By Madeleine Girard
Would you be surprised to learn that some of the floors you have walked on may contain several large plastic “air bubbles” made from recycled materials? Would you be more surprised to learn this type of floor system is the best on the market? In his talk on March 1, 2019, Professor Jose Pincheira, from the University of Wisconsin-Madison’s Department of Civil and Environmental Engineering, gave a talk on Vibration Characteristics of a Voided Concrete Slab Floor System. He described his research on a unique type of floor system that is appearing in some new construction projects in the U.S. market because it offers several advantages.
The flooring system is one of the greatest construction costs of a typical building project; therefore, structural engineers aim to utilize the most efficient design possible. In the U.S., solid slabs have been the most commonly used floor system, but the disadvantages of this type of slab include it’s substantial material usage, weight and cost. Other flooring systems use ribbed slabs, which have a waffle like pattern. Ribbed slabs use less material than solid slabs saving on material, weight, and cost.
However, there is an even better flooring system known as the voided slab system which is ingeniously designed with embedded large hollow spheres made from recycled plastic to form voids in the slab. In a typical floor layout, the voided slab system is used in combination with solid slabs around columns to handle the larger stresses at the connection points. A voided slab weighs 30-50% less than a solid slab, creating the opportunity for longer spans without beams. Furthermore, with a significant reduction in material usage, this floor system reduces building costs by 8-10%.
Since the voided slab floor system is not yet routinely used in the U.S., there is little to no vibration data available to help determine if the movement of people inside the building will be disturbing to others in the building. Therefore, Pincheira conducted a field study to gather such information. In the study, he measures the vibration response of different panel sizes inside of a building that was under construction. Excitation locations, where someone would jump on top of the slab, were determined and accelerometers, placed in other locations around the slab, were used to measure the associated vertical accelerations or vibrations.
The results showed the largest amplitude was found upon impact and then the vibrations damped quickly over time. When looking at how the different sized panels handled the jumps, Pincheria mentioned that the smallest, most stiff panel observed the highest frequency of vibration, while the largest, more flexible panel had the lowest frequency measured. Overall, the vibrations exhibited frequencies above the acceptable threshold of floor motion. The findings of the field study allowed Pincheria and his team to deem the building to be within the recommended limits for human comfort.
To supplement his field study, Pinceria created numerical simulations which was validated with the experimental results collected. This allowed him to model the mechanical properties of the voided slab system such as stiffness, mass and damping for estimating floor vibrations. The goal is to develop proposed design recommendations for projects before they are built, saving time, money, and materials in the future.