Hi Shelia,
As I don't know any specifics about your arch and haven't seen a photo it's difficult to tell. The most common reason for sagging in the middle is that the frame is not sturdy enough to support the weight of the balloons. Whereas for smaller and hanging designs, the 1/4" aluminium rod is strong enough, for a self-supporting walk through arch you will need 1/2" aluminium rod for the frame. If the frame has to be extra sturdy (e.g. for outdoor arches), I would even recommend using 1/2" conduit. It's more difficult to work with, but it will give your arch the sturdyness it needs.
