The Dragons (11-3) will go for their second consecutive City League championship when they play Perry (10-4) at 1 p.m. May 30 in the title game at McGibbney Field.
Perry beat Carrick, 12-2, in Wednesday's other semifinal game, with Amee Hilson going 4 for 4 with a double and a triple.
Allderdice (11-3) scored all four runs of its runs against Brashear in the bottom of the sixth.
Brashear fell to 5-12.
Norwin dropped to 9-11.
North Allegheny will play at a time, date and site to be determined against Connellsville (15-3), which beat Penn Hills, 1-0.
Dana Perry and Jess Givens drove in runs with fielders' choices in the first, and Ashley Nikithser doubled in a run in the second for North Allegheny.
Connellsville scored the game's only run in the fourth inning when Shawna Little doubled and scored on a single by Natalie Bukovac.
Losing pitcher Amber Pazzaccoli struck out 10 for Penn Hills.
Burrell will play Riverside (16-1) in the semifinals in a rematch of last year's championship game.
Kayla Hornack tied the score with a two-run single, then Kara Wojcik scored the game-winner on Jen Stiveson's grounder to first.
Jessi Speer hit a two-run double for Burrell in the first inning.
Amanda Skura singled twice an RBI.
Mike McAfoose singled home winning pitcher Art Yerecic on a 1-2 pitch with two outs in the top of the seventh inning to tie the score, and Bryan McAfoose followed with a run-scoring double to rally Franklin Regional to a 3-2 upset victory against third-ranked Trinity in a WPIAL Class AAA first-round playoff game.
'We saw an opening and took it,' said Franklin Regional coach Larry Wayman. 'The two deciding runs in the seventh inning proved to be the difference.'
With Trinity leading 2-1, Yerecic opened the seventh inning with a single to left field. Yerecic then made it to second after a sacrifice bunt by brother Mike Yerecic.
'We played good, sound defense with strong pitching,' Wayman said. 'Both teams played a great game, with the focus of each team being on the strong pitching.'
Yerecic struck eight and walked one. Losing pitcher Jim Blue struck out five.
Burrell (17-0) will play Mohawk (13-5) in a Class A second-round game.