Abstract: A foldable modular structure for a fast-erecting shelter comprising the assembly of profiled sections that form at least two opposing arches linked by at least two purlins, including one ridge purlin. A first end of the ridge purlin is linked by a hinge to a first ridge part of a first arch, said hinge comprising first pivoting means allowing said ridge purlin to pivot on a transverse axis, parallel to the plane of the arch, in order to allow the purlin to fold parallel to said arch, and second pivoting means allowing the ridge purlin to pivot on an axis perpendicular to the plane of the arch, and the second end of the ridge purlin comprises linking means for linking with a second ridge part of a second arch, said linking means being arranged to provide a dismountable but rigid link between the ridge purlin and said second linking part.