a core a group of mutually parallel first wire cables or chord members designated 32 a in fig9 are laid out in a suitable jig or fixture not shown so as to extend in a direction parallel to the intended longitudinal axis of the wire core and a group of mutually parallel second wire cables or chord members designated 32 b in fig9 are laid out in the same fixture crosswise over the first chord members a first zig zag wire truss 34 a fig4 is then arranged along a first one of the longitudinal wire chord members 32 a in a substantially upright position in a plane which is slightly inclined relative to the vertical plane of the first longitudinal chord member in a direction away from the next adjacent longitudinal chord member with the bottom vertices or apexes of the first wire truss 34 a located at respective intersections of the first longitudinal chord member with the cross chord members a second zig zag wire truss 34 b see fig5 is then placed next to the first wire truss is 34 a with the bottom vertices of the second truss being located at the same intersections between the underlying longitudinal and cross chord members 32 a and 32 b as the bottom vertices of the first truss and with all those elements.