Marco Djuricin had a number of early chances as Brentford were left frustrated in Lee Carsley’s first half in charge of the club.
The Bees striker missed the target from close range twice in the opening 15 minutes before forcing Birmingham keeper Tomasz Kuszczak into two low saves before the half-hour mark.
Alan Judge also forced a save from Kuszczak just before the interval as Carsley, who replaced the sacked Marinus Dijkuizen on Monday, made two changes from the side that lost at home at the weekend.
Jack O’Connell replaced the suspended James Tarkowski while Ryan Woods came in for Josh Clarke.
Konstantin Kerschbaumer was moved to the right-hand side of midfield as Carsley opted for a 4-4-2 system that included Lasse Vibe up front with Djuricin.
Brentford: Button; McCormack, O’Connell, Dean, Bidwell; Diagouraga, Kerschbaumer, Woods, Judge; Vibe, Djuricin.
Subs: Bonham, Gogia, Yennaris, Barbet, Udumaga, Clarke, Canos.