Query Builder order by lenght

Hi ,i have a field on db alphanumeric (A01,A20,A110) , to sorting properly  i must sort by LENGHT(nome)   , nome  but if i do this :

Code:

$scaffali_model->orderBy('LENGHT(nome)  ASC , nome ASC')->findAll();

I have this error : FUNCTION my_magazzinoes.LENGHT does not exist

use SELECT to get the length, then can use the sort

select(' length(nome) as L')

orderBy('L asc')