A method of producing an assembly for use in a fibre reinforced structural element, said method comprising the steps of: i) providing an elongated core element (12 XIII), said elongated core element (12 XIII) including an inner core of a first material, and, a cover circumferentially encircling said inner core and being made of a second material being a fibre reinforced material compatible with the materials of said fibre reinforced structural element, said elongated core element (12 XIII) having an end part, said end part having a conical or partially conical shape, such as a truncated conical shape, said end part defining a central end face exposing said inner core and a circumferential end face encircling said central end face and exposing said cover, ii) providing a bolt fixture (22 XIII) or bolt comprising an end recess having a conical or partially conical shape, such as a truncated conical shape, said end recess being congruent with said end part of said elongated core element (12 XIII), iii) producing a subassembly by receiving and centring said end part of said core element in relation to said end recess of said bolt fixture (22 XIII) or bolt, iv) fixating said bolt fixture (22 XIII) or bolt to said end part of said core element in a pultrusion process by pulling said subassembly through a pultruder, by circumferentially covering said subassembly with reinforcing fibres and resin and by heating and curing said resin for causing said resin to provide in conjunction with said reinforcing fibres a casing circumferentially encircling said subassembly, or alternatively fixating said subassembly by adhesion to said casing produced in a separate pultrusion process, and v) machining said subassembly circumferentially encircled within said casing of said reinforcing fibres and said cured resin for providing said assembly including said elongated core element (12 XIII) and said bolt fixture (22 XIII) or bolt. i) providing an elongated core element (12 XIII), said elongated core element (12 XIII) including an inner core of a first material, and, a cover circumferentially encircling said inner core and being made of a second material being a fibre reinforced material compatible with the materials of said fibre reinforced structural element, said elongated core element (12 XIII) having an end part, said end part having a conical or partially conical shape, such as a truncated conical shape, said end part defining a central end face exposing said inner core and a circumferential end face encircling said central end face and exposing said cover, ii) providing a bolt fixture (22 XIII) or bolt comprising an end recess having a conical or partially conical shape, such as a truncated conical shape, said end recess being congruent with said end part of said elongated core element (12 XIII), iii) producing a subassembly by receiving and centring said end part of said core element in relation to said end recess of said bolt fixture (22 XIII) or bolt, iv) fixating said bolt fixture (22 XIII) or bolt to said end part of said core element in a pultrusion process by pulling said subassembly through a pultruder, by circumferentially covering said subassembly with reinforcing fibres and resin and by heating and curing said resin for causing said resin to provide in conjunction with said reinforcing fibres a casing circumferentially encircling said subassembly, or alternatively fixating said subassembly by adhesion to said casing produced in a separate pultrusion process, and v) machining said subassembly circumferentially encircled within said casing of said reinforcing fibres and said cured resin for providing said assembly including said elongated core element (12 XIII) and said bolt fixture (22 XIII) or bolt.