Collapsible security grille, grille system, bar, and method

A collapsible security grille, comprising a multiplicity of successive bars that are configured to be stacked in a retracted state of the grille and are configured to be spaced apart with respect to each other in an extended state of the grille, and comprising a multiplicity of elongated connecting elements for interconnecting respective bars, wherein the bars of the multiplicity of successive bars extend parallel to each other and each of the bars of the multiplicity of successive bars extends horizontally and is provided with series of corresponding through holes or passages for receiving the elongated connecting elements, wherein at least a number of the bars comprises one or multiple parts formed from bent sheet metal, wherein each of the at least a number of the bars is formed as a hollow bar, wherein each of the at least a number of the bars is put together from at least a first part formed from a first blank of sheet metal and a second part formed from a second blank of sheet metal, wherein at least the first part is a bent profile, and/or wherein at least the second part is a bent profile and wherein the first part is at least partly enclosing the second part, and/or wherein the first part defines a hollow room for accommodating the second part at least partly in an assembled state of the bar, the first part comprising one or more multiple supports or retaining elements for supporting the second part and/or for facilitating the second part at least partly within the first part, the first part being configured to have a cross-section shaped as an inverted U, the one or more multiple supports or retaining elements being formed by inwardly extending supports at the ends of legs of the inverted U-shaped first part.