Why Not?

That’s the underside of a mezzanine in an industrial building in an industrial complex, built in the 1910s. The slab above is one-way spanning from the beam on the right over the window to the beam on the left above the electrical conduit. There’s a column on the right, just past the window, and a girder running left from the column…why is the girder arched? This is reinforced concrete, the arch is too shallow to work well as an arch, and there is nothing on the right to withstand an arch’s thrust, so this is a beam carrying load in bending, not a structural arch.

Why is the girder arched? Because it could be and someone wanted it to be. Cool.