June 2016 Newsletter: A Problem With Walking

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.


Examples of simulated slab flexion for 4 modal frequencies

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.

Examples of peaks and troughs resulting from modal vibrations across beams

Oftentimes, base building designs are reviewed for vibrational risk before construction documents are issued. This review is considered essential for slabs where rhythmic activities such as dance, aerobics, jumping exercises and fitness activities are planned. Other vibration concerns are related to floors where sensitive equipment such as microscopes, lasers, and operating rooms are located, which require very low levels of vibrations. Unfortunately, footfall excitation is often overlooked when designing typical office spaces, commercial mixed-use spaces, educational buildings, and multi-family buildings, for example.

Examples of slab vibration prediction models

In new buildings, complaints generally begin after several months when occupants become sensitized to floor vibrations. Once a building is occupied, remedial strategies can be costly, often beyond the savings provided by the lighter design.

In conclusion, low cost designs can cost more when the results are unacceptable. Maybe a novel solution might be gliding along on a Segway from the elevator to your desk. This of course could solve the problem and add a spark of enjoyment.

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