Vibration monitoring of structures in the light of the Polish and international requirements

In most of the legislations, the threshold values of the peak particle velocity (PPV) are presented in the frequency domain for different types of structures. The limit values cannot be exceeded during the exposition of structure to dynamic excitation. Four definitions of PPV are used for vibration monitoring, while the most common concerns the maximum observed component of the velocity vector for the measurements carried out in three orthogonal directions (e.g., BS 5228-4, BS 7385-2, DIN 4150-3 – for the measurements made on the foundation level) [21,22,23]. Other definitions are addressed to a maximum of two measured horizontal components (e.g., DIN 4150-3 measurements of the structure dynamical response in the upper ceiling level), vertical component (DIN 4150-3 measurements of the structure dynamical response in the middle of ceiling), square root of the sum of the squares of the maximum values observed in all three orthogonal directions (SRSS), or the true vector sum of the three-velocity vector components (TVS) (used in SN640312 [24]). Each definition leads to a different estimation of the structure damage risk. According to [1], PPV based on the maximum value of the three components of the velocity vector gives a value up to 25% smaller than TVS. On the other side, the application of SRSS can lead to even 50% greater values than TVS.

According to the majority of the standards analyzed in the paper, points located near the ground surface or foundation should be observed during vibration monitoring. Measurements should be carried out on the structural wall next to the vibration source in three orthogonal directions. Moreover, the whole structure should be observed during monitoring to avoid the resonance effect of the whole structure or its selected elements, especially in the case of long-term vibrations (BS 7385-2, DIN 4150-3, Circulaire 23/07/86). The critical points for structure, where the largest dynamical response is predicted, are located on the highest floor: –

in the middle concerning the vertical component and

The observed value of PPV is compared to the threshold value given by the appropriate standard, which depends usually on the type of the examined structure and characteristics of the excitation force (frequency, long- or short-term vibrations).

In Figs 1 and 2, the threshold values of PPV are compared in the light of different international standards [21,22,23,24,25,26,27,28,29]. Two opposite cases (the most and the least conservative ones) are presented: –

structures sensitive to the dynamical excitations subjected to long-time vibrations (Fig. 1) and

Figure 1:

Figure 2:

Generally, the acceptable vibration level is lower in the first analyzed case. Long-lasting continuous vibrations may cause material fatigue and initiate the resonance effect of the whole structure or its elements. As the natural frequency of typical structures is relatively small (less then few Hertz), the threshold values of PPV are smaller for the excitation frequencies close to the range of frequencies typically observed for buildings. For large values of excitation frequencies, the vulnerability of the structure to damage caused by the resonance effect is smaller. Therefore, larger values of the acceptable velocity components can be observed for both continuous (Fig. 1) and transient (Fig. 2) vibrations. In the case of impulse type of excitation (like blast), the permissible vibration level is much bigger than in the case of long-time excitations, as the possibility of the resonance effect and structure or material fatigue decreases (Figs 1 and 2).

Generally, for long-time vibrations and dynamically sensitive structures, the vibration PPV limits are very small values (1.25 mm/s) for low frequencies (<10 Hz) and more significant (4–5 mm/s) for the frequencies apart from the natural frequencies of structures (>30 Hz). The most conservative estimation of the damage possibility is presented in Swiss code (SN640312) (Fig. 1).

The limit value if velocity is 1.5 mm/s in the wide frequency range covering the natural frequencies of structures (<30 Hz). It has to be emphasized that not only the threshold values given by SN640312 are lower compared to other codes, but also they are related to the sum of the velocity vector components, not the maximal values of three orthogonal components, like in the vast majority of the codes analyzed (DIN 4150-3, BS 7385, BS 5228-2, Circulaire 23/07/86, and others). On the other hand, French code (Circulaire 23/07/86) requires the largest values of velocity vector components; however, the threshold values are given for measurements carried out not only on the foundation level, but also on the floors [25]. In other codes, the threshold values defined for ceilings are usually bigger compared to the ones given for monitoring made on the foundation level (compare DIN 4150-3 and BS 5228-2).

In the case of transient vibrations of the dynamically insensitive structures (Fig. 2), the acceptable limit of vibrations is less conservative compared to the case described above (Fig. 1). Moreover, the wider range of the values given by the different guidelines can be observed (Fig. 2), compared to the case of continuous vibrations (Fig. 1). The most conservative approach is presented in French code (Circulaire 23/07/86), where acceptable velocity components differ from 8 mm/s for lower frequencies (<8 Hz) to 15 mm/s for the frequencies away from the natural frequencies of structures (>30 Hz), when the resonance effect is less possible. The highest values of vibration limits are presented by British and Swiss codes, where the threshold vales for the velocities are greater than 45 mm/s. However, it must be emphasized that the threshold value given by Swiss code is related to the velocity vector and not to its component, so the relative estimation is more conservative.

The criterion for assessing the impacts on structures is not yet widely disseminated and used in civil engineering. Substantially, there is also no provision in the legal regulations related to the obligation to perform an assessment of dynamic impacts. There are, however, certain legal regulations in Poland that relate to the analyzed issue.

Specific legal regulations can be found in the PN-B-02170: 2016-12 standard [30], based on which the impact of vibrations on buildings is tested. The second standard item is PN-B-02171: 2017-06 [31], based on which the impact of vibrations on the people in buildings is determined.

The measurement methodology can be divided into four cases, depending on the existence (or not) of a building object and the source of vibrations [30] as follows: –

the building is designed, the source of vibrations is designed;

The most common case is when both the building is in operation and the source of the vibration is known. In this case, the focus should be on determining the influence of vibrations by making actual field measurements. In the case of actual measurements and their analysis, the diagnosis is based on vibrograms recorded with the use of specialized equipment, that is, vibration charts showing the dependence of displacement, velocity, or acceleration on time.

However, according to [30], the influence of vibrations transmitted to the ground may be neglected if the building is located –

25 m from the axis of the railway line,

If the building is located closer to the vibration source, it is recommended to check the influence of dynamic effects on structure. According to the Polish code, measurement sensors (accelerometers) should be installed at least in three places on the site from the vibration source [32]. The measuring points should be located on the foundation or on the wall at the ground level. The sensors should not be placed in places with a dilated sense from the building and in places with high vibration amplitude, for example, on a wall under the landing of the stairs. The sensors should also not be placed on the ceiling above the basement because it may have overstated values [33]. The measurement should be performed for the horizontal vibration components on the foundation or load-bearing walls at the ground level, separately for the longitudinal and transverse directions. It is assumed that the reliable vibration duration is the time during which the amplitude values are greater than 0.2 of the extreme value. Measurement results should be filtered in the frequency range 1–100 Hz. The analysis is performed in one third octave bands for the center frequencies. A separate detailed standard analysis is performed for each building. It is, therefore, necessary to determine the extreme amplitudes for 21 different center frequencies and plot them on the Dynamic Influence Scale (DIS) graph (see Fig. 3), and then read the zone in which the object is classified. The standard [30] applies to residential, brick buildings, constructed in traditional technology, which are divided into two groups as follows: 1)

compact buildings with small external dimensions of the horizontal projection (maximum length 15 m), with one or two storeys; the height of the building should not exceed the dimensions of the horizontal projection and

Figure 3:

DIS I has been created for residential buildings belonging to the first group and DIS II for buildings meeting the assumptions of the second group (Fig. 3).

Influence zones are defined as follows [30]: –

zone I (below A line in Fig. 3): no impact of vibrations on the building;

The presented example concerns the influence of vibrations on the structure located close to the vibration source during diaphragm wall realization (Fig. 4). The dipper (~20 T) suspended on ropes was lowered freely to finally hit the ground. In this way, dynamic impulse with rather low velocity amplitudes and low frequencies occurs. After the bucket is closed, the soil is removed. In this way, the trench is gradually deepened. The process of one-segment realization takes ~30–60 min, so it causes long-term or permanent vibrations according to the definitions given in the different standards [21,22,23,24,25,26,27,28,29,30]. The monitored facility is a school building with a traditional brick structure, built in the years 1890–1892. The building does not have concrete lintel beams, ring beams, or other modern bracing ensuring spatial stiffness. The catastrophic flood in 1997 in Wrocław, Poland did not cause structural damage directly, but had an adverse effect on the building. Moreover, the development on the adjacent property resulted in further adverse impacts on the building. The location of the building in the city center, the impact of other sources of vibration from road and rail traffic, as well as its existing damage allow to classify the structure as an object sensitive to dynamic influences. Vibrations were measured ~0.5 m above the ground level in three orthogonal directions (Fig. 5), using the three-channel geophone (Digital IR filter) Profound’s VIBRA+ device. With Profound’s VIBRA+, vibrations are measured reliably in accordance with the national and international standards (like DIN 4150-2 and -3, BS 5228-2, BS 7385-2). VIBRA+ also determines the dominant frequency in accordance with the advanced FFT (Fast Fourier Transform) method. Velocity range 0–100 mm/s with the resolution display 0.01 mm/s.

Figure 4:

Figure 5:

The results are presented in Fig. 5, where PPV exceeding according to the most and the least restrictive codes are presented. It can be seen that application of different codes can result in different conclusions.

For FTA (Federal Transit Administration) starndards, only few potentially unsafe exceedings appear. This is opposite to the analyses base on BS 5228-4, where the situaltion looks more dengerous. According to BS, geotechnical works should be stopped, whereas in the light of FTA, the situation is safe. During trench excavation, some additional dangerous effects may appear due to the variations in the level of the groundwater table, the pore pressure, or due to the soil collapse, weak soil lenses, suction forces, etc. [37]. Distinction of the cause of failure between the vibrations themselves and the above-mentioned factors can be difficult because the mentioned defects usually appear simultaneously [38].

The case of short-term dynamic interactions characterized by high velocity amplitudes and high frequencies is presented using the example of plunging the steel profiles of the soldier pile wall (Fig. 6).

Figure 6:

According to the technical specification of the ABI vibration hammer used, it works in the frequency range ~29–35 Hz and generates a centrifugal force ~237–324 kN on the profile IPE 300 profiles being deepened. A monitored residential building with a reinforced concrete structure, still in the construction phase, without finishing elements, is resistant to dynamic effects (Fig. 6). Vibration monitoring was conducted using the device Profound’s VIBRA+. Measurements were performed on the structural walls at the ground level in three orthogonal directions, consistent with the direction of the structural elements. The effect of the short-term dynamic excitation on the structure is presented in Fig. 7 and Table 1. Large vibration amplitudes and frequencies can be observed in the presented case. However, only few transgressions can be seen considering BS 7385-2, unlike the FTA where the situation looks much more dangerous.

Figure 7:

In the presented case, long-term vibrations are analyzed in the light of Polish legislations [30], wherein the obligation to conduct tests applies only to cases where the building is located 15 m from the axis of the first category road (see Section 2.2). In the presented example, it is a local road; so, there is no legal obligation to carry out surveys. However, the research has been carried out because owners reported vibration discomfort when vehicles passed in front of the house. The examined building is a single-family house made of concrete blocks, with reinforced concrete foundations, is newly built, and its dimensions exceed 15 m. It is located on a road made of reinforced concrete slabs, which is in a very bad condition (Fig. 8a). Piezoelectric acceleration sensors have been connected to the measuring apparatus, which had been properly configured. The tests have used six sensors mounted on the foundation wall of the building parallel to the street, just above the ground level. After connecting the sensors and setting up the apparatus, calibration has been carried out. On a computer connected to the “Vimea” measurement apparatus (see Fig. 8b), velocity time courses have been recorded for various vehicles: passenger cars, buses with two axes, vans and trucks with a total weight of up to 10 t, and buses and trucks with a weight of over 10 t and more than two axes.

Figure 8:

After measurements were conducted in the field, each time for each tested building, an in-depth analysis was performed by subjecting the time courses to filtration.

Fig. 9 shows the results of vibration monitoring. Research conducted on the building showed that the situation could be classified as zone II, which means that the vibrations are felt by the building, but are not harmful.

Figure 9:

Cracks and peeling off of the paint may occur. The tests were carried out at the request of the investor because the vibrations had been felt by him. However, it turned out that the dynamic impact on the building was small. This is because the threshold of human perceptibility to vibrations is much lower than the threshold of perception of vibrations by a building. After performing field tests using specialized equipment, it often turns out that there is no direct threat to the structure. Therefore, carrying out such tests for all buildings located along the road may prove to be unprofitable from an economic point of view.