Steel Construction – Design Guide 11.
A finite element model of the floor was built and enhanced vibration analysis was conducted with consideration for slow walking footfall within the imaging rooms, moderate walking footfall along the ghost corridors and fast walking along the main corridors.
Finite element model (FEM) of the floor plate (including the 32-ft span bridge structure) was built and FEM-based footfall vibration analysis was performed. Maximum peak bridge deck accelerations were calculated and compared against the vibration criteria. Required reinforcement on the existing floor framing and the optimum structural design of the bridge deck were realized by iterating on the design parameters to meet the vibration criteria.
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