Lattice Girders

LATTICE GIRDERS FULFILL IMPORTANT FUNCTIONS IN TUNNEL CONSTRUCTION

Lattice Girders have been developed for the special demands of tunnel construction. The Lattice system has rigorously tested and successfully installed in many tunnel projects throughout the world.

Contrary to standard rolled steel sections and regardless of shape, the Lattice system, in conjunction with shotcrete is totally surronded by concrete forming a complete homogeneous composite structure with no unconsolidated areas, voids or fissures visible. The overall result in a tunnel lining that significantly reduces ground settlement and prevents the ingress of water.

REINFORCING ELEMENT OF THE SHOTCRETE LINING

Due to the total integration with the shotcrete spray, the Lattice Girders are completely encased in concrete and form a complete composite reliable structure. The quality of the bond between the shotcrete and the Lattice Girders is determined essentially by the mechanical properties of the shotcrete (compressive and tensile strength) and the geometry of the Lattice Girders. This quality is essential for the tunnel lining to achieve its designed deflection performance.

DRILLING SYSTEMS FABRICATION

  1. Lattice Girder segments shall be composed of low alloy steel and used for concrete reinforcement and intended for special applications where bending and welding are of importance.

  2. Minimum physical properties of steel:
    Tensile Strength min - 80 ksi
    Yield Strength min - 70 ksi
    Ultimate Elongation min - 10%

  3. The connection elements at the end of the grinder shall be constructed of structural angle or plate meeting the minimum properties of A36. Bolts and nuts shall meet the requirements of A325 unless otherwise noted.

  4. Welding process to meet the requirements set forth by AWS for gas metal arc welding (GMAW). All welders must be certified in accordance with AWS D1.1. All welds run parallel to the retaining bars a minimum length of 1”.


ADVANTAGES OF LATTICE GIRDERS

  • Immediate roof support over the excavated section.
  • Act as a template assuring aminimum thickness when applying shotcrete.
  • Become part of the reinforcement in the shotcrete lining.
  • Provide support for spiling, if needed, over the next excavation.
  • Work equally well with machine mining or drill and shoot tunneling
  • no need for major investment in special equipment.
  • Are practical supports for a wide range of ground conditions.
  • Can be erected within a few miutes by a small crew without special handling equipment.


LATTICE GIRDERS/ 3 BARS

With spaced diagonals, 3 bars are assembled to form a girder. The single bar can be located against the ground or towards the inside of the tunnel, fitting a specified design geometry. The spaced stiffening elements reduce the local buckling length of the bars, assure a high normal and bending moment resistance and secures a transfer of lateral forces in the lining before the shotcrete has reached its design strength.


LATTICE GIRDERS/ 4 BARS

With spaced diagonals, 4 bars are assembled to form a girder. Fabrication of the girder to the design geometry is possible. The spaced stiffening elements reduce the local buckling length of the bars and assure a high normal and bending moment resistance and a secure transfer of lateral forces in the lining before the shotcrete has reached its design strength.


WALLPLATE BEAMS

The 4 bar Girders are also used as longitudinal wall plates for heading and bench advances. They serve as bases and fixed points for the height and sides of the arch assembly. At the same time, they can be considered as a statically effective reinforcing of the base beam. The butt connection is rigid in bending and allows easy assembly. The arch spacing is variable.