All framed structures, including stairs, have an inherent natural frequency defined by their material composition, shape, and support system. Human activities, such as walking, running, and jumping also occur on these structures at a certain speed and frequency. When the frequency of human activity produces oscillating forces that match the natural frequency of the structure, vibration resonance occurs. This resonance will produce accelerations that can be palpably sensed by stair users. The sensation produced is generally uncomfortable and in extreme cases alarming.
Slender monumental stairs, consisting of heavier treads and guardrails combined with slender stringers and very little damping provided by non-structural finishes, often have very low natural frequencies. Consequently, this trait makes them more susceptible to vibration issues caused by human movement because there is a higher probability that the walking frequency of the stair users will match the natural frequency of the stair, driving vibrations. The subtle motions of these structures and the transmission of that movement to other parts of the building, can create vibration serviceability issues. This produces complaints of occupant discomfort and in extreme cases, potential safety concerns.
To best avoid vibration performance issues and the need to stiffen the structure post-construction, stair designers must be able to predict the dynamic properties and vibration response of monumental stairs under users’ loads during the design phase. Identifying and mitigating these issues long before fabrication and installation can save time and money, all while upholding the original architectural vision for the design.
Historically, vibration behavior was predicted using a series of conservative empirical mathematical formulas based on structural characteristics such as mass and natural frequency. In 2016, the American Institute of Steel Construction (AISC) published an updated consensus procedure based on cutting-edge research by Brad Davis, Ph.D. The paper evaluated expected vibration responses of high-performance steel structures using finite element analysis, which analyzes the response characteristics of a structure using three-dimensional computer modeling. This procedure permits design engineers to more closely predict how a structure will behave in the real world in response to vibrations by using sophisticated structural analysis software, such as SAP2000. Programs like this can predict the magnitude of accelerations due to human-induced vibration.
“McLaren is at the forefront of using these new techniques to evaluate vibration response and has been collaborating with the architects and contractors who design and build these high-end monumental stair structures,” says Brad Fallon, PE, structural engineer specializing in the design of building structural components as well as the analysis of existing structures. “This analysis is performed as part of our design process and anticipated acceleration levels in relation to common threshold values reported back. If the analysis indicates a particular concern for vibration resonance, we can use our process to quickly evaluate design options to minimize impact to the overall architectural vision.”
Brad Fallon, PE, is a structural engineer out of McLaren Engineering Group’s Baltimore office has 12 years of structural engineering analysis and design experience. Recently, Fallon used finite element modeling technique for the design of a monumental stair system for a well-known corporate headquarters in New York City. Additionally, he specializes in detailed design of structural systems and the preparation of contract drawings and specifications. Fallon has worked on facilities of various sizes and complexity ranging from aquariums to multi-story medical, residential, and mixed-use structures, as well as power plants, pedestrian bridges, and casinos. His project portfolio spans nationwide and includes: Two Light Tower (Kansas City, MO), Inmar Headquarters at PTRP (Winston-Salem, NC); Live! Maryland Casino & Hotel (Hanover, MD); National Aquarium (Baltimore, MD), and Teachers Village Workforce Housing (Newark, NJ).His NYC healthcare engineering work and oversight includes projects for the City’s top area systems including Mount Sinai Healthcare Facility, Northwell Health; New York Presbyterian Hospital; Beth Israel Medical Center; Lutheran Hospital; Lawrence Hospital and more.