Why is the T-beam effective width a function of the span length? Or, to be more precise, for relevant codes - a function of the points of contraflexure for sagging bending moments?
It’s instructive to think about this in strut and tie terms. The compression in the flange originates down at the tension face and spreads out laterally as it makes it’s way up. It takes a bit of beam length in order for that load spreading to kick in.
Above is a snippet.