DIRECT FIXATION TRACK DESIGN

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DIRECT FIXATION TRACK DESIGN

• Laurence E. Daniels, P.E.
• TRB Session 734
• Sponsored by AP080 (A2M04) Rail Transit System
  Design
• January 14, 2004

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Alternative Title Choosing Fastener
StiffnessPresentation Order

• Direct Fixation definitions
• Fastener Stiffness Design Goals
• Fastener Stiffness and Rail Deflections
• Fastener Vibration Attenuation
• Vibration Sources
• Fastener Stiffness Fastener Natural Frequency
• Precision of Fastener Stiffness
• Summary Choosing a Fastener Stiffness

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Credit

• The data on Direct Fixation fastener stiffness
  in this presentation is a small portion of that
  research reported under TCRP Project D-5, Direct
  Fixation Fastener Performance.
• Jim Tuten is gratefully acknowledged as the
  Project Manager of that work.

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Direct Fixation Definition

• Applicable Configuration for this presentation
• Ballastless track using discrete rail fasteners.
• Configurations not addressed
• Fully embedded rail
• Continuously supported rail
• Open deck bridges
• Ballasted track

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Tangent Stiffness
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Design Goals for Direct Fixation Fastener
Stiffness

• Protect the support from high impact loads
• Restrain the rail within acceptable deflection
  limits
• Attenuate vibrations (a very distant third in
  importance to the railways performance)

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Fastener Stiffness and Rail Deflections

• Lateral fastener stiffness is influenced by the
  vertical stiffness.
• Vertically softer fasteners will generally allow
  larger lateral rail deflections than a stiffer
  fastener.
• Lateral Stiffness (and gauge deflection) is
  influenced by vertical load.
• Precession wave mechanics away from wheel load
  are critical (very high L/V ratios near rail
  uplift)

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Fastener Lateral Stiffness vs. Vertical Load
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Rail Clip Selection and Fastener Stiffness

• Rail clips and fastener specifications for rail
  deflection must be compatible.
• Example If the specifications for rail
  deflection are held uniformly, the fastener for a
  low longitudinal restraint system should be
  different from a high longitudinal restraint
  system.
• (Better solution Adjust the deflection
  specification for each system).

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Point Fastener Stiffness Selection

• Choose allowable rail deflections first.
• Then, work backward to fastener stiffness from
  those maximum gauge deflections and the expected
  load range.
• Notes
• Knowing lateral stiffness relation to a vertical
  stiffness for a fastener is problematic and not a
  current specification requirement.
• Rail restraint capacity (rail clip) must be
  compatible with resulting decisions.

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Fastener Vibration Attenuation
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Important Vibration Sources

• Axle and Truck Precession Waves
• Vehicle Kinematic Frequencies
• Pitch Mode
• Bounce Mode
• Yaw Mode
• Track Natural Frequencies
• Wheel/Rail Interaction (Curving, Hunting, Rail
  Corrugation behavior)
• Rail Natural Frequency

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Vibration Source Frequency and Energy Levels
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Fastener Vibration Filtering Concept

• Direct Fixation Fasteners are Low Pass
  vibration filters
• A Low Pass filter allows vibration frequencies
  that are below the fasteners natural frequency
  to be transmitted unimpeded.
• Vibration frequencies above the fasteners
  natural frequency are filtered.
• A fasteners natural frequency is derived from
  the fasteners stiffness.

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Fastener Transfer Function
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Transfer Function Illustration
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Stiff Fastener Transfer Function
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Soft Fastener Transfer Function
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What is the Precision of Fastener Stiffness
Values?

• This question
• Effects underlying considerations of
  specification stipulations.
• Allows insight into the realities of fastener
  manufacturing variations.
• Centers our expectations on fastener properties
  and performance.

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Fastener Stiffness Variation
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Fastener Tangential Stiffness
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Stiffness Variation Other Manufacturers
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Fastener Natural Frequency
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Fastener Stiffness Tolerance Insights

• The fastener manufacturing can introduce
  significant variations in fastener stiffness (not
  currently controlled in DF QC specifications).
• Of the 4 manufacturers with multiple tests on one
  fastener model, one exhibited poor repeatability
  of fastener stiffness and natural frequency
  properties (not currently controlled by
  specifications).
• Rail modes (Light Rail, Heavy Rail, Freight) are
  effected differently by fastener variations
  (inconsistencies).

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Insights on Fastener Stiffness Mechanics

• Fastener stiffness varies with applied load
• Fastener stiffness varies as the wheel load
  approaches the fastener .
• Rail modes invoke diffesrent fastener stiffness
  ranges.

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Fastener Natural Frequency and Source Vibrations
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Fastener Effects on Vibration Attenuation

• DF fasteners can not attenuate primary rail
  sources of ground vibration, the truck (or wheel)
  passing frequencies and kinematic vehicle
  motions.
• DF fasteners can attenuate most or all wheel/rail
  interactions (curving, hunting and rail
  corrugation behavior), but these generally have
  low energy levels relative to the primary rail
  sources of ground vibration.

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Summary Choosing Fastener Stiffness

• Fastener stiffness is determined primarily from
  allowable rail deflections and expected loads
  (safety criteria).
• No current Direct Fixation Fastener model has a
  natural frequency capable of attenuating the
  primary sources of track vibrations.
• Fastener stiffness is imprecise
• Stiffness varies with wheel approach to fastener
• Manufacturing is imprecise for some manufacturers.

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Direct Fixation Fastener and Track Design
Information Source

• www.TrackEngineer.com