TR Circular E-C058_9th LRT Conference_2003.pdf - Florida ...

Carman, Smoluchowski, and Berliner 411
FLOATING SLAB DESIGN ISSUES
In basic terms, an FST 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 Figure 3. 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 (1) demonstrated this, in particular
when the FST is under vehicle load.
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
M = ISOLATED MASS
K = SUPPORT SPRING STIFFNESS
C = DAMPING COEFFICIENT
M
f n =
2 1 K
M
K
C
FIGURE 3 Idealized spring-mass and damper system.