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 (6a, 6b, 6c), 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 (6a, 6b, 6c) 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 (28a, 28b) 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 (38a, 38b) 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 (38a, 38b) that connects the supporting arms and the bails (28a, 28b) 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 (6a, 6b, 6c), 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 (6a, 6b, 6c) 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 (28a, 28b) 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 (38a, 38b) 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 (38a, 38b) that connects the supporting arms and the bails (28a, 28b) that connects the driving arms.