Considering your image says "Recommended length"...I'd guess there isn't one. Most codes for structural steel define certain aspects of the members that have to be checked, either for section strength capacity or peak stress against an allowable, and then have more nuanced checks for connections. But very few are so prescriptive as to define something like this. Minimum 2 bolts, stiffeners at unframed ends, things like that...but this will be based on the stability of the building and strength of the components involved.

I suspect the author of that text has found either though personal experience or industry study that 10% usually ends up being the most economical arrangement for that sort of design.

Pls look EN 1993, Eurocode 3. Design of steel structures. General rules and rules for buildings.
https://www.phd.eng.br/wp-content/uploads/2015/12/...
The haunch length ( 10% portal frame span ) is a common practice and chosen such that , the resisting hogging moment at haunch sharp edge starting point is the same with max. sagging moment.

I have copy and pasted a snap from SCI_P397-Elastic design of single bay steel EC 3,


I will suggest you to look to the following doc.


....

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

• https://files.engineering.com/getfile.aspx?folder=56fa3282-333d-4aeb-9268-6