First, shore the structure if it's failing.
The contractor can't do something without the supervision of an engineer signing responsibility for the work, talk to this engineer and show his neck is on the line.
If there is no engineer, you'll need to redesign the strucutre, NOT THE LINTEL, THE...
Depends, there are codes with only 20mm cover for beams, and stirrups small as 4.2mm
That leaves up to 100mm for main reinforcement.
Assuming 25mm rebars(#8), we are left with 50mm for the concrete in 3 or 4 layers.
Many water tanks have similar cross sections for their walls.
I can't imagine anyone doing a 15m cantilever, and yet here we are.
The biggest span I ever achived was 35m simply supported, and the beam was I section, 1920mm tall.
In bridge construction, we can see girders of many times that size.
a 15m cantilever should be at least 2 meters tall, assuming it's braced, we could have a width of 0.15m
let's say the load is around 0.3 tons/meter, and the beam weighs 0.75 tons/meter.
1.05 x 15^2 / 2 = 118 ton x meter
deflection will be around 30mm instantly, it could reach 100mm on the long...
