mujoco

<mujoco model="Gothic arch">
  <visual>
    <global azimuth="140" elevation="-20"/>
  </visual>

  <!-- Frictional stability requires elliptic cones and large impratio. The
       discrete integrator treats the contact rows implicitly, so the
       authored contact stiffness is honored at an 8 ms timestep and the
       settled arch stays quiet (implicitfast needs 2 ms for the same drift).
       noslip_iterations=2 removes the slow frictional creep. -->
  <option timestep="8e-3" integrator="discrete" cone="elliptic" impratio="10" noslip_iterations="2"/>

  <!-- Equilateral pointed arch: two 2m-radius arcs, each centered at the
       opposite springing. Each flank is one static springer and 4 replicated
       voussoir pairs; the keystone spans both arcs at the apex. Wedge faces
       are shrunk so neighboring voussoirs sit 0.05mm apart. The base poses
       and the replicate step are the arch as settled under load (the exact
       120/11 degree pattern with its joint gaps closed), so it loads at rest. -->
  <asset>
    <texture name="ibl" type="cube" builtin="gradient" rgb1=".25 .3 .45" rgb2=".55 .5 .45" width="32" height="32"/>
    <texture type="skybox" builtin="gradient" rgb1=".12 .16 .28" rgb2=".5 .45 .4" width="32" height="512"/>
    <texture name="grid" type="2d" builtin="checker" rgb1=".1 .2 .3" rgb2=".2 .3 .4" width="100" height="100"/>
    <material name="grid" texture="grid" texrepeat="8 8" texuniform="true" reflectance="0.06"/>

    <mesh name="voussoir" vertex="-0.15 -0.0891990 -0.1271229
                                  -0.15  0.0891990 -0.1271229
                                  -0.15 -0.1010922  0.1225941
                                  -0.15  0.1010922  0.1225941
                                   0.15 -0.0891990 -0.1271229
                                   0.15  0.0891990 -0.1271229
                                   0.15 -0.1010922  0.1225941
                                   0.15  0.1010922  0.1225941"/>
    <mesh name="keystone" vertex="-0.15 -0.414478  0.082401
                                  -0.15         0  0.364576
                                  -0.15  0.414478  0.082401
                                  -0.15  0.162361 -0.208560
                                  -0.15         0     -0.23
                                  -0.15 -0.162361 -0.208560
                                   0.15 -0.414478  0.082401
                                   0.15         0  0.364576
                                   0.15  0.414478  0.082401
                                   0.15  0.162361 -0.208560
                                   0.15         0     -0.23
                                   0.15 -0.162361 -0.208560"/>
  </asset>

  <default>
    <geom type="mesh" density="2500" solref="0.005 1" solimp="0.95 0.99 1e-4"/>
  </default>

  <worldbody>
    <light name="key" pos="-1.35 -2.7 5.4" dir=".3 .6 -1" cutoff="25" softness="1" range="30" ambient=".15 .15 .15" specular=".15 .15 .15" intensity="3000000"/>
    <light type="image" texture="ibl" intensity="7500"/>
    <camera name="front" pos="3.6 0 1.1" xyaxes="0 1 0 0 0 1"/>

    <geom name="floor" type="plane" size="4 4 .01" material="grid"/>

    <!-- Left flank: arc centered at the right springing. -->
    <frame pos="0 1 0">
      <geom name="springer_left" mesh="voussoir" pos="0 -1.9977348 0.0951706" euler="87.27273 0 0" rgba=".45 .44 .46 1"/>
      <replicate count="4" euler="-10.89287 0 0">
        <body name="left_a" pos="0 -1.981148 0.28557" euler="82.19661 0 0">
          <freejoint/>
          <geom mesh="voussoir" rgba=".71 .71 .73 1"/>
        </body>
        <body name="left_b" pos="0 -1.944908 0.4723266" euler="76.74274 0 0">
          <freejoint/>
          <geom mesh="voussoir" rgba=".80 .80 .82 1"/>
        </body>
      </replicate>
    </frame>

    <!-- Right flank: arc centered at the left springing. -->
    <frame pos="0 -1 0">
      <geom name="springer_right" mesh="voussoir" pos="0 1.9977348 0.0951706" euler="-87.27273 0 0" rgba=".45 .44 .46 1"/>
      <replicate count="4" euler="10.89287 0 0">
        <body name="right_a" pos="0 1.981148 0.28557" euler="-82.19661 0 0">
          <freejoint/>
          <geom mesh="voussoir" rgba=".71 .71 .73 1"/>
        </body>
        <body name="right_b" pos="0 1.944908 0.4723266" euler="-76.74274 0 0">
          <freejoint/>
          <geom mesh="voussoir" rgba=".80 .80 .82 1"/>
        </body>
      </replicate>
    </frame>

    <body name="keystone" pos="0 0 1.549614">
      <freejoint/>
      <geom mesh="keystone" rgba=".87 .79 .63 1"/>
    </body>
  </worldbody>
</mujoco>