Una soluzione potrebbeessere questa
`
\documentclass{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\draw[help lines] (-2,-4)grid(9,5);
\def\lI{4cm}
\def\rIfrazI{.4}
\pgfmathsetmacro\rIfrazII{1-\rIfrazI}
\def\rl{2}
\draw[line width = 2pt,
double distance = \lI-\pgflinewidth] (0,0)–(\rl,0);
%%%%%%%%%%
% RAMO I %
%%%%%%%%%%
\def\rIl{5}
\draw[line width = 2pt,
double distance = \rIfrazI*\lI-\pgflinewidth] (\rl,{(1/2-\rIfrazI/2)*\lI})–++(\rIl,0);
% …poi continui tu!
%%%%%%%%%%%
% RAMO II %
%%%%%%%%%%%
\def\rIIl{2.7}
\draw[line width = 2pt,
double distance = \rIfrazII*\lI-\pgflinewidth]
(\rl,{(-1/2+\rIfrazII/2)*\lI})–++
(\rIIl,0)arc[radius = {(\rIfrazII*\lI-\pgflinewidth)/2+1cm},
start angle = 90,
end angle = 0]
coordinate(r);
% …poi continui tu!
\end{tikzpicture}
\end{document}
`
Ovviamente da migliorare e sviluppare.
Saluti