What’s wrong with walking? To get somewhere it’s fine unless it causes discomfort or annoyance to those elsewhere in the building. Walkers don’t intend to disturb others, nor do they imagine that their footsteps might be noticed by others. However, walking that causes disturbances is becoming more and more common today and one reason is the trend that involves the use of lighter weight concrete and thinner slabs on upper level floors to reduce base building costs. Read on to learn more about walking problems and to stop the earth from moving under your feet.
Lightweight concrete on metal deck weighs much less than normal weight concrete. The resulting structural design allows the steel beams, joists, and columns to be sized smaller and lighter, allowing a cost reduction. The problem is that this reduction in structural weight can reduce the natural frequency of the floor system to below 10 Hz (cycles per second).
Walking can be classified as slow, medium or fast with step rates of roughly 1.5, 2.0, and 2.8 steps per second, respectively. Multiples of these step rates include the frequencies of 3.0, 4.0, 4.5, 5.5, 6, and 8.25 Hz. Any of these frequencies can set a lightweight floor system into walker-induced vibrations.
Floor systems with fundamental frequencies of 3 Hz or less should be avoided as they easily align with walking step rates. To ensure satisfactory performance in terms of human comfort, floor fundamental frequencies should be above 10 Hz.
Additional weight on the floor system in the form of partitions and high-density paper files can reduce the vibrations in the floor system. But in today’s workplace, most of the floor is open plan offices and file storage is electronic; therefore, the pace of the footsteps can excite a thin slab to vibrate strongly when the walking rate is approximately aligned the fundamental resonant frequency of the slab or its harmonics since not much is there to dampen the slab. This is often felt throughout the floor, but even more so mid span between beams, girders and columns.