Sanayei-2019-Experimental-Study.Pdf

Engineering Structures 198 (2019) 109473

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Engineering Structures

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Experimental study of train-induced vibration in over-track buildings in a metro depot T ⁎ Ziyu Taoa, Yimin Wanga, Masoud Sanayeic, , James A. Moored, Chao Zoub a School of Civil Engineering and Transportation, South China University of Technology, Guangzhou, Guangdong 510641, China b School of Civil and Transportation Engineering, Guangdong University of Technology, Guangzhou, Guangdong 510006, China c Department of Civil and Environmental Engineering, Tufts University, Medford, MA 02155, USA d Acoustic & Vibrations, Acentech Inc., Cambridge, MA 02138, USA

ARTICLE INFO ABSTRACT

Keywords: Over-track buildings in Chinese metro depots have recently been designed to accommodate the rapid urbani- Train-induced vibration zation and massive construction of urban rail transit systems. However, train-induced vibration and noise im- Metro depot pacts in over-track buildings need to be carefully assessed to provide comfortable working or living environ- Experimental study ments for the building's occupants. Vibration and noise measurements were carried out in a typical 28-story Low-rise steel-framed over-track building load-bearing wall supported residence and a 4-story steel-framed office building during train pass-by events in High-rise load-bearing concrete wall supported the Qianhai metro depot in Shenzhen, China. Measured points were set at ground level adjacent to the building over-track building fl USA and Chinese standards support structures as well as on upper oors. Train induced noise levels were within both FTA and Chinese limits. Floor vibration levels in the low-rise building were within FTA limit but not the more stringent Chinese limit. Ground-borne vibration signals in the throat area contained high level, short duration impacts generated as cars passed over discontinuities in rails at a switch. Ground vibration levels near the columns under the low-rise building were significantly greater than at the load bearing walls of the high-rise building by 20 dB or more. Levels within the buildings near columns or walls were only 5 dB higher in the low-rise building. The low-rise building transfer structure apparently significantly reduced transmission into the building above the platform. Vertically aligned load bearing walls in the high-rise building more effectively transmitted vibration. The measured vibration reduction from floor to floor was meaningfully less than recommendations in FTA guidelines. This research enriches the database of train-induced vibration and noise in over-track buildings, provides insights into the vibration transmission process into and within over-track buildings and is of value for the design and construction for similar over-track buildings in future train depots.

1. Introduction 4511.3 km for metros [2]. Metro depot, which is used for subway train storage, cleaning, maintenance and performance test, is a basic ancil- Increase of urbanization and development of urban rail transit are lary facility of a metro system and usually covers a large land area [19]. interdependent. Some inevitable issues arise with urbanization such as With the massive construction and development of metro systems, road traffic congestion, air pollution and land scarcity etc. However, traditional metro depots of lower building density but large land oc- urban rail transit, which encompasses metros, light rail, monorail, cupation aren't an economical use of urban land. To relieve the issue of trams and maglev, is an effective solution to road traffic congestion and urban land scarcity and also offer financial support to the operation of air pollution problems. Many cities in China have experienced rapid metros, over-track buildings built above metro depots or stations are urbanization with population growth and economic development since increasing. More than 27 cities in mainland China have built or have the recent social reforms [33]. The urbanization level in China is ex- begun construction of over-track buildings recently. pected to reach 66% by 2050 [29]. By the end of 2018, a total of 35 Newer metro depot incorporates a large-scale reinforced concrete cities in mainland China have constructed urban rail transit systems and platform over the tracks that covers the full extent of the depot. The put them into service with a total length of 5766.6 km, containing platform is supported from the ground by columns and load bearing

⁎ Corresponding author. E-mail addresses: [email protected] (Y. Wang), [email protected] (M. Sanayei), [email protected] (J.A. Moore), [email protected] (C. Zou). https://doi.org/10.1016/j.engstruct.2019.109473 Received 26 March 2019; Received in revised form 10 July 2019; Accepted 1 August 2019 0141-0296/ © 2019 Published by Elsevier Ltd. Z. Tao, et al. Engineering Structures 198 (2019) 109473 walls that are located in-between tracks. The platform is segmented track buildings of a metro depot, one of 14 floors and another of 25 horizontally by construction joints to accommodate thermal expansion floors, and suggested that vertical vibrations and noise impacts be and to create modular seismic-resistant structures. Over-track buildings carefully examined during the design of over-track buildings within in metro depots normally form an entire community including re- 40 m on the platform above the throat area. An impedance model for sidences, offices, schools, clinics, stores, restaurants, etc., and accom- efficiently predicting train-induced vibration in over-track buildings modate thousands of citizens. has been proposed and verified through data of field measurements However, along with the increased urbanization and growth of rail [35]. infrastructure [27], the distance between urban area and railway cor- Over-track buildings in metro depots typically have different ridors is decreasing, which results in growing concerns about railway- structural configurations, including concrete and steel-columns, con- induced vibrations [5]. Although the speed of the trains in the metro crete load-bearing walls as well as transfer structures between the depots is low, the train-induced vibration can transmit from the tracks primary ground columns and those in the over-track building. Reliable through the columns and walls into the platform and over-track in-situ measurements in over-track buildings with different construc- buildings where it can impact the buildings' occupants [36]. tion during train pass-bys in the metro depot below are needed to assess Train-induced vibration may cause discomfort and annoyance to the impact of the rapid development and massive construction of metro people living or working in over-track buildings in terms of feelable systems in China. vibration and noise [20,7]. It can also adversely affect the operation of The primary objective of this research is to assess the impact of vibration sensitive equipment such as those found in medical labora- train-induced vibration and noise on occupants in over-track buildings tories and hi-tech manufacturing. Additionally, side effects due to ex- at Qianhai metro depot in Shenzhen, China. A 4-story steel-framed cessive vibration and noise exposure include the price depreciation of building with a transfer structure and a 28-story load-bearing wall over-track real estate property. As a result, it is important to under- supported building were chosen for the measurements, which are stand, predict and assess the impact of train-induced vibration on oc- compared with both US and Chinese standards for feelable vibration cupants of the over-track buildings to develop mitigation measures if and noise. Insights into how vibration transmits from the ground into needed. the different building constructions were also developed. Past studies have focused on the characterization, assessment and prediction of vibration in the nearby environment and buildings due to 2. Description of Qianhai metro depot high-speed railway trains [11,8,14,4,32], high-speed Maglev [31,16], underground subway trains [17,18], and elevated railway trains Shenzhen has developed to one of the biggest modern cities in China [12,15], etc. with a population of about 12 million people and serves as the head- Prediction of train-induced vibration propagation through build- quarters to many high-tech companies. Its subway system is currently ings, have included empirical formulae [28], numerical modeling [6] extensive and growing. Plans are for the urban railway system in and theoretical analysis [35]. All of these prediction models need to be Shenzhen to grow to include 16 lines from the current 11 lines with a verified by field measured data. There are also a number of field total length of 596.9 km (371 miles) by the year of 2020 [21]. measurements carried out on train-induced vibration [26,13,3]. Xia Qianhai metro depot of Shenzhen Metro is where subway trains are et al. [30] conducted an experimental study of train induced vibration parked, prepared, repaired, and tested with a total land use area of on a low-rise masonry building near the Beijing-Guangzhou railway about 489,700 m2, a total over-track building area of 350,000 m2 and a track. The dependence of vibration levels on train speed, type, and at total overall floorage of 1,410,000 m2. different floor elevations and locations within the building were mea- The over-track buildings in the Qianhai metro depot (Fig. 1) include sured. Sanayei et al. [22,23] measured vibration levels induced by more than 25 high-rise buildings of 18–35 floors in which people live surface trains and underground subway trains respectively, both in and over 30 low-rise buildings of 4–6 floors where people work. It is open fields and on the foundation slabs inside nearby buildings, with estimated the community hosts more than 16,000 residents. the intent of quantifying the difference between ground vibration levels The elevation view of Qianhai metro depot with typical over-track in an open field and in a building foundation at the same distance from buildings is shown in Fig. 2. The train tracks are on the ground level, the tracks. The measurement of ground vibration before construction of the platform level (9 m above the ground) is a mixed-function zone with a building can then be adjusted to account for the effect of the foun- restaurants and stores where people can gather, socialize or shop and it dation on vibration levels in the foundation after construction. Train- also acts as the car garage level for over-track buildings, and the plaza induced vibrations in a full-scale four-story building were also mea- level (16 m above the ground) is used as a green space for over-track sured for comparison with predictions of a modeling procedure being buildings. The platform covers the whole metro depot with width of developed. 387 m and length of 1521 m. Recent studies have measured the impact of vibration due to surface Fig. 3 shows the plan view of tracks within Qianhai metro depot. and subway trains on the surrounding environment and over-track Throat area is a sector area with curve tracks and switches, linking the buildings of metro depots. Xie et al. [34] evaluated vibration service- entrance/exit lines and the parking or maintenance area, where tracks ability of an 11-story over-track building with frame structure built over split one into two and trains fan out into parking slips. Although trains the parking area through finite element modeling, finding the service- pass through the throat area at relatively low speeds of between 10 and ability of the over-track building could not meet relative standard re- 20 km/h, the curved lines and switches can cause undesirable impacts quirements and two vibration mitigation measures were proposed. Cao resulting in higher vibration and noise in the over-track buildings. The et al., [1] conducted field measurements in a 7-story over-track building test line is a straight track along the edge of the metro depot which is with concrete frame structure, which was built over an elevated metro used for testing performance of trains where they are operated over a depot. The trains were running on the 2nd or 3rd floor of the elevated range of speeds from 20 to 80 km/h. metro depot at speeds below 5 km/h. Guo et al. [9,10] proposed and verified numerical and theoretical models based on Cao's research. 3. Description of over-track buildings Our research team have also conducted comprehensive researches related with vibration and noise problems [25] of modern metro depot There are typically two types of supporting method of over-track with over-track platform structure since 2015. According to functional buildings. The structures of buildings in the throat areas of metro de- partition of a metro depot, the throat area and test line are usually pots are generally supported by irregularly spaced ground columns that considered to be the most serious vibration and noise sources [37]. Zou are located between the densely spaced tracks. The ground columns et al. [36] measured train-induced vibration transmission in two over- that support the platform cannot be conveniently aligned with the

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Fig. 1. Over-track community in the Qianhai metro depot. building columns that are arranged on a regular and rectangular grid. 1.2–1.4 m) through transfer girders (Fig. 2a). Generally, under the Thus, it is necessary to design transfer girders that span between the ground a rectangular cushion cap and several piles of diameter of 1 m ground columns to support the building columns at locations along are connected with each concrete platform column base. A low-rise 4- their spans. This is a key reason why only low-rise over-track buildings story building (Building #36, Fig. 4) was chosen as a typical office are built over throat areas on top of platforms with transfer girders, building for carrying out measurements because it is located just over which would not be not feasible to support a heavy high-rise over-track the switches and curve track segments which are typical of the throat building. High-rise buildings, located in areas with less dense tracks, are area track. supported by deep foundation and load bearing wall segments ex- The high-rise over-track buildings are reinforced concrete load- tending from the ground contiguously through the platform all the way bearing walls supported structures for residential use. Among all the up into the building. high-rise over-track buildings, a 28-story high-rise building (Building In Qianhai metro depot, the low-rise over-track buildings are steel- #20, Fig. 5) was selected as the typical residential building for the framed structures with reinforced concrete floor slabs for office use. measurements because it is located between the throat area tracks and They are constructed just above the throat area and supported by the intermediate section of the test line track with the highest train concrete platform columns of circular cross section (diameter of speed. The instrumented load-bearing walls include two cross-sections,

(a) Low-rise office building (b) High-rise residential building

Fig. 2. Elevation sketches of typical over-track buildings in the Qianhai metro depot.

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Fig. 3. Plan view of the Qianhai metro depot.

(a) Elevation view (b) Supporting concrete columns of platform

Fig. 4. Typical low-rise office building. one with a rectangular cross-section (S wall) and the other with an L- platform columns G1-G3 are shown in Fig. 7. These three columns are shaped cross-section (L wall). The former is designated as Straight/S under or near the footprint of the test room of the low-rise building. wall shown in Fig. 6a and the latter is designated as L-shape/L wall in Train induced vibration propagates across the ground into the bases of Fig. 6b. Deep foundations are used under the load-bearing walls of the columns G1-G3, and up through the columns into the low-rise building high-rise building. through the transfer structure and building columns. A question in mounting accelerometers on the ground adjacent to building columns 4. Measurement program or walls is whether the measured vibration is the same as the axial vibration within the column/wall, or whether the ground motion is The test train in this research is A-type metro train with 6 cars. Each potentially partially coupled with the column/wall in some frequency car has a length of 22 m and a weight of 48,800 kg. The test line track is ranges depending on soil properties and foundation type/dimensions. composed of 60 kg/m seamless rail, a concrete sleeper, with two-layer This issue is of interest to the authors for future investigation. ballast and resilient non-separated rail fasteners. The throat area track Measurement locations on the 2nd floor room of the low-rise consists of 50 kg/m seam rail, a wood sleeper, single-layer ballast and building are shown in Fig. 7 and include floor vibration levels at dif- resilient separated rail fasteners. ferent locations including on the balcony, adjacent to the room columns B1-B4, and four locations in the middle of the room floor. Vibration 4.1. Test plan measurements were also made on the open floor, adjacent to room column B3 and staircase column S1 on different floors. For the low-rise building, vibration measurement locations are Several sets of measurements were made with the wireless units that given in Table 1, and include ground locations adjacent to the concrete contained two accelerometers, one measuring vibration in the vertical platform columns, locations adjacent to steel columns within the direction and the second one in horizontal direction (set perpendicular buildings at different floors, locations on the open floors at different to the track). Vibration measurements on open flooring were made only elevations and microphone locations for measuring noise levels in a in the vertical direction for bending deformation of the floor. room on the 2nd floor. Unlike the low-rise building with the transfer structure the S wall Measurements of vibration at track level adjacent to three concrete and L wall segments between floors are aligned from ground at the

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second for horizontal direction), and a JM3873 Data Acquisition System (Fig. 9d). Each wireless unit has a time synchronizer with ac- curacy of 1 ms and a gigabyte storage card. The wireless units also have an input connector for a third external accelerometer signal that is sampled at the same sampling rate as the other two accelerometers and which is also stored within the unit. Prior to the field measurements, all wireless units were time-synchronized side-by-side using a laptop with the DAQ software through a wireless gateway. The wireless units were then left running to maintain the synchronization among units and mounted in place at their measurement locations. Potential concerns with interference/blockage issue were avoided during testing as the internally sampled/stored time signals were not transmitted during the measurements. The one gigabyte storage memory was sufficient to store the data at the 512 Hz sampling frequency for up to about 12 h. After completion of the measurements the time-synchronized data was downloaded from each wireless unit. The sampling frequency was set to 512 Hz for each wireless unit which provides spectral information to the Nyquist frequency at 256 Hz. The dominant frequency range for train-induced floor vibration in the two buildings in this study was below 80 Hz. The Chinese code used in this research for train-induced vibration and secondary-radiated noise assessment stipulates a frequency range up to 200 Hz, thus the 512 Hz sampling rate chosen was adequate for these measurements. For the 4-channel RION digital data recorders the sampling frequency was set to 12,800 Hz to provide spectra of the microphone noise Fig. 5. Elevation view of typical high-rise residential building. signals in the 2nd floor room to 5 kHz. The low rise building in which the noise measurements were made was unfurnished, unoccupied, and track level throughout the full height of the building. Measurement very quiet. Above 80–100 Hz, ambient noise levels in between train locations of the high-rise building are listed in Table 2 and shown in pass-bys were comparable to the train-induced noise levels. Below Fig. 8. Wireless units set near the wall base collect both vertical and 80–100 Hz the train-induced noise levels exceeded ambient levels by up horizontal vibration levels. An accelerometer was epoxied to the wall as to 10 dB for a particularly robust pass-by (see Section 5.4). shown in Fig. 8 in order to measure out-of-plane bending vibration in Digitized acceleration time signals were downloaded from both the the wall. Vibration measurements were made only near the load- wireless units and the Rion data recorder and read by a Matlab script to bearing walls outside the residences because the building was occupied compute frequency spectra. The Matlab script computes average power and there was no access to measure floor vibration within the re- spectra over the relatively steady period of the acceleration time signals sidences. A total of 13 train pass-bys on track 1 in the throat area and on during train pass-bys. Narrowband spectra in 1 Hz bands were com- the test line track were measured for the high-rise building. For pass- puted and 1/3 band levels were synthesized from the narrowband bys on the test line track with train speeds of 20 km/h and 60 km/h spectra. All spectra are shown in 1/3 octave bands. were measured.

5. Measurements in a low-rise building supported by columns 4.2. Instrumentations and signal processing through a transfer structure

The instrumentation used in testing is shown in Table 3. It consists The next sections present and discuss results for the vibration of 941B ultra-low frequency accelerometers (Fig. 9a), microphones measurements in the low-rise building. All vibration measurements in (Fig. 9b), 4-channel digital data recorders (Fig. 9c), wireless units with following sections refer to the vertical direction, unless otherwise 2 built-in magnetoelectric accelerometers (one for vertical direction, a stated.

(a) S wall (b) L wall

Fig. 6. Typical load-bearing walls in the high-rise building.

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Table 1 Measurement locations of the low-rise building.

Setup Locations Sensor Locations Measurements Total pass-bys

Track-level 3 On ground adjacent to concrete platform columns G1-G3 Vertical and horizontal acceleration levels 29 Test room 9 Different locations in test room of 2nd floor Vertical acceleration levels 11 Test room 2 Noise levels 11 Staircase 5 Adjacent to staircase column S1 within the building from platform level to 4th floor Vertical and horizontal acceleration levels 11 Room column 6 Adjacent to column B3 from platform level to roof 18 Open floor 4 On room floor from plaza level to 4th floor 18

Fig. 7. Vibration and noise measurement locations at track level and within the low-rise building.

Table 2 Measurement locations in the high-rise building.

Setup Locations Sensor Locations Measurements

S wall 3 Adjacent to S wall (ground, platform, 2nd ﬂoor) Horizontal, vertical and out-of-plane vibration levels L wall 8 Adjacent to L wall at diﬀerent elevations Horizontal and vertical vibration levels

5.1. Train-induced time signals and spectra the train changes direction through the switch from Track 1 onto Track 1-1 as shown in Fig. 7. Among the 29 train pass-bys measured in the low-rise building, one, The spikes are presumably produced as the train passes through the pass-by #4, had distinct, large amplitude, short duration spikes in the switch while changing direction (pass-by #4) as opposed to passing measured acceleration time history at ground column G1, as shown in straight through (pass-by #22). For the train to change direction at a Fig. 10. By comparison, measured time signals at ground column G3 switch the lead truck of car must first rotate in order to change the and at the base of columns B3 and B4, and on the open floor in the 2nd direction of the car. The rotation occurs as the rim of the lead outer floor room were of lesser amplitude with a more varied array of spikes. wheel passes onto the transition rail in the switch. The inner wheels As seen in Fig. 7 ground location G1 is near a switch on track 1, pass smoothly around the inner rail. An impact likely occurs between which is the track that pass-by #4 was on. It is suspected that the spikes the lead outer wheel and the transition rail that is transmitted into the are generated as each truck set passes through the switch. For a six-car ground through the ballast and resilient rail fasteners. Ground-borne train there are 12 trucks with 12 spikes at G1 as shown in Fig. 10a. transmission likely is in the form of Rayleigh surface waves which ex- However, individual spikes aren't as obvious in the measurements on hibit orbital motions of the ground in horizontal and vertical directions the 2nd floor (Fig. 10b). Each spike generated at the switch travels which are comparable on the surface of the ground. across the ground to all columns that transmit vibration into the Fig. 12 compares vertical vibration spectra for pass-by 4 and pass-by building, through the transfer structure to the 2nd floor building col- 22. Spectral levels at ground column G1 during pass-by 4 are 5–19 dB umns and floor. Vibration at 2nd floor locations includes contributions higher than for pass-by 22 over the wide frequency band from 10 to from multiple paths that arrive at different times. 200 Hz. Both spectra peak in the 63 Hz in 1/3 octave frequency band. Fig. 11 compares acceleration time signals in both vertical and horizontal directions at ground column G1 during train pass-by 4 with 5.2. Vibration levels for different pass-bys through the throat area in the that during train pass-by 22. These two pass-bys were on the same track low-rise building (Track 1), for same type of metro train (A-type) at similar speeds where the pass-by 4 has much higher spikes. However, for pass-by 22, the train The next four sections show comparisons of overall vibration levels passed straight through the switch onto Track 1-2, while for pass-by 4, and 1/3 octave band spectra for trains on different tracks. The overall

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Fig. 8. Experimental measurements in the high-rise building.

Table 3 Instrumentation.

No. Type Model Information

1 Vibration sensors 941B ultra-low frequency sensor 1. Select velocity or acceleration time signals for output 2 Wireless magnetoelectric accelerometer 1. Vertical and horizontal measurement directions 2. Select velocity or acceleration time signals for output 3 Data recorders Rion DA-20, 4-channel digital data recorder 1. Capable of recording acoustic and vibration time signals 2. Recorded signals saved in waveﬁle format on CF cards 4 JM3873 Data Acquisition System (including the 1. Wireless unit and data acquisition system exchange information through wireless wireless unit) communication 2. Vibration time signals are stored in the wireless units for subsequent download to the data acquisition system 5 Microphones 1. B&K 4189 ½ '' Type I mic cartridge 2. PCB 426EO1 Preamp 3. PCB 480E09 signal conditioner, ICP power supply

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Fig. 9. Sensors and data acquisition system. levels are computed by summing the spectral energies in 1/3 octave location is to the track which the pass-by was on, the higher will be the bands. overall vibration level. Overall vibration levels at both G1 and G3 are lower for pass-bys on Track 3 due to the greater separation between it 5.2.1. Overall vibration level comparison of ground level measurements and Tracks 1 and 2 as shown in Fig. 7. There are three main throat area tracks (Tracks 1–3) under the low- rise building. Track 1 is adjacent to columns G1 and Track 2 is next to 5.2.2. Vertical vibration transmission along the height of the low-rise G3, both in-between G1 and G3. Track 3 is beyond column G3. Overall building vibration levels adjacent to columns G1 and G3 indicate which track the Figs. 14 and 15 show overall vibration level comparisons for pass- train was running on for each pass-by. The overall vibration levels at bys on different tracks and at different locations within the low-rise ground locations adjacent to columns G1 and G3 for Tracks 1, 2, or 3 building. Of note is that differences in overall levels are not particularly are shown in Fig. 13(a)–(c), respectively. The closer the measured great regardless of which track the pass-bys were on, including the

Fig. 10. Vertical acceleration time signals during pass-by 4.

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Fig. 11. Time history comparison between pass-by 4 and pass-by 22 at G1.

Tracks 1 or 2 for pass-bys on either of these tracks. As seen in Fig. 14(a) and (b), overall vibration levels at staircase column S1 were higher than at room column B3 for elevations above the platform and plaza. It is likely that the heavier column/wall structures near the staircase and elevator transmit vibration across the plaza floor to and through the upper floors more effectively than oc- curred through the lighter building columns, such as B3. The transmission through column B3 to upper floors was more effectively atte- nuated by the intervening floors. Overall vibration levels in open areas on the 2nd room floor, shown in Fig. 15, are generally higher than that at the 2nd floor building columns which transmit vibration to the floor. This is attributed to amplification due to resonances within the floor, which is discussed further on in the paper. Fig. 16 shows vibration transmission along staircase column S1 and room column B3 of the low-rise building during pass-by 4. In the frequency range of 10–25 Hz, the acceleration levels in the building measured at different floors are comparable to levels at the ground columns. Above 25 Hz the acceleration levels in the building drop off Fig. 12. Vertical vibration spectra for pass-by 4 and 22 at column G1. rapidly compared to those at the ground. It is likely that this is related to the reduction in transmission through the transfer structure at the more distant Track 3. Levels within the building, as opposed to on the platform level to the floors above the platform. ground, are due to contributions from all ground columns that support FTA guidelines for floor to floor changes in vibration level are about the platform and all transmit vibration into the transfer structure under 2 dB per floor for lower floors. As shown in Figs. 15 and 16, overall the low-rise building. For instance, pass-bys on Track 3 would transmit vibration levels of the low-rise building at columns and on the floor in higher vibration levels into ground columns near Track 3, which while the primary bands from 20 to 50 Hz drop off at a lesser rate than sug- not measured, would likely transmit equally well into the platform and gested by the FTA guidelines. As described in a subsequent section, this transfer structure and the low-rise building as from columns near was also the case for the high-rise building showing an even smaller

Fig. 13. Overall vibration levels at G1 & G3.

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Fig. 14. Overall vibration levels of columns S1 and B3.

Fig. 15. Overall vibration levels at open ﬂoor locations. Fig. 17. Spectra of vertical vibration at G1 and G3 during pass-bys on diﬀerent tracks.

Fig. 16. Column vibration levels during pass-by 4 at diﬀerent ﬂoors in the low-rise building.

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Fig. 18. Frequency spectra comparison of measurements at column G1 and B3 for different pass-bys. drop off rate from the platform level to the higher floors relative to estimates based on the FTA guidelines.

5.2.3. Spectra of ground measurements at columns G1 & G3 for pass-bys on different tracks Fig. 17 shows the differences of frequency spectra at different ground columns and tracks. Pass-bys 29, 27 and 15 were selected as being typical pass-bys on tracks 1, 2 and 3, respectively. The distances of G1 and G3 to the track center line are 4.2 m and 8.8 m, while for Track 2, the distances to G1 and G3 to the track center line are 9.2 m and 3.8 m, respectively. Vibration levels at G1 and G3 at frequencies below 40 Hz were comparable while above 40 Hz levels 8–21 dB dis- crepancies show up. It is inferred that along the direction perpendicular to the track line in a distance of 4–9 m, vertical vibration components above 40 Hz reduce significantly, while low-frequency vibrations barely reduce in this range.

5.2.4. Frequency spectra comparison between ground measurements at G1 and building measurements at B3 Fig. 19. Column to floor transfer function on the 2nd floor of the low-rise Fig. 18 compares frequency spectra between ground and building building. measurements. For pass-bys on each track, the spectra are close to each other with some variation due to differences in the trains, speeds, and direction of travel. It was also shown that track location is a more sensitive factor for vibration levels at ground in comparison to that within the building.

5.3. Transfer functions from column to ﬂoor of the test room on the 2nd room ﬂoor

Fig. 19 shows transfer functions from average vibration levels at four room columns to average vibration levels at two open floor locations for individual pass-bys. The floor measurement locations were near the middle of the floor. Vibration levels for each pass-by at the two open floor locations were of comparable amplitudes, as was the case for the four columns (B1-B4). Peaks levels on the floor are significantly higher than at the columns. The amplification at the peaks is due to low order floor resonances whose mode shapes have high response amplitudes near the middle of the floor. The pronounced dips suggest that the locations in the middle of the floor may be at nodes or low modal response locations on the mode shapes of other resonances in the fre- Fig. 20. Relationship between room noise level and floor velocity level in the quency region of the dips. A more representative transfer function in- 2nd floor room. cluding resonances that have high modal response at locations away from the middle of the floor may vary smoothly between the peaks shown.

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5.4. Noise levels induced by floor vibration velocity 6.2. Vertical vibration levels at different floors in the high-rise building

Fig. 20 shows the average noise level between two microphone lo- Fig. 22 shows vertical vibration levels adjacent to the L wall at cations in the 2nd floor room during the robust pass-by # 4 which had different elevations in the high-rise building. Vibration levels at dif- the greatest potential to excite noise levels relative to ambient levels. ferent floors extending to near the top on the 22nd floor of the high-rise Vibration induced radiated noise is proportional to the average vibra- building were comparable to levels at the ground up to 80–100 Hz, tion velocity of the floor. The noise levels within a room depend on the above which the levels in the building drop off compared to those at the vibration induced radiated sound power as well as the noise absorption ground. This is in contrast to the results for the low-rise building where within the room, for a given radiated sound power in a room with building levels drop off from ground levels at 25 Hz and above. This is greater absorption, i.e., an acoustically deader space, the resulting noise likely due to the transfer structure in the low-rise building where the S levels would be less. The 2nd floor room during the measurements had and L walls in the high-rise building are aligned continuously from the bare concrete surfaces including the floor, walls and ceiling, and no ground up. furnishings. The proportional relationship then involves what are, in As was observed for the high-rise building the floor to floor reduc- general, frequency dependent factors related to vibration induced noise tion in levels is significantly less than suggested by the FTA guidelines radiation and sound absorption within the room. The intent was to for overall vibration levels which indicate a 1 dB drop per floor above determine if there might be a noise issue relative to Chinese or FTA the 5th floor and on the order of 2 dB per floor at lower elevations. The noise standards, where the measurements indicated that there was not a FTA guidelines suggest a total decrease of nearly 30 dB at the 22nd floor concern. Noise levels with the room when fully outfitted and occupied which is considerably more than measured as seen in Fig. 22. would likely be lower than what we measured. 6.3. Power transmission comparison between longitudinal and bending 6. Measurements in the high-rise building supported by load- waves bearing walls Fig. 23a shows measured vertical vibration adjacent to the wall on The following three sections discuss results for the high-rise the floor and out-of-plane bending vibration on the wall. These are for building. For measurements with the trains on the test line track there different types of motion within the wall so a direct comparison isn't was little variation in time signals and spectra between different pass- meaningful. If the measured levels characterized the amplitudes of bys. The trains were operated at steady speeds of 20 and 60 km/h in waves within the wall in one direction, a meaningful comparison would both directions, with no variation for travel in opposite directions. be between the power transmission for longitudinal and bending waves Overall levels at 20 km/h were about 10 dB less than at 60 km/h. based on the measured vibration levels and the appropriate elastic Therefore, only the higher speed data are reported. Data was also re- parameters for each motion type. The actual vibration measurements corded for all measurement locations during train pass-bys on Track 1 involve waves propagating in both upward and downward directions in the throat area. for each type, with different amplitudes. Shown in Fig. 23b, the power transmission by vertical (axial) vi- 6.1. Vertical vibration levels adjacent to the L and S walls bration is much higher than that carried by out-of-plane bending vibration. This crudely suggests that axial wave transmission through As seen from comparisons between levels in Fig. 21a and b, vertical supporting wall segments is the more important mode of transmission vibration levels at the ground adjacent to the S and L walls were within the building. comparable. The distance from the Track 1 in the throat area and from the test line track to the base of the walls is 19.3 and 28.1 m respec- 7. Vibration transmission reduction due to the transfer girders/ tively (Fig. 8). For train pass-bys on Track 1 in the throat area, the floor measured vibration levels peak at lower frequencies in the 31.5–50 Hz bands compared with peak levels on the test line in higher frequency Fig. 24 shows comparison of transfer functions from ground to bands above 63 Hz. The test line has seamless or welded rails without platform of the low- and high-rise building induced by throat area pass- any switches, so there are no spikes related to impacts within switches. bys (Track 1 and Track 2) and test line pass-bys with speed of 60 km/h,

Fig. 21. Vertical vibration levels at the ground for trains on the test line and Track 1 in the throat area.

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Fig. 22. Vertical vibration at diﬀerent elevations in the high-rise building.

Fig. 23. Power transmission comparison between longitudinal and bending waves in the S wall.

the ground is comparable to that at the platform level for frequencies below 31.5 Hz. However, vibration levels at frequencies above 40 Hz drop off significantly. For the high-rise building, vibration reduction from ground to the platform drops off for higher frequencies above 63 Hz. The transfer floor effectively reduces vibration levels at higher frequencies within the low-rise building above the platform relative to transmission in the high-rise building. This is due the fact that for the low-rise building (1) there is no direct contiguous vertical path from the ground columns into the building columns for axial wave transmission, and (2) the transmission reduction due to the conversion of axial waves in ground columns into bending waves in transfer floor and also the conversion to axial waves in building columns from bending in the transfer girders/floor.

8. Vibration and noise impact assessment

The US standard [7] and Chinese standard (JGJ/T 170-2009) are chosen to assess the impact of train-induced vibration and noise levels Fig. 24. Transfer function comparison between the low-rise and high-rise on human comfort of occupants in the low- and high-rise buildings. The building. US code uses an overall rms velocity level while the Chinese code uses frequency-weighted maximum acceleration level (VLmax) in 1/3 octave respectively. The low-rise building is supported by the transfer floor at bands. The US standard has different limits corresponding to different the platform level, unlike the high-rise building that is supported di- frequency of events per day. Based on the frequency of train operation rectly at the ground level. For the low-rise building, vibration level at events in Qianhai metro depot, the velocity level limit of 75 dB for

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Fig. 25. Vibration assessment for the low-rise building during pass-by 4.

30–70 events per day is appropriate. The Chinese standard sets different day-time and night-time limits based on the different functions of the building. For vibration and noise sensitive buildings such as research institutes, hospitals, residential buildings, etc., the limit is more stringent. Considering that the over-track buildings are multi-functional, including office buildings, residential buildings and commercial buildings, etc., a day-time acceleration level limit of 70 dB for the low-rise office building and a night-time acceleration level limit of 67 dB for the high-rise residential building are chosen for assessment. For residences both the US and Chinese standards use an A- weighted noise limit of 35 dBA.

8.1. Vibration and noise impact assessment for the low-rise building

A-weighted noise levels in the 2nd floor room during the high amplitude, pass-by 4 measured at Loc A and Loc B were 34.8 dB (A) and 34.0 dB (A), respectively and satisfy limits of both the US and Chinese standards. Less robust pass-by events would exhibit lower noise levels and the levels would also be somewhat lower if the room had been fully Fig. 26. Low-rise floor vibration levels and 95% confidence upper bound for all outfitted with carpets, acoustical ceiling tiles, and furniture. pass-bys. Fig. 25 shows measured 1/3 octave band spectra of floor and column vibration velocity levels for comparison with the FTA limit of floor locations. It would not be appropriate to apply the transfer func- 75 dB overall for occasional pass-by events and vibration acceleration tion from column vibration to open floor vibration for the low-rise levels for comparison with the day-time Chinese limit of 70 dB, both building because there are significant differences in the building sup- during the most unfavorable pass-by 4 with high amplitude. Although port structures between the two buildings, i.e. load bearing concrete fl the overall oor velocity levels are within the limit of the FTA limit, the walls in the high-rise building and steel columns in the low-rise fl oor acceleration levels exceed the Chinese limit by around 10 dB. The buildings, and with different spacings. lower column levels without amplification due to floor resonance are comfortably below both limits. 9. Conclusions The standard deviation of overall open floor vibration levels within the low-rise building for all measured 11 pass-bys was about 2.91 dB. This paper presents train-induced vibration and noise levels in over- fl Fig. 26 shows the average oor acceleration level along with levels track buildings at Qianhai metro depot in Shenzhen, China. The mea- at ± two standard deviations about the average level in 1/3 octave sured levels were compared with Chinese and US FTA standards for bands. Two standard deviations about the average statistically includes potential impact on the buildings’ occupants in terms of annoyance in about 95% of pass-bys which has a maximum acceleration level of terms of feelable vertical floor vibration and radiated noise. Response to – fl 67 78 dB. It is seen from Fig. 26 that the average open oor accelera- wind and earthquake excitation occurs at considerably lower fre- tion level is about 2 dB higher than the limit. quencies than are of concern for train induced vibration. Useful insights have been developed in understanding vibration transmission into and 8.2. Vibration impact assessment for the high-rise building within low-rise over-track buildings with steel-frame columns and high- rise buildings with concrete load-bearing walls. The findings presented fl Vertical oor vibration levels at measured locations adjacent to provide useful vibration level estimates prior to construction in de- walls in the high-rise building were well below the Chinese limit, by signing similar buildings in metro depots in the future. least 12 dB or more (Fig. 27). Without any measurements on the fl ooring within the residences of the high-rise building it is not possible (1) For the Qianhai metro depot in Shenzhen recorded noise levels to estimate whether the FTA or Chinese limits are exceeded at open

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Fig. 27. Acceleration levels at the L/S walls of the high-rise building based on the Chinese limit.

were within both FTA and Chinese limits. In the low-rise building a ground column(s) that contribute to transmission into the floor vibration levels were within the FTA limit but exceeded the building. more stringent limit in the Chinese code. (5) The transfer structure is apparently responsible for significantly (2) For the low-rise buildings in the throat area recorded impacts reducing vibration levels within the low-rise building above the caused by switches in the tracks significantly contributed to the platform. The building columns are supported by the transfer gir- generation of ground-borne vibration that transmits into the ders and are not aligned with the ground columns that support the buildings. It is speculated that higher impacts are generated as platform from the ground. Conversely, in the high-rise building the wheels pass onto the transition rail of the switch when it is posi- load bearing walls are vertically aligned throughout the full height tioned so that the train changes direction through the switch. from the ground to the roof resulting in higher vibration trans- (3) Vibration levels at the ground columns that support the low-rise mission. building due to train pass-bys in the throat area are significantly (6) Vibration attenuation in vibration levels from floor to floor within greater, by more than 20 dB, than levels at the ground adjacent to both buildings are less than FTA guidelines. As a result, the FTA load bearing wall support structures of the high-rise building for guidelines significantly overpredict the vibration transmission loss trains on the test line at high speed (60 km/h). However, levels from the platform to the top of each building compared to the adjacent to the building columns within the low-rise building are measured change in each level. only somewhat greater by about 5 dB than at floors within the high- rise building. (4) While vibration levels at a ground column in the throat area can Acknowledgements vary considerably by more than 10 dB depending on its proximity to the track with the pass-by, levels within the low-rise building above This research was supported by the science and technology project the transfer structure do not vary much for pass-bys on different of Guangdong province, China (No. 2017A050501005) and the tracks. Transmission into the building includes contributions from National Natural Science Foundation of China (No. 51908139). The several ground columns that are equally effective in transmitting authors would like to thank all collaborators helping to conduct the vibration into the building. The tracks in the throat area are all near field measurements and data analysis.

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