• Brad Davis, Ph.D., S.E., Associate Professor, University of Kentucky; Owner, Davis Structural Engineering, LLC
• Alex Brown, P.E., MBA, CDT, Product Champion, Nucor – Vulcraft/Verco
• Dennis Montgomery, P.E., S.E. Marketing Engineer, New Millennium

Understanding the Floor System

Floor vibration can affect occupant comfort and, in some applications, sensitive equipment. To achieve acceptable performance, engineers must account for more than the joists themselves; framing layout, connections, slab properties, and nonstructural components such as partitions also shape vibration response. “A floor joist is only a singular component of the floor system,” Montgomery says.

According to Davis, floor vibration is evaluated by comparing the predicted acceleration due to walking versus the vibration tolerance limit. If the predicted acceleration does not exceed the limit, then the likelihood of complaints is very low. The predicted acceleration depends on the natural frequency (higher is better), floor mass (higher is usually better), and damping (higher is better). The limit depends on what the occupants are doing and the ambient environment. Occupants are much more sensitive when they are sitting in a quiet office and much less sensitive when they are walking on an outdoor structure. A secondary criterion has to do with natural frequency only. The commonly referenced 3 Hz lower limit helps provide separation from the first harmonic of walking and prevents “vandal jumping,” a term used to describe occupants intentionally exciting a floor after noticing its movement. 

Brown points to another challenge when addressing vibration: misconceptions about what options of joist design are available to affect vibration performance, leading to less efficient and indirect measures. He notes that some engineers rely on deflection criteria as a stand-in for vibration analysis. Instead, he recommends using anticipated frequency and acceleration to guide design decisions, whether that leads to a stiffer joist or a different connection approach.

Addressing Vibration During Design

In Davis’ experience, almost all investigations of problematic vibration begin with occupant complaints. To reduce the likelihood of post-construction modifications, which are expensive and difficult, Davis has one “critically important” piece of advice: “The EOR has got to check vibration during design.”

Montgomery echoes Davis’ message, cautioning against waiting until the floor system has already been established to address vibration performance. “A common misunderstanding is that your floor joists are the main, sole basis of the design of your floor system,” he says. “Leaving it to the joist engineer to go ahead and design for vibration is probably not your best method to achieve a good floor system.”

Vibration considerations can also affect decisions such as joist spacing, slab thickness, and girder sizing, which is why Montgomery recommends performing at least a preliminary vibration evaluation before those parameters are finalized.

When vibration problems do need to be corrected, the solutions are often straightforward in principle. “Usually what we’re doing is adding steel to increase the natural frequency or mass of the floor,” Davis says. That may involve adding beams, reinforcing existing members or introducing additional support.

Flush-Framed Connections and Performance

Flush-framed connections are one option for improving vibration performance in steel joist floor systems. Brown says, “Flush frames are just an absolutely brilliant invention, and they’ve seen really widespread use in recent years.”

This type of connection places the joist top chord in the same plane as the top of the supporting girder, typically through a bolted joist-to-girder connection. Rather than bearing on a traditional joist seat, the connection allows the joist to fit within the framing depth and can allow for composite action of the girder with the floor system, if needed.

“It massively improves vibration performance just on account of having that stiffer connection, which makes joists a lot more feasible for a floor system,” Brown says.

The benefit extends beyond the connection itself. Davis explains that traditional seated systems often require designers to increase girder sizes significantly to satisfy vibration requirements. With flush-framed connections, “it’s a much, much more balanced design with strength, deflection, and vibration about as likely to control. Vibration performance comes up quite a bit.”

Because flush-framed connections affect both the joist and the supporting girder, their design often involves coordination between the EOR and the joist manufacturer. “There really is a lot of benefit to have some early coordination with the joist manufacturer to make sure dimensions, how the plate will interact with the chord gap, etc. all go together,” Montgomery says.

Resources for Engineers

Several resources are available to engineers evaluating floor vibration, ranging from design guides and spreadsheets to software developed specifically for vibration analysis.

Davis says SJI’s Technical Digest 5 (TD5) and AISC’s Design Guide 11 (DG11) were developed to provide practical methods that engineers can use without having to work directly from the research literature. TD5 is specialized for floors with concrete slabs supported by open-web steel joists. DG11 covers other topics also, including stairs, footbridges, and sensitive equipment. “For the vast majority of the cases, the EORs can figure out how to check their floor for vibration using Technical Digest 5 or Design Guide 11,” Davis says.

Davis also highlights the FloorVibe 3.1 software, developed by his longtime mentor and collaborator, Dr. Tom Murray, whom Davis describes as the “father of floor vibration.” Engineers can input information such as joist series and design load, allowing the program to estimate properties needed for vibration analysis, including chord areas, centroid location and moment of inertia. Davis describes the software as “practical and easy to use” and says “the quality control is very, very high.”

Brown discusses manufacturer-developed calculators that automate portions of the analysis process described in SJI Technical Digest No. 5, allowing engineers to evaluate alternatives more quickly during design. He also notes that SJI provides a downloadable spreadsheet, Floor Bay Comparison Tool – With Vibration, for vibration calculations.

Educational resources remain available as well. Davis points to AISC NASCC presentations on a wide variety of vibration topics including overviews of DG11, floors with open-web steel joists with flush-framed connections, sensitive equipment application, finite element analysis methods, monumental stairs, retrofit strategies, and more.

Brown also points engineers toward the technical expertise within the joist industry, encouraging project teams to get involved earlier in the design process. “Pick your favorite joist supplier, become friends with one of their engineers and start bouncing questions off of them,” Brown says. “I don’t want joist to be seen as a black box, and there’s a lot you can analyze upfront.”