Simple concatenate?

I’m trying to achieve something i thought would be extremely easy, but i can’t get it to work!

for (var i = 0; i<6; i++) {
    if (_root.b1_mc._currentframe == '1') {

I want to take the following line:

    if (_root.b1_mc._currentframe == '1') {

and make the ‘b1_mc’ variable change with the i variable. I thought it would be…

if (_root.["b" + i ]_mc._currentframe == '1') {

but that doesn’t work. Any tips on how to make this work?