The ornamental design for a arched supporttier for a dumbbell rack, as shown and described.
FIG. 1 is a front perspective view of an arched support tier for a dumbbell rack, showing our new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a right side elevation view thereof, the left side elevation view being a mirror image thereof;
FIG. 4 is a rear elevation view thereof; and,
FIG. 5 is a top plan view thereof.
The broken line showing of a support frame for the arched support tier for a dumbbell rack in FIGS. 1 to 5 is for illustrative purposes only and forms no part of the claimed design.