A fishplate, or joint bar, carries bending moment and shear across a bolted rail joint, so its contact geometry, section profile and bolt pattern decide whether the joint stays tight or starts to fret. Steel joint bars are commonly rolled or forged from structural steel such as EN 10025-2 S355J2 or GB/T 1591 Q355B, and the dimensional requirements for North American practice are published in the AREMA Manual for Railway Engineering, Chapter 5. Three design moves now dominate new joint specifications: arc-shaped contact, variable cross-section, and staggered bolt layout.
Arc-Shaped Fishplates and Contact Compatibility
An arc-shaped fishplate is contoured on a large radius, typically about 1000 mm, so its contact face follows the rail head and rail foot profile rather than sitting against them on flat faces. Contact compatibility improves by roughly 30 percent compared with a rectangular design, and the load spreads more evenly into both the bar and the rail.
Finite element analysis of the same joint shows that the arc profile lowers peak stress by about 25 percent and extends fatigue life by roughly 50 percent. On a busy mainline where the design was adopted, the joint crack rate fell from 12 percent to 4 percent, which reduced both maintenance frequency and cost. The curved contact also damps the small relative movements between bar and rail that generate joint noise and loosen bolts.
Variable Cross-Section Fishplates and Bending Resistance
A variable cross-section fishplate thickens the zones that matter, around bolt holes and the mid-span, while thinning the zones that carry little load. In a typical heavy-haul bar the section at the bolt holes grows from 24 mm to 30 mm, and the transition is blended rather than stepped so that stress does not concentrate at the change.
Bending stiffness increases by about 40 percent compared with a uniform section of the same nominal weight.
Deformation under heavy-haul traffic with axle loads of 25 t and above is reduced by roughly 35 percent.
On a heavy-haul line that converted to variable cross-section bars, joint fractures fell by about 70 percent.
The gain comes from putting steel where the bending moment is highest, which is more efficient than simply making the whole bar heavier.
Staggered Bolt Arrangement and Load Sharing
A traditional single-row bolt pattern concentrates force on the first bolts in the row. Staggering the bolts, so that alternate fasteners sit above and below the neutral axis, spreads shear and tensile demand more evenly through the joint.
Theoretical calculation shows roughly 60 percent better dispersion of shear and tensile force between individual bolts, which reduces the risk of fatigue fracture. In one case a line using single-row bolts recorded an 18 percent bolt fracture rate after one year of service; after conversion to a staggered layout the replacement cycle extended from one year to three years. Lateral stability also improves, so the joint resists rail displacement under lateral load.
Composite Fishplates for Weight and Corrosion
Carbon fibre reinforced composite fishplates, built from carbon fibre and epoxy resin, weigh about 60 percent less than steel bars of similar duty, which suits urban transit and bridge decks where every kilogram of track weight matters. Being non-metallic, they do not corrode, and in coastal or acidic environments they last around three times as long as steel.
The elastic modulus is lower than steel, so the laminate is tailored to compensate. Optimised fibre layup using 0, plus and minus 45 and 90 degree orientations with balanced layer design produces bending strength above 800 MPa. A metro operator that adopted composite bars reported a 30 percent reduction in life-cycle cost with less interference maintenance.
Tolerance, Torque and Joint Inspection
Contact surface tolerance between bar and rail must be held within about plus or minus 0.1 mm for a tight fit. A tolerance of 0.3 mm reduces the contact area, weakens the connection and allows relative slip under passing trains, which then loosens the bolts. A site with poor tolerance control recorded a 20 percent bolt loosening rate within three months and abnormal vibration at the joints; after reworking the bars to specification the loosening rate fell to 3 percent.
| Inspection point | Target | Why it matters |
|---|---|---|
| Bar to rail contact tolerance | Within plus or minus 0.1 mm | Maintains contact area and prevents slip |
| Bolt pattern | Staggered, matched to bar design | Distributes shear and tensile load |
| Bolt condition | No visible thread wear or cracking | Preserves clamping force |
| Joint geometry | Aligned rail ends, no step | Limits impact loading |
FAQ
Q: What radius is used for an arc-shaped fishplate?
A common contour radius is about 1000 mm, selected to follow the rail head and foot profile closely enough to raise contact compatibility by roughly 30 percent against a flat rectangular bar.
Q: How much does a variable cross-section raise bending stiffness?
Thickening the bolt hole zones from 24 mm to 30 mm while thinning the neutral zones increases bending stiffness by about 40 percent and cuts deformation by roughly 35 percent on heavy-haul track.
Q: Why stagger the fishplate bolts?
Staggering distributes shear and tensile force roughly 60 percent more evenly across the bolt group, which reduces fatigue fracture risk and extends the bolt replacement cycle from about one year to three years.
Q: Are composite fishplates strong enough for mainline track?
With an optimised laminate layup, composite bars reach bending strength above 800 MPa, which suits urban transit and bridge applications where light weight and corrosion resistance outweigh the lower elastic modulus.
Q: How tight should bar to rail contact tolerance be?
Hold it within about plus or minus 0.1 mm. Allowing 0.3 mm reduces the contact area, promotes slip under traffic and drives bolt loosening, which has been measured at 20 percent within three months on poorly controlled sites.
Q: What standard governs joint bar dimensions?
In North American practice the AREMA Manual for Railway Engineering, Chapter 5 sets the joint bar requirements, while the steel itself is ordered to a structural steel standard such as EN 10025-2 S355J2 or GB/T 1591 Q355B.