Are I-beams as strong as girders?

Strong.

For flexural members, I-beams with the same weight are better than circular tubes and I-beams. Because the moment of inertia of I-beam is larger than that of circular tube, the bending bearing capacity of I-beam is higher.

I-beam, also known as universal beam, is a long strip steel with I-shaped cross section. I-beams are divided into ordinary I-beams and light I-beams.