How should I fit a piecewise regression with interaction effect using brms?

Interfaces brms
1

I learned the picewise regression in brms by 1 and 2.

But I still don’t get how build the formula in brms.

This is my date
date.csv (129.1 KB)

first I fitted a simple model like this:

fit<-brm(Length~type*num+(1|location)+(1|G),
                 family = gaussian(link="identity"),
                   data=data,
                   seed=1,
                   cores = getOption("mc.cores", 10),
                   chains=4,
                   iter=5000,
                   warmup=2000,
                   thin=1)

Now, I want to break up the num to 0-20, 20-35, 35-70 using piecewise regeression with a interaction efffect (type), I tried several times, bu failed all of them.

How should I fit it?

Thank you guys!

2
3

help please~~~~~~~~~~~~~~~~