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

412 Transportation Research Circular E-C058: 9th National Light Rail Transit Conference
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.
Once a suitable natural frequency for the FST is determined, the appropriate stiffness of
the springs and mass of the slab trackbed can be determined. The NERL MOS-1 FST system
design uses “natural rubber” springs and a hybrid pre-cast concrete slab combined with a cast-inplace
(CIP) slab. Natural rubber has been used for over 100 years as a material for structural
isolation elements, and has been found to be highly durable and an ideal spring material for FST
after its first use in North America in this manner in 1970.
A generic FST system concept is shown in cross section in Figure 4. The rubber pads rest
on either an invert or the bottom of a concrete tub as in the case of NERL MOS-1. 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 FST system for NERL MOS-1 presented several design issues and challenges not
typically encountered. Two of the main issues involving groundborne noise and vibration
projections were
• Lack of an existing fleet of vehicles to use for measurements, and
• No at-grade operations for measurements.
FIGURE 4 Typical FST configuration.