Renzo Emili
Stridori e Vibrazioni
Hilton Portland Hotel. Oregon 2003 - Mr. Emili while is presenting at the session "Civil Design" the tramway design and works in Rome, on APTA invitation.
Introduction
In basic terms, an FST (Floating Slab Trackbed) system is comprised of springs and masses, which are designed to isolate vibration coming from wheel or rail interaction, and decrease its transmission to the surrounding track support structure. The amount of isolation necessary depends on the amount of vibration reduction required by the particular circumstances of each situation. The circumstances involve many factors, including:
•Sensitivity of the affected building;
•Speed of the transit vehicles;
•Rail roughness;
•Dynamic interaction between the vehicle’s trucks and the rail system;
•Response of the soil underlying the track;
•Ease of propagation of vibration through the soil between the track and the building;
•Response of the building to ground vibration, and manner in which vibration is transmitted through the building.
The amount of vibration reduction that can be achieved using an FST depends on the dynamic characteristics of the transit vehicle, but is dictated to a large degree by the primary natural frequency of the FST system. The FST can be idealized as a simple spring-mass and damper system as depicted in Fig.1.
In actuality, it is a more complex dynamic system than this, but for determining the basic performance of the FST it often suffices to model it in this manner.
Field tests performed on a full scale FST mock-up demonstrated this, in particular when the FST is under vehicle load.
In basic terms, an FST (Floating Slab Trackbed) system is comprised of springs and masses, which are designed to isolate vibration coming from wheel or rail interaction, and decrease its transmission to the surrounding track support structure. The amount of isolation necessary depends on the amount of vibration reduction required by the particular circumstances of each situation. The circumstances involve many factors, including:
•Sensitivity of the affected building;
•Speed of the transit vehicles;
•Rail roughness;
•Dynamic interaction between the vehicle’s trucks and the rail system;
•Response of the soil underlying the track;
•Ease of propagation of vibration through the soil between the track and the building;
•Response of the building to ground vibration, and manner in which vibration is transmitted through the building.
The amount of vibration reduction that can be achieved using an FST depends on the dynamic characteristics of the transit vehicle, but is dictated to a large degree by the primary natural frequency of the FST system. The FST can be idealized as a simple spring-mass and damper system as depicted in Fig.1.
In actuality, it is a more complex dynamic system than this, but for determining the basic performance of the FST it often suffices to model it in this manner.
Field tests performed on a full scale FST mock-up demonstrated this, in particular when the FST is under vehicle load.
Fig.1: Idealized spring-mass and damper system.
The FST system will have a natural frequency determined by the stiffness of the supporting springs and the amount of mass the springs support. Contrary to an occasionally expressed opinion, the mass of the transit vehicle does not affect the natural frequency of the FST, except by changing the stiffness of the support springs due to its static loading on the FST.
The secondary suspension, located between the truck and vehicle, has a low natural frequency (typically 1 to 2 Hz) effectively decoupling the vehicle from the FST. However, the mass of the truck or some portion of it (e.g., wheelset), depending on the nature of the primary suspension system, will contribute to the dynamic mass of the FST system.
A generic FST system concept is shown in cross section in Fig. 2. The rubber pads rest on either an invert or the bottom of a concrete tub. The concrete slabs (masses) are placed on top of the rubber pads. To restrain lateral and longitudinal motion of the concrete slabs in the horizontal plane, discrete rubber pads or continuous rubber strips are used around the edges and in-between adjacent slabs.
The secondary suspension, located between the truck and vehicle, has a low natural frequency (typically 1 to 2 Hz) effectively decoupling the vehicle from the FST. However, the mass of the truck or some portion of it (e.g., wheelset), depending on the nature of the primary suspension system, will contribute to the dynamic mass of the FST system.
A generic FST system concept is shown in cross section in Fig. 2. The rubber pads rest on either an invert or the bottom of a concrete tub. The concrete slabs (masses) are placed on top of the rubber pads. To restrain lateral and longitudinal motion of the concrete slabs in the horizontal plane, discrete rubber pads or continuous rubber strips are used around the edges and in-between adjacent slabs.
Fig.2
Application of Floating Slab Trackbed systems in Rome's historic center
The city of Rome is world renowned for its massive presence of ancient monuments, churches, and historical buildings, mostly dating from the Roman period to the Baroque Age. The noble marbles and the facades of those monuments have been severely damaged by air pollutants, mainly emitted by private and public vehicles.
To face this problem, ATAC (Agenzia per il Trasporto Autoferrotranviario del Comune di Roma), the public company which manages public transportation in Rome and in its region, began a long-term project in 1993 called “Zero Pollution Public Transportation in the Center of Rome,” which aims to convert all fossil fuel operated public transportation into electric transportation through the introduction of battery operated small buses, trolley buses, and, above all, light rail transit (LRT) systems. To accomplish this, new problems had to be faced and solved.
LRT systems crossing the historical center must have minimal environmental impact; that is, unobtrusive overhead wire systems and sites for substations are necessary, as are, above all, vibration reducing track structures, since vibrations can severely damage ancient buildings. This paper will deal about the experience in Rome since 1994 with designing and testing different techniques for vibration reducing track structures.
First: the design of the two main vibration reducing track structure systems that were produced and tested will be detailed, focusing on the differences between them.
Second: results of the measurements carried out at two sites in the center of Rome, before and after the installation of the vibration reducing track structures (“before works” and “after works”), will be presented.
Finally: a comparison table showing the vibration reduction and the cost of each system will be presented, the solutions adopted on the recently constructed tramway line n°8 in Rome will be shown.
To face this problem, ATAC (Agenzia per il Trasporto Autoferrotranviario del Comune di Roma), the public company which manages public transportation in Rome and in its region, began a long-term project in 1993 called “Zero Pollution Public Transportation in the Center of Rome,” which aims to convert all fossil fuel operated public transportation into electric transportation through the introduction of battery operated small buses, trolley buses, and, above all, light rail transit (LRT) systems. To accomplish this, new problems had to be faced and solved.
LRT systems crossing the historical center must have minimal environmental impact; that is, unobtrusive overhead wire systems and sites for substations are necessary, as are, above all, vibration reducing track structures, since vibrations can severely damage ancient buildings. This paper will deal about the experience in Rome since 1994 with designing and testing different techniques for vibration reducing track structures.
First: the design of the two main vibration reducing track structure systems that were produced and tested will be detailed, focusing on the differences between them.
Second: results of the measurements carried out at two sites in the center of Rome, before and after the installation of the vibration reducing track structures (“before works” and “after works”), will be presented.
Finally: a comparison table showing the vibration reduction and the cost of each system will be presented, the solutions adopted on the recently constructed tramway line n°8 in Rome will be shown.
The vibration problem
While in motion, light rail transit (LRT) vehicles dynamically impact the numerous track structure components, thereby generating unwanted vibrations, which propagate through the ground and reach the foundations of the buildings close to the LRT line (1).
From the foundations, vibrations extend to all the structural components of the buildings, and may also create objectionable noise levels in the apartments where people live. Typically, there are two types of vibrations: vertical vibrations and transverse vibrations.
Technical standards in force
For vibration limits, one must respect Technical Rule UNI 9614 (Vibration measurement in buildings and annoyance evaluation) in Italy, which is substantially in compliance with other international rules (such as ISO 2631, DIN 4150/2, and BS 6472).
For damaging effects caused to the buildings by vibrations in Italy, one must respect Technical Rule UNI 9916 (criteria for the measurements of vibrations and the assessment of their effects on buildings), which is also substantially in compliance with international rules ISO 4866, DIN 4150/3, and BS 6472.
The vibration thresholds to limit the disturbance to people inside their homes are
•Daytime-10.0 mm/s2 (80 dB) for vertical acceleration; 7.2 mm/s2 (77 dB) for transverse acceleration;
•Nighttime-7.0 mm/s2 (77 dB) for vertical acceleration; 5.0 mm/s2 (74 dB) for transverse acceleration.
Floating Platform System
The floating platform system is basically made up of several layers.
The technical drawing of the floating platform system is shown in Figure 3, where the several layers are shown:
1.Stabilized base, about 5 to 10 cm (2 to 4 in.) thick;
2.Reinforced concrete platform, about 20 cm (8 in.) thick;
3.Anti-vibration mat (neoprene), about 2.5 cm (1 in.) thick, installed below the precast concrete platform;
4.Precast concrete platform, about 600 cm (236 in.) long, 230 cm (90 in.) wide, and 25 cm (10 in.) thick;
5.Side/central precast concrete slabs, which are joined to the platform by means of
bolts;
6.Block pavement or asphalt;
7.Specifically mixed rubber sections inserted along the rails, whose main functions are to reduce transverse vibrations and to take into account slight movements of the track structure.
The rails are joined to the precast concrete platforms by means of elastic fasteners. Besides the vibration abatement, the main feature of this system is that the concrete platforms and the concrete slabs are precast, and then they are sent to the construction site where they are simply installed, dramatically reducing construction time. Maintenance operations on tracks are also made easier: in the case of rail substitution, basically all the operators need to do is loosen the nuts and bolts that keep the precast concrete slabs in their place, raise the slabs up by a crane, substitute the rails and then put everything back in its place.
SITE 1- Regina Margherita Avenue in Rome
Regina Margherita Avenue represents one of the major avenues radiating from downtown, and has been a tramway route since 1930.
From 1994 to 1995, large refurbishment works were carried out, with the substitution of the floating platform system for the traditional track structure system.
An aerial view of the site during the construction is shown in Figure 4, while the final result of the works is shown in Figure 5, with vehicles operating revenue service.
By means of accelerometers, vertical and transverse vibration levels were measured while the tramway vehicles were passing by, before and after the works. The measurement conditions were the same, before and after the works: same vehicles, same acceleration and velocity. Also, the “before works” rails did not show any particular signs of wear or roughness on the running surface.
The measurement points were all at ground level, at three different positions along the line. At each position, three measurements were made, 7, 10, and 13 m (7.7, 11, and 14 yd) away from the tracks (nine measurement points in all). Since measurements were made before and after the works, a total of 18 measurements were taken. The building are 13 m (14yd) away from the tracks, so the 13 m measurement gives a very good estimation of the vibration level inside the buildings.
The results of the measurement campaign are shown in Figure 6. In the Y-axis the level L5 is shown ( which is the conventional level not exceeded by the modulus of the vertical and transverse vibration for no more than 5% of the measuring time).
The results show excellent vibration reduction. The accelerometer closest to the buildings shows an average value for the conventional level L5 of 76.3 dB after works, versus 83.7 dB before works, which is approximately a 60% reduction.
Regina Margherita Avenue represents one of the major avenues radiating from downtown, and has been a tramway route since 1930.
From 1994 to 1995, large refurbishment works were carried out, with the substitution of the floating platform system for the traditional track structure system.
An aerial view of the site during the construction is shown in Figure 4, while the final result of the works is shown in Figure 5, with vehicles operating revenue service.
By means of accelerometers, vertical and transverse vibration levels were measured while the tramway vehicles were passing by, before and after the works. The measurement conditions were the same, before and after the works: same vehicles, same acceleration and velocity. Also, the “before works” rails did not show any particular signs of wear or roughness on the running surface.
The measurement points were all at ground level, at three different positions along the line. At each position, three measurements were made, 7, 10, and 13 m (7.7, 11, and 14 yd) away from the tracks (nine measurement points in all). Since measurements were made before and after the works, a total of 18 measurements were taken. The building are 13 m (14yd) away from the tracks, so the 13 m measurement gives a very good estimation of the vibration level inside the buildings.
The results of the measurement campaign are shown in Figure 6. In the Y-axis the level L5 is shown ( which is the conventional level not exceeded by the modulus of the vertical and transverse vibration for no more than 5% of the measuring time).
The results show excellent vibration reduction. The accelerometer closest to the buildings shows an average value for the conventional level L5 of 76.3 dB after works, versus 83.7 dB before works, which is approximately a 60% reduction.
Fig.3-floating platform system.
Fig.4-Aerial view of the construction site (last phase) on Regina Margherita Avenue.
Fig.5- final result of the works on Regina Margherita Avenue.
Fig.6-Vibration measurement results in Regina Margherita Avenue.
Floating Mass System
The floating mass system is basically made up of several layers of concrete, precast concrete sleepers, and an anti-vibration mat (neoprene).
The main difference, with respect to the floating platform system, is that the floating mass system is not precast; instead, its construction coincides with the laying of the tracks.
The technical drawing of the floating mass system is shown in Figure 7, where the several layers are shown:
1.Stabilized base, about 5 to10 cm (2 to 4 in.) thick;
2.Reinforced concrete platform, about 20 to 25 cm (8 to 10 in.) thick;
3.Anti-vibration mat (neoprene), about 2.5 cm (1 in.) thick;
4.First layer of concrete, about 10 to 15 cm (4 to 6 in.) thick;
5.After the first layer of concrete is laid, concrete sleepers, about 27 cm (10.6 in.) long, 230 cm (90 in.) wide, and 16 cm (6.3 in.) thick, are put in place;
6.After laying, leveling, and alignment of the tracks a second layer of concrete is laid, about 16 cm (6.3 in.) thick, up to the top level of the concrete sleepers;
7.Specifically mixed rubber sections inserted along the rails, whose main functions are to reduce transverse vibrations and to take into account slight movements of the track structure; and, finally, block pavement or asphalt is applied.
The noise and vibration abatement performances of the floating mass system are excellent. Also, a floating mass system is highly adaptable for curves and rail intersections. Unfortunately, construction time can be long and difficulties may arise when it comes to leveling and aligning the tracks.
The floating mass system is basically made up of several layers of concrete, precast concrete sleepers, and an anti-vibration mat (neoprene).
The main difference, with respect to the floating platform system, is that the floating mass system is not precast; instead, its construction coincides with the laying of the tracks.
The technical drawing of the floating mass system is shown in Figure 7, where the several layers are shown:
1.Stabilized base, about 5 to10 cm (2 to 4 in.) thick;
2.Reinforced concrete platform, about 20 to 25 cm (8 to 10 in.) thick;
3.Anti-vibration mat (neoprene), about 2.5 cm (1 in.) thick;
4.First layer of concrete, about 10 to 15 cm (4 to 6 in.) thick;
5.After the first layer of concrete is laid, concrete sleepers, about 27 cm (10.6 in.) long, 230 cm (90 in.) wide, and 16 cm (6.3 in.) thick, are put in place;
6.After laying, leveling, and alignment of the tracks a second layer of concrete is laid, about 16 cm (6.3 in.) thick, up to the top level of the concrete sleepers;
7.Specifically mixed rubber sections inserted along the rails, whose main functions are to reduce transverse vibrations and to take into account slight movements of the track structure; and, finally, block pavement or asphalt is applied.
The noise and vibration abatement performances of the floating mass system are excellent. Also, a floating mass system is highly adaptable for curves and rail intersections. Unfortunately, construction time can be long and difficulties may arise when it comes to leveling and aligning the tracks.
Site 2-Vittorio Emanuele Square in Rome
Until last year, one of the most important open-air markets in Rome was situated in Vittorio Emanuele Square, a big square very close to Termini Station, the main railway station in Rome. Now, the market has moved and the square has gone back to its old look, a crowded place with a park in the middle.
The square underwent heavy reconstruction, which included substitution of the floating mass system for the traditional track structure system.
The site during the track substitution is shown in Figure 8 and Figure 9.
The before works and after works measurements have been taken at ground level, during tram traffic, at three different points along the tracks, 1.5 m (1.6yd.) away from the tracks. Two accelerometers were used, one for vertical vibrations and another for transverse vibrations (compare with photo n° 6 of the chapter "Vibration Containment" on this web site).
As for Site 1, the measurement conditions were the same, before and after the works (same vehicles, same acceleration and velocity), and the before-works rails did not show any particular signs of wear or roughness on the running surface.
The results of the measurement campaign are shown in Figure 10. In the Y-axis the average levels for vertical and transverse acceleration are shown, both before works and after works.
Also in this case, the results show excellent vibration reduction: about 12.5 dB for vertical acceleration and 14 dB for transverse acceleration, which is approximately 75%-80% reduction in both cases, in terms of absolute values of the magnitudes in play (In the case of Vittorio Emanuele Square, fig.9, from about 318 mm/sec2 measured before works, to about 76 mm/sec2 measured after works, at the distance of 1,5 m from the more near track (compare with photo n° 7 of the chapter "Vibration Containment" on this web site).
The average frequency spectra of the measured signals are shown in Figure 11. A constant decrease can be seen, throughout the whole spectrum. Moreover, a slight shift towards low frequencies is present, due to the increased mass of the track system.
Until last year, one of the most important open-air markets in Rome was situated in Vittorio Emanuele Square, a big square very close to Termini Station, the main railway station in Rome. Now, the market has moved and the square has gone back to its old look, a crowded place with a park in the middle.
The square underwent heavy reconstruction, which included substitution of the floating mass system for the traditional track structure system.
The site during the track substitution is shown in Figure 8 and Figure 9.
The before works and after works measurements have been taken at ground level, during tram traffic, at three different points along the tracks, 1.5 m (1.6yd.) away from the tracks. Two accelerometers were used, one for vertical vibrations and another for transverse vibrations (compare with photo n° 6 of the chapter "Vibration Containment" on this web site).
As for Site 1, the measurement conditions were the same, before and after the works (same vehicles, same acceleration and velocity), and the before-works rails did not show any particular signs of wear or roughness on the running surface.
The results of the measurement campaign are shown in Figure 10. In the Y-axis the average levels for vertical and transverse acceleration are shown, both before works and after works.
Also in this case, the results show excellent vibration reduction: about 12.5 dB for vertical acceleration and 14 dB for transverse acceleration, which is approximately 75%-80% reduction in both cases, in terms of absolute values of the magnitudes in play (In the case of Vittorio Emanuele Square, fig.9, from about 318 mm/sec2 measured before works, to about 76 mm/sec2 measured after works, at the distance of 1,5 m from the more near track (compare with photo n° 7 of the chapter "Vibration Containment" on this web site).
The average frequency spectra of the measured signals are shown in Figure 11. A constant decrease can be seen, throughout the whole spectrum. Moreover, a slight shift towards low frequencies is present, due to the increased mass of the track system.
Fig.7- Floating Mass System
Fig.8- Works on Vittorio Emanuele Square in Rome
Fig.9- Vittorio Emanuele Square. Floating Mass System under construction. On the left rail, the first layer of concrete has been laid. On the right rail, the second layer of concrete has been laid, up to the top level of the concrete sleepers.
Fig.10-Vibration measurement results in Vittorio Emanuele Square.
Fig.11-Vibration frequency analysis results in Vittorio Emanuele Square.
Conclusions
Comparison of the floating mass system and the floating platform system is summarized in fig.12. Both systems exhibit excellent vibration abatement, as the before works and after works as the measurements presented in the paper have shown.
In fig.12 information is given about the cost of the systems and the cost differential with respect to traditional track structures.
Even if the cost differential is considerable, it should be borne in mind that usually vibration reducing track structures are needed just for relatively short sections of an LRT line, so the incremental cost may be acceptable.
The higher cost of the floating platform system is due to the precast concrete platforms and slabs, but this cost is compensated by shorter construction time and easier maintenance operations on tracks.
Comparison of the floating mass system and the floating platform system is summarized in fig.12. Both systems exhibit excellent vibration abatement, as the before works and after works as the measurements presented in the paper have shown.
In fig.12 information is given about the cost of the systems and the cost differential with respect to traditional track structures.
Even if the cost differential is considerable, it should be borne in mind that usually vibration reducing track structures are needed just for relatively short sections of an LRT line, so the incremental cost may be acceptable.
The higher cost of the floating platform system is due to the precast concrete platforms and slabs, but this cost is compensated by shorter construction time and easier maintenance operations on tracks.
Fig.12-Summary table of the characteristics and costs of the two systems placed in comparison
REFERENCES
1.Bono, G., C. Focacci, and S. Lanni. La Sovrastruttura Ferroviaria. Collegio Ingegneri Ferroviari Italiani, Roma, 1997.
2.Transactions from the Congress Impatto Vibroacustico e Ambientale delle Ferrovie Metropolitane. Seriate (BG), June 2-3, 1994.
3.Transactions from the Symposium Traffico e Ambiente: Inquinamento chimico, acustico e da vibrazioni. Trento, February 21-25, 2000.
4. Harvey L.Berliner, David W.Campo, Charlas N.Dickerson, Glenn Mack "Design and Construction of the Weehawaken Tunnel and Berghen line Avenue Station for the Hudson-Berghen Light Rail Transit System- 9thNational Light Rail Transit Conferenc, Portland 2003.
1.Bono, G., C. Focacci, and S. Lanni. La Sovrastruttura Ferroviaria. Collegio Ingegneri Ferroviari Italiani, Roma, 1997.
2.Transactions from the Congress Impatto Vibroacustico e Ambientale delle Ferrovie Metropolitane. Seriate (BG), June 2-3, 1994.
3.Transactions from the Symposium Traffico e Ambiente: Inquinamento chimico, acustico e da vibrazioni. Trento, February 21-25, 2000.
4. Harvey L.Berliner, David W.Campo, Charlas N.Dickerson, Glenn Mack "Design and Construction of the Weehawaken Tunnel and Berghen line Avenue Station for the Hudson-Berghen Light Rail Transit System- 9thNational Light Rail Transit Conferenc, Portland 2003.