DISCOMFORT OF VIBRATIONS
When there are noise nuisance complaints very often occur simultaneously vibration nuisance complaints. Being the assessment of discomfort a difficult task, due to the subjective component of this human reaction, This difficulty increases because the, contrary to what occurs with noise, the criteria for discomfort of vibrations are not well established in Portugal. So who makes the assessment is often, in difficulty to draw conclusions. In this article pass in review the factors that influence the discomfort due to vibration, criteria used in evaluation and your requirements of measurement instrumentation.
2 HUMAN RESPONSE TO VIBRATION IN BUILDINGS
The following are the factors that influence human reaction to discomfort vibrations in buildings:
• Thresholds of perception
• Frequency response of the perception threshold of the human being
• vibration level
• Exposure time
• Type of vibration
• Type of use of the building
• Time of day and activity of people
• The attitude of the people
Thresholds of lack
Lack to vibration thresholds vary greatly from individual to individual. According to the NP ISO 2631-1:20071 "50% of people (alert and in good physical shape) they can sense a vibration weighted weighted Wk with a peak amplitude equal to 0.015 m/s2. There is a very large variation, from person to person, the lack of vibration capability. While the average lack threshold is approximately equal to a peak amplitude of 0.015 m/s2, range of responses may vary from about 0.01 m / s2 to 0.02 m/s2. "
Frequency response of human beings lack thresholdThe
The way people feel the vibrations in buildings depends on several factors, included the frequency of vibration and direction. When evaluating the human response are so necessary different weights depending on the frequency axis of movement. The ISO 2631-2;20032 It is recommended that Wd weighting and Wk to horizontal and vertical directions respectively, If you meet the posture of occupants, or the weighting Wm, If it is not known. On BS 6472-1;20083 It is recommended that Wd and Wb weighting for the horizontal and vertical directions respectively. The Wm weighting is used when you don't know the direction of excitation, as is very common in situations of discomfort.
According to NP ISO 2631-1 _ 2007 (Annexed 3 – C. 3 reactions in terms of comfort with vibration environments), "Experience shows that, in many countries, often, the occupants of residential buildings complain of the vibration amplitudes provided that exceed some threshold of lack. "
Time of exposure
According to the NP ISO 2631-1:2007 (8.2.1), "There is no conclusive evidence that supports the universal dependence in time of the effects of vibration in comfort."
Kind of vibrations
The vibrations can be classified in many manerias. However from the point of view of reaction of discomfort there are two types diferentes4:
• Vibrations due to explosions
• Other vibrations
The vibrations caused by explosions can cause shock and reactions in countries with seismic activity, as our, can be confused with earthquakes.
The evaluation criteria for these two types of vibration must be completely distinct.
Type of use of the building
In premises, It is probable adverse comments occur on vibrations in the building when vibration levels, to which the occupants are exposed, they are only slightly above the thresholds of lack. In the workplace, the adverse comments often appear quite higher levels, Although people with a sedentary occupation react more as occupiers of residences.
The criteria to be used to assess human response requires consideration of the type of expected occupancy in the building (homes, offices or workshops) and the activity of the occupants.
The vibrations can also be ready due to Visual effects and ruído4.
So how people react to a particular building vibration level, may also depend on other effects related to vibrations, that occur simultaneously. The most common effects are:
• Born in Noise due to propagation structure via solid;
• Rattling noise induced;
• Third-party Influences.
Period of the day and activity of people
An individual, in a building, feel a vibration of a certain amplitude, depends on what the person is doing, where is in the building, as the person is bound to the building and the dominant direction of vibration. Someone who moves is less sensitive to vibrations than a person who does not move, and that's in a given site. If the person is standing the coupling is more direct. If you are sitting the coupling is less straight, particularly if the seat is pot roast. The construction of the seat may result in some vibration isolation, or on the contrary, an MRI can amplify the vibrations. The same can refer for beds.
As people's activity is associated with the period of the day, These two factors influence the human response.
The attitude of the people
Whether at home or at work, the tolerance of people varies in a wide range of values, Depending on the specific values of social and cultural factors, psychological attitudes and the expected degree of intrusion.
In addition to this the human reaction to situations of comfort today, a well studied technical discipline, among other things because, for example, vehicle manufacturers make major investments to ensure the best conditions of comfort for its occupants. That is, is established scientifically that the discomfort is subjective, and the human reaction to situations of comfort depends on several factors, independent of physical agent that causes the reaction.
3 EVALUATION CRITERIA
In the absence of legal limits, in Portugal, to evaluate the discomfort of vibrations are used different standards and recommendations, including the following:
• The designated "criterion of LNEC" 5
• ISO 2631 Part 1 e 2
• The norm BS 6472 (2008)
The criterion of LNEC
This criterion is defined in article 1 of 2001, and reflects years of experience of the national laboratory of Civil Engineering. The parameter used to evaluate discomfort continued vibration is the effective speed, and the following table:
A ISO 2631 Part 1 e 2
A ISO 2631 No evaluation criterion of discomfort. Features, However, relevant indications on this issue, indirectly, can be used as a criterion to determine if the vibration levels exceed the threshold of lack. and if they are susceptible of generating discomfort.
"Experience shows that, in many countries, often, the occupants of residential buildings complain of the vibration amplitudes provided that exceed some threshold of lack. " (Annexed 3 – C. 3 reactions in terms of comfort with vibration environments NP ISO 2631-1 _ 2007)
"50% of people (alert and in good physical shape) can detect a weighted vibration with weighting wk, with a peak amplitude equal to 0.015 m/s2. There is a great variation from person to person in the perception capacity of vibrations. Since the average perception threshold approximately equal to 0.015 m/s2, range of responses may vary from about 0.01 m / s2 to 0.02 m/s2 (peak amplitud)." (Annexed 3 – C.3 Perception, NP ISO 2631-1_2007)
Has that levels are less than 0.010 m/s2 peak amplitude, measure with weighting Wk, It is unlikely there is discomfort due to vibrations, These levels are below the threshold of lack. Above this value, discomfort is more likely to occur.
The standard BS 6472 (2008) – Guide to evaluation of human exposure to vibration in buildings. Vibration sources other than blasting
As stated in your introduction, This arises from the need to establish a clearer evaluation criterion of discomfort of vibrations, than the established in ISO 2631. Is divided in Part 1 e 2; the Part 1 refers to vibration than those caused by explosions, and the Part 2 refers to vibrations caused by explosions. Greatness is the acceleration, with the weights Wd and Wb for the horizontal and vertical directions respectively.
The parameter used to avaliaçã, in Part 1, is the VDV (Vibration Dose Value).which is defined as follows:
VDV b/d,day,night is the vibration dose value (in m. s-1.75)
a(t) is the frequency-weighted acceleration (in m. s-1.75), using the weights Wb or Wd as appropriate.
T is the total time of the day or night (in s) during which the vibration can occur.
This parameter gives more weight to the level of vibrations that the exposure time. Is a cumulative measure that increases as the exposure time increases and not an average.
In this standard is presented in the following table with the evaluation criteria:
In the following table is a comparison between these critérios6.
4 RECOMMENDATIONS FOR MEASUREMENT OF VIBRATION INSTRUMENTATION
Nowadays ISO and BS standards of measurement of human body vibration using always ISO 80417 as reference standard. In this standard are defined weightings in frequency of human response to vibrations and many other parameters.
In addition to what is stated in ISO 8041 measurement equipment should have the following characteristics:
• The sensor must be in order to measure the three-axis three orthogonal directions simultaneously;
• The sensor must have the ability to measure at least vibration levels 10 times lower than the threshold of sensitivity of humans. The sensors used must be of high sensitivity, to allow measuring the "residual" vibration in order to highlight clearly the vibrating relevant events;
• The equipment shall have the ability to record the detailed temporal history (Example: a measure per second) that allows to understand the physical phenomena that are at the origin of the complaints of discomfort. Obtaining the registration of the temporal history of peak values, in the three orthogonal directions, for extended periods, is an essential help to understand the phenomena concerned and establish with the greatest possible objectivity, the relevance of the existence of discomfort due to vibration. The exclusive use of average values for assessing the existence of discomfort is always an inappropriately to evaluate the discomfort being more advisable the simultaneous use of peak values. It is well known that human reaction refers to what you feel, or if you think you feel, that's more related to peak values, and not with average values.
• The weighting of the exposure time is always relevant because despite the human response be little influenced by this factor, something must be permitted so as not to choke the company economically,
High sensitivity triaxial accelerometer (1 V/g) mounted on appropriate vibration measurement base on buildings.
From experience we know that there may be complaints of discomfort and vibration levels not even reaching the threshold of lack, or on the contrary, they are exceeded and there is no complaint. Therefore the evaluation of discomfort can never be something 100% goal, and such is already reflected in standardisation.
Any evaluation of discomfort must, at least, determine whether the threshold of lack is reached or exceeded.
The source of the vibrations and side effects is very relevant to the understanding and evaluation of complaints of discomfort of vibrations, due to the strong influence they have on human reaction for vibrations in buildings.
The instrumentation used to evaluate discomfort must comply with the requirements of ISO8041, and use triaxias high-sensitivity accelerometers.
Among the evaluation criteria compared, the most comprehensive is defined in BS 6472 (2008).
1 NP ISO 2631-1 _ 2007 mechanical vibration and shock – Exposure assessment of human body vibration, Part 1 – General requirements
2 ISO 2631-2_2003 Mechanical vibration and shock — Evaluation of human exposure to wholebody vibration — Part 2: Vibration in buildings (1 Hz to 80 Prophet)
3 BS 6472 (2008) – Guide to evaluation of human exposure to vibration in buildings. Vibration sources other than blasting
4 Predicting the Feeling of Vibration in Buildings, MJ Griffin, Proceedings of the Institute of Acoustics, 2007, Vol.29 Pt.4.
5 Vibrações ambientes. Critérios de danos e de incomodidade. Actualidade e perspectivas futuras. Schiappa de Azevedo, Fernando; Patrício, Jorge, TECNIACUSTICA 2001
6 – Avaliação de Incomodidade devido a Vibrações – Comparação de Critérios Carlos Aroeira, TECNIACUSTICA 2015
7 ISO 8041:2005 Human response to vibration — Measuring instrumentation