Abstract: The bolt assembly comprises a traction shaft 1 with an anchoring extremity 13, an expansion ring 2 formed by means of expansible leg elements 18,19, and a brace 3 disposed around the shaft 1 and supported against a shoulder 24 provided upon the expansion elements 18, 19 in order to provoke expansion of leg portions 22, 23 with an undercut portion of an anchoring hole. The expansion elements 18, 19 extend between the shaft 1 and the brace 3 so as to give the latter a bearing surface of the same length as the legs 22, 23. The shoulder zone 24 defines an articulation region for the legs 22, 23.The bolt assembly is used for the fixing of pieces within a zone subject to tensile forces within a building material.