Used heavily in the episode “Blue on Blue” (S1 Ep5) from the beginning of the episode and a few scenes after.

There was a lot of us, about 120, working 3 days on these scenes. Photo © CBS
Both the bridge and under the bridge were used in “Blue on Blue”.
I personally got a lot of face-time on this episode (in the purple shirt and blue-gray jacket):

Writing in my book to communicate with those outside the dome during visitor’s day. Photo © CBS

From the opening credits, walking with Holly who does some great crazy arms in a later scene. Photo © CBS

Conga line! The girl next to me who was supposed to be my sister. Photo © CBS
Actual Location:
6th St. Bridge between Campbell St. and Brunswick St.
Also Filmed Here:
One Tree Hill
Actual Location: No known location or no longer exists
This location was used on or around May 2nd, 2013 for the production Under the Dome (TV 2013-2015).