Biblatex, alphabetically enumerated

Question

I hope that you can help me to get customized labels for my bibliography.
I want to get a specific format for labels in bibliography, using \biblatex; only two characters, alphabetically ordered:

1. The first character of the author's name.
2. A number, which is the list for the first character.

I understood that I have to change the \DeclareLabelalphaTemplate; but I don't know how, I'm using this code:

\documentclass[11pt,a4paper]{article}

\usepackage[spanish, es-noshorthands]{babel}

\usepackage{csquotes}
\usepackage[backend=biber,style=alphabetic,backend=biber]{biblatex}
\usepackage{hyperref}
\addbibresource{bib.bib}

\DeclareLabelalphaTemplate{
  \labelelement{
    \field[final]{shorthand}
    \field{label}
    \field[strwidth=1,strside=left,ifnames=1,compound=false]{labelname}
  }
  \labelelement{ %I have to change or add here, I suppose.
    \field[]{}
  }
}

\begin{document}

These are my references, \cite{bael}, \cite{is}, \cite{macgregor}, \cite{morales}, \cite{nationalacademy-joints}, \cite{pfeiffer}.

\printbibliography


\end{document}

This is my bibliography data:

@book{morales,
    author    = {R. Morales},
    title     = {Diseño Estructural Sismorresistente},
    publisher = {Fondo Editorial ICG},
    year      = {2002},
    address   = {Lima},
    pages     = {4-5},
    }

@book{macgregor,
    author    = {J. MacGregor and J. Wight},
    title     = {Reinforced Concrete Mechanics and Design},
    publisher = {Prentice Hall},
    date      = {2011},
    location  = {Upper Saddle River, New Jersey},
    pages     = {71-71},
    }

@report{pfeiffer,
    author    = {Michael J. Pfeiffer, and David Darwin},
    title     = {Joint Design for Reinforced Concrete Buildings},
    institution = {University of Kansas Structural Engineering and Materials Laboratory},
    date      = {1987},
    location  = {Lawrence, KS},
    pages     = {73},
    }

@book{seae,
    author    = {Departamento de Tecnología Industrial},
    title     = {Código Técnico de la Edificación. Documento Básico SE-AE Seguridad Estructural: Acciones en la Edificación},
    location  = {Madrid},
    date      = {2007},
    pages     = {9},
    }

@book{bael,
    title    = {Règles de techniques de conception et de calcul des ouvrages et construction en béton armé suivant la méthode des états limites},
    author   = {Ministère de l'Equipement des Transports et du Logement},
    location = {Paris},
    date     = {1999},
    pages    = {86-87},
    }

@book{is,
    title    = {Plain and Reinforced Concrete - Code of Practice IS 456-2000},
    edition  = {Fourth},
    author   = {Bureau of Indian Standards},
    location = {Nueva Delhi},
    date     = {2000},
    pages    = {86-87},
    }

@report{pca-joints,
    title    = {Buildings Movements and Joints},
    author   = {Portland Cement Association},
    location = {Skokie, Illinois},
    date     = {1983},
    pages    = {36},
    }

@report{nationalacademy-joints,
    title    = {Expansion Joints in Buildings},
    author   = {Standing Commitee on Structural Engineering of the Federal Construction Council},
    number   = {Technical Report No. 65},
    institution    = {National Academy of Sciences},
    location       = {Washington},
    date           = {1974},
    pagetotal      = {43},
    }

@book{vallecilla,
    author    = {C. Ramiro Vallecilla},
    title     = {Fuerzas Sísmicas Principios y Aplicaciones NSR-98},
    publisher = {Editorial Bauen},
    location  = {Bogotá},
    date      = {2003},
    pages     = {192-201},
    }

@online{senhami,
    author          = {Servicion Nacional de Meteorología e Hidrología},
    organization    = {Ministerio del Ambiente},
    title           = {Datos Históricos},
    url             = {http://www.senamhi.gob.pe/main_mapa.php?t=dHi},
    }

This image depicts, what I want,

Answer 1

I really hope someone can come up with a cleaner solution than this, but here we go.

Based on Audrey's answer to How to split the bibliography alphabetically? and an answer of mine to Define bibchecks using a loop heavily making use of Audrey's answer I would suggest the following.

We will use the numeric style and its prefixnumbers option for this solution (the latter requires defernumbers be enabled).

First, we create a bibliography category for each letter of the alphabet, making sure that skipped entries do not cause trouble

\makeatletter
\def\ifskipbib{\iftoggle{blx@skipbib}}
\makeatother

\def\initlist{}
\forcsvlist{\listadd\initlist}{A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z}
\forlistloop{\DeclareBibliographyCategory}{\initlist}
\AtDataInput{\ifskipbib{}{\addtocategory{\thefield{sortinit}}{\thefield{entrykey}}}}

Then, in the document, we loop over all the categories and print them: each category gets its own bibliography prefixed via prefixnumbers by the relevant letter (of course, we would also like a heading)

\printbibheading
\foreach \letter in {A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z}{%
  \begingroup\edef\myprintbib{\endgroup\printbibliography[category=\letter,prefixnumbers=\letter,heading=none]}\myprintbib
}

The slightly weird \edef definition is due to expansion (I could not get it right otherwise, so better ideas are very welcome).

MWE

\documentclass[11pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[spanish, es-noshorthands]{babel}
\usepackage{filecontents}
\usepackage{csquotes}
\usepackage{pgffor}
\usepackage[backend=biber,style=numeric,backend=biber,defernumbers]{biblatex}
\usepackage{hyperref}
\begin{filecontents*}{\jobname.bib}
@book{morales,
  author    = {R. Morales},
  title     = {Diseño Estructural Sismorresistente},
  publisher = {Fondo Editorial ICG},
  year      = {2002},
  address   = {Lima},
  pages     = {4-5},
}

@book{macgregor,
  author    = {J. MacGregor and J. Wight},
  title     = {Reinforced Concrete Mechanics and Design},
  publisher = {Prentice Hall},
  date      = {2011},
  location  = {Upper Saddle River, New Jersey},
  pages     = {71-71},
}

@report{pfeiffer,
  author    = {Michael J. Pfeiffer and David Darwin},
  title     = {Joint Design for Reinforced Concrete Buildings},
  institution = {University of Kansas Structural Engineering and Materials Laboratory},
  date      = {1987},
  location  = {Lawrence, KS},
  pages     = {73},
}

@book{seae,
  author    = {{Departamento de Tecnología Industrial}},
  title     = {Código Técnico de la Edificación. Documento Básico SE-AE Seguridad Estructural: Acciones en la Edificación},
  location  = {Madrid},
  date      = {2007},
  pages     = {9},
}

@book{bael,
  title    = {Règles de techniques de conception et de calcul des ouvrages et construction en béton armé suivant la méthode des états limites},
  author   = {{Ministère de l'Equipement des Transports et du Logement}},
  location = {Paris},
  date     = {1999},
  pages    = {86-87},
}

@book{is,
  title    = {Plain and Reinforced Concrete - Code of Practice IS 456-2000},
  edition  = {Fourth},
  author   = {Bureau of Indian Standards},
  location = {Nueva Delhi},
  date     = {2000},
  pages    = {86-87},
}

@report{pca-joints,
    title    = {Buildings Movements and Joints},
    author   = {{Portland Cement Association}},
    location = {Skokie, Illinois},
    date     = {1983},
    pages    = {36},
}

@report{nationalacademy-joints,
  title         = {Expansion Joints in Buildings},
  author        = {{Standing Commitee on Structural Engineering of the Federal Construction Council}},
  number        = {Technical Report No. 65},
  institution   = {National Academy of Sciences},
  location      = {Washington},
  date          = {1974},
  pagetotal     = {43},
}

@book{vallecilla,
  author    = {C. Ramiro Vallecilla},
  title     = {Fuerzas Sísmicas Principios y Aplicaciones NSR-98},
  publisher = {Editorial Bauen},
  location  = {Bogotá},
  date      = {2003},
  pages     = {192-201},
}

@online{senhami,
  author          = {{Servicion Nacional de Meteorología e Hidrología}},
  organization    = {Ministerio del Ambiente},
  title           = {Datos Históricos},
  url             = {http://www.senamhi.gob.pe/main_mapa.php?t=dHi},
}
\end{filecontents*}

\addbibresource{\jobname.bib}

\makeatletter
\def\ifskipbib{\iftoggle{blx@skipbib}}
\makeatother

\def\initlist{}
\forcsvlist{\listadd\initlist}{A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z}
\forlistloop{\DeclareBibliographyCategory}{\initlist}
\AtDataInput{\ifskipbib{}{\addtocategory{\thefield{sortinit}}{\thefield{entrykey}}}}

\begin{document}
These are my references, \cite{bael}, \cite{is}, \cite{macgregor}, \cite{morales}, \cite{nationalacademy-joints}, \cite{pfeiffer}.
\printbibheading
\foreach \letter in {A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z}{%
  \begingroup\edef\myprintbib{\endgroup\printbibliography[category=\letter,prefixnumbers=\letter,heading=none]}\myprintbib
}
\end{document}