All around us, there are various vibrations that adversely affect the comfort and livability of our lives. What causes these vibrations and what are their characteristics?
Classification of vibration sources
Vibration sources of vibration phenomena that occur around us can be divided into two main categories: natural vibration sources and artificial vibration sources.
Natural vibration sources are caused by natural phenomena. Examples include strong winds and long-period seismic tremors (slow, large shaking caused by earthquakes).
Artificial vibration sources, on the other hand, are caused by human activity. This can be further subdivided into internally generated and externally generated vibration sources, with the building as the primary source. Vibrations generated inside buildings are called “internal artificial vibration sources,” and include vibrations caused by people walking, moving, and operating equipment and machinery. Vibrations generated outside the building are called “external artificial vibration sources,” and include vibrations transmitted by road traffic, railroads, factories, and other nearby sources.
Figure 1 Classification of vibration sources
(Reference 1) Architectural Institute of Japan: Guide for Environmental Vibration Design to Ensure Occupant Performance, 2020.6
Natural Vibration Source
Natural vibration sources include wind and earthquakes.
Wind primarily shakes skyscrapers. Wind speed and wind pressure increase with altitude, so when the swaying period of a high-rise building matches the wind strength, this can cause significant and prolonged swaying of the upper floors.
Earthquakes exhibit different characteristics depending on the periodic component of their vibrations. For earthquakes occurring in remote areas, the short-period component (short and small tremors) weakens and the tremors become smaller, but the long-period component does not weaken and travels through the ground, causing high-rise buildings (especially high-rise floors) to shake slowly and significantly. Therefore, when considering the occupancy environment of high-rise buildings, long-period seismic motions are mainly considered.
External artificial vibration source
Traffic vibration
Traffic vibration can be caused by road traffic and railroads. When cars and trains pass nearby, vibrations are generated and are transmitted from the ground to the building, causing it to sway.
Road traffic vibration is caused by ground vibrations generated by vehicle travel. The condition of the road surface and viaducts, the type of vehicle (weight and speed), and the volume of traffic affect this phenomenon.
Railroad vibration is affected by the weight and speed of the train and passengers, the wear condition of the wheels, and the condition of the rails. There are also many impact vibrations generated by rail joints, which can cause solid-state noise (noise derived from vibration) such as rumbling as well as shaking of buildings.
Vibration caused by factory/production machinery
Vibration sources in factories and other production facilities include metalworking machines, compression machines, molding machines, crushing machines, weaving machines, and traveling cranes. These machines generate impact vibration from presses and vibration from rotating motors.
Vibration inside and outside the building can be a problem because these machines not only shake the inside of the building in which they are installed, but also travel through the ground and cause neighboring buildings to shake.
Internal Artificial Vibration Source
Vibration caused by equipment
Equipment in buildings includes elevators, mechanical parking garages, air conditioning equipment, water supply and drainage equipment, and boiler equipment. Vibration is generated by the rotation and reciprocating motion of these equipment. Vibration can also be caused by water flow and air currents inside machinery.
These vibrations are not only felt as shaking, but also transmitted to walls, ceilings, and floors, where they are heard as sound (solid sound) and can become a noise problem.
Vibration caused by human movement
Vibration is caused by human motion, such as walking or exercise.
The excitation force generated by human walking itself is not very large, but the excitation force contains a frequency component that is an integral multiple of the rhythm (period) of walking. When the periodic component of one to five times the walking tempo approaches the period at which the floor tends to sway, the floor may sway significantly (resonance phenomenon). Long spans and lightweight floors are particularly prone to resonance. In addition, steel-framed buildings tend to take longer than wood-framed buildings for vibrations to settle. The combination of these conditions can cause significant and uncomfortable shaking, which can be problematic.
Aerobics and other types of fitness in which a large number of people perform jumping and bending exercises in rhythm with each other have high excitation force and, like walking, resonate when the integer multiple of the periodic component in the exercise cycle approaches the period when the floor or building tends to sway, resulting in a large sway. As a result, the swaying is transmitted to the floors above and below the building (and in some cases, to floors several floors apart), which can cause vibration problems.