A View of NCSX Structural System and Load Path for the Base Support Structure

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A View of NCSX Structural System and Load Path
for the Base Support Structure
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TF Support Frame

• Upper and lower TF support frames are symmetry
  to the mid-plane.
• The inner frame and the outer frame are connected
  by T-shape beams to resist the TF vertical EM
  load.

• The frames are formed by casting pieces and
  bolted together to form continuous elements in
  the toroidal direction.
• The frames are bolted to the feet of the
  modular coil shell structure.
• The upper frame and the lower frame do not
  connected by any structural members. Loads from
  the upper frame will go through the shell
  structure to the base support.
• The frames also support the PF coils
  eccentrically.

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Inner Support Frame

• The bending and torsion rigidity of the
  ring-shape frames are best provided by the
  continuity of the cross section along the
  toroidal direction when the castings are securely
  tightened
• Bolt joints between casting pieces are weak for
  bending and torsion

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Outer Support Frame

• The frame provide better stiffness in the radial
  direction, but weaker in the toroidal direction.
  
• Only the upper plate is continuous in the
  toroidal direction.

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Center Stack OH Coils

• The center stack for the OH assembly is preloaded
  and mounted on the top inner frame

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• Loads and Load Paths
• All loads will be either balanced each other or
  carried down to the base support structure.
• The distributions of the loads to the adjacent
  components depend on the type of the loads and
  the rigidity of the interacting structural
  components
• Dead weight - As the shell structure is more
  rigid than the TF support frame, most of the
  weight will be carried through the shell to the
  base support structure
• EM loads Because of the symmetry nature of
  loads, some loads will be balanced each other,
  some will be transferred through the shell, and
  some at the lower TF support frames will be
  reached the base support directly.
• Thermal load Uniform cooldown to 80K will cause
  minimum stress if structure is allow to move
  freely in the radial direction in the same plane.
  Therefore, if sliding point is designed, it
  shall be verify that the joint will be able to
  slide during cooldown.
• Seismic load Horizontal loads will be restrained
  by the support posts that do not slide in the
  force direction. .The vertical loads will be
  similar to the dead load.

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Bolt Joint Design Considerations

• The NCSX structure transfers forces from one
  component to the other component by bolted
  joints.
• Bolt joints are best located at where the bending
  moment is small, so the tension in the element
  will not reduce the bolt preload.
• Because of the EM loads are not static, It is
  recommended (or required) to design the bolt
  joint as a friction joint, not the shear joint.
• The bolt preload depends on the bolt capacity.
  The determination of the preload magnitude should
  also consider the preload change due to thermal
  expansion of different materials caused by
  temperature variation and the creep of the
  insulation material at the joint if any.
• The bolt joint shall be locked in position
  because of transit in the load nature

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Shear Force in the Shell Due to Bending

• As the TF support frame do not provide much
  continuous areas in the toroidal direction, The
  bending rigidity of the system will mostly depend
  on the shell.
• For a beam with continuous cross section, the
  shear stress due to vertical shear force in the
  cross section is
• Szx VQ/bI Where V shear force
  Q area moment b
  Thickness of total shell walls I
  moment of inertia of the cross section
• As Q is maximum in the mid-plane, without the
  port opening in the shell, the maximum vertical
  shear stress will be near the mid-plane.
• There are no bolts available at the shell inboard
  joint. The compression at the inboard will
  depend on the bolt preloads at the upper and
  lower flanges.
• In shell cross section, bolts on the top and
  bottom flanges take most of the bending stress.
• The shear force V and the bending moment are
  primarily determined by the magnitude of loadings
  and the spacing between the supports

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Design of the Base Support - Simple Beam Analogy

• The center of gravity of the NCSX module is
  located approximately one third of the distance
  between inner post and outer post, Therefore,
  for the dead weight, the vertical load in the
  outer post is about twice as big as the load of
  the inner post.
• The equal spacing base post design provide not
  only small bending stress but also small shear
  force in the cross section

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Summary

• The modular coil shell structure become the
  primary structure to transfer loads to the base
  support structure because the upper TF support
  frame and the lower TF support frame do not
  connected by any structural members
• The shell structure is basically a circular tube
  in shape. In additional to the stress in the
  shell, it strength and rigidity depends on the
  capability of the bolt joints.
• Bolt joints are best located at where the bending
  moment is small, so the tension in the element
  will not reduce the bolt preload.
• The equal-spacing base post design provide not
  only small bending stress but also small shear
  force in the cross section
• The base support locations and moving
  constraints, such as sliding joint will affect
  the structural responses of the upper structure.
• Because of the EM loads are not static, It is
  recommended (or required) to design the bolt
  joint as a friction joint, not the shear joint.
• It is suggested that the center stack for the OH
  assembly is mounted at the bottom inner frame to
  lead the loads directly to the base support