Mike Clarke wrote:

The beam is not weaken any less by having a U shape.

The stress concentration factor for a sharp corner is _much_ greater
than a corner with a generous circular profile. How do you account for
the stress difference not affecting the strength?

He doesn't need to, Mike. He's IMM. Stress concentrations and crack
propagation don't happen in his world - talk of them is just a
conspiracy by the landowning classes from the snot universities. In his
material world, magickalmegnetickal descalers work, hacksaws cut without
burrs, and for all I know little fairies run to hold together composite
beams whenever their altheometers detect a cr*ck starting to spr**d.

In the dull ol' world the rest of us live in, portholes are round rather
than square...