| 摘要 |
A top frame assembly having an upper member having a central opening defined therethrough and a generally planar upper wall, a first plurality of ribs extending downwardly therefrom, and a first mating surface. Also included is a bottom member having a central opening defined therethrough and a generally planar lower wall, a second plurality of ribs extending upwardly therefrom, and a second mating surface, wherein the first and second mating surfaces are attached to each other. Also included is at least one insert member disposed between the upper wall and the lower wall.
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