The vibrations we feel inside a building can be divided into two main types of shaking. One is horizontal (sideways) vibration and the other is vertical (up and down) vibration. These are called horizontal vibration and vertical vibration, respectively. The mechanism of vibration generation and how people feel the shaking differ depending on the direction of shaking. The following is an easy-to-understand explanation of these differences.
Causes of horizontal and vertical vibration
There are several different types of shaking that can be felt in a building. For more information on this, please refer to our previous tidbit,“Causes of Vibration - A Complete Guide”.
Figure 1 shows the relationship between vibration sources and the direction of vibration generated in a building. Horizontal vibrations are generated by strong winds, long-period seismic tremors (slow shaking with a long period), traffic vibrations from cars and trains, and the operation of production machinery in factories. Vertical vibrations are generated by human movements such as walking, jumping, dancing, etc., and by the operation of equipment (internal man-made vibrations), as well as by traffic vibrations such as car and train vibrations and the operation of production machinery in factories (external man-made vibrations). Traffic vibration and the operation of factory production machinery are potential sources of both horizontal and vertical shaking vibration, but in reality vertical vibration is more often a problem.
Figure 1 Relationship between vibration source and direction of vibration generated in the building
(Reference 1) Architectural Institute of Japan: Guide for Environmental Vibration Design to Ensure Occupant Performance, 2020.6
How buildings sway due to the difference between horizontal and vertical vibration
Vibrations transmitted to a building affect it differently depending on the direction of sway. Figure 2 shows how a building shakes (horizontal and vertical vibration). Horizontal vibration resonates with the entire building frame, shaking the entire building horizontally. Vertical vibration travels through the building frame and resonates with the target beams and floors, causing the building to sway vertically. Horizontal vibration shakes the entire building, so the magnitude of the shaking does not vary much from place to place on the same floor. Vertical vibration is localized, so even within the same room, there are places that sway and places that do not sway much.
Figure 2: How the building shakes (horizontal and vertical vibration)
Frequency of horizontal and vertical vibration
I will discuss the frequency of horizontal and vertical vibrations.Long-period seismic motions and strong winds, which cause horizontal vibrations, are infrequent, occurring only a few times a year at most. However, strong winds can last from a few hours to more than half a day.
Vertical vibration, on the other hand, occurs many times each day because it is caused by human activity, traffic, equipment, and plants.
図3 振動の発生頻度
How people feel the difference between horizontal and vertical vibration
Depending on the direction of the shaking, people feel differently. For differences in the way people feel, there is a reference response (sensitivity to shaking at different frequencies), which quantifies the characteristics (ease of feeling) of horizontal and vertical vibrations. Figure 4 is a graph of the reference response of vertical and horizontal characteristics against frequency (period of shaking) (see JIS C 1510 Vibration Level Meter). The horizontal axis is the frequency of the vibration, and the vertical axis is the amount of correction for the sensation of vibration acceleration level. The green and blue colors represent the horizontal and vertical directions, respectively. From this,
- Horizontal vibration is most easily felt at 1 to 2 Hz
- Vertical vibration is most easily felt at 4 to 8 Hz.
- Horizontal vibrations are more easily felt at frequencies lower than 3 Hz, while vertical vibrations are more easily felt at higher frequencies.
Vertical vibration is more sensitive at higher frequencies.
Figure 4 Reference response for vertical and horizontal characteristics