An extendable, load-bearing structure , wherein: an extent of the extendable, load-bearing structure is arranged to be changeable in both directions between a first position, in which the extendable, load-bearing structure has a smallest extent, and a second position, in which the extendable, load-bearing structure has a largest extent; the extendable, load-bearing structure comprising a main structure , a driving element , a lock mechanism , and a supporting structure ; the main structure including at least four driving arms , wherein each driving arm includes two driving-arm elements connected to each other by a joint ; and the driving element is placed in connection with the driving arms to drive the extendable, load-bearing structure between said positions, whereby the driving arms of the main structure are parallel in pairs, wherein the driving arms of each pair of driving arms is connected by a bail extending between the joints of the driving arms ; the supporting structure includes at least four supporting arms that each includes two supporting-arm elements connected to each other by a joint ; the supporting arms of the supporting structure are parallel in pairs, wherein the supporting arms of each pair of supporting arms is connected by a bail extending between the joints of the supporting arms ; the supporting arms of the support structure are shorter than the driving arms of the main structure ; the supporting-arm elements of the supporting arms substantially are in line when the extendable, load-bearing structure is in its second position; the lock mechanism is arranged to releasably lock the supporting-arm elements of the supporting arm in position when these are in line, such that the extendable, load-bearing structure in its second position forms a truss; and the lock mechanism comprises the bails that connects the supporting arms and the bails that connects the driving arms . an extent of the extendable, load-bearing structure is arranged to be changeable in both directions between a first position, in which the extendable, load-bearing structure has a smallest extent, and a second position, in which the extendable, load-bearing structure has a largest extent; the extendable, load-bearing structure comprising a main structure , a driving element , a lock mechanism , and a supporting structure ; the main structure including at least four driving arms , wherein each driving arm includes two driving-arm elements connected to each other by a joint ; and the driving element is placed in connection with the driving arms to drive the extendable, load-bearing structure between said positions, whereby the driving arms of the main structure are parallel in pairs, wherein the driving arms of each pair of driving arms is connected by a bail extending between the joints of the driving arms ; the supporting structure includes at least four supporting arms that each includes two supporting-arm elements connected to each other by a joint ; the supporting arms of the supporting structure are parallel in pairs, wherein the supporting arms of each pair of supporting arms is connected by a bail extending between the joints of the supporting arms ; the supporting arms of the support structure are shorter than the driving arms of the main structure ; the supporting-arm elements of the supporting arms substantially are in line when the extendable, load-bearing structure is in its second position; the lock mechanism is arranged to releasably lock the supporting-arm elements of the supporting arm in position when these are in line, such that the extendable, load-bearing structure in its second position forms a truss; and the lock mechanism comprises the bails that connects the supporting arms and the bails that connects the driving arms . <EOS>