how is this a string raise???

[canis582]

What did the dealer say?

Seems like a raise to me. But you could have been clearer.

[Rottersod]

Since your first chip was less than half the size of a required raise it could be considered an illegal raise. I'm not much of a limit player though.

[Twistofsin]

If no one has raised in front of you I don't consider it possible to string from the BB.

[crashjr]

I've had a string raise called on me before when I put out 15 chips in a 4 chip 8 chip game, since I didn't have the amount sufficient to raise in my hand. My intention could not have been more clear, just like in OP's case.

Some rooms/floors are just plain dumb. This is a raise.