A collapsable structure including a folding strut one end of which is pivoted to a rigid element to enable said strut to be rotated from a retracted position to an extended position, said strut having an abutment surface spaced away from the pivot and extending in a plane substantially parallel to the axis thereof, said surface being shaped to engage a co-operating engagement surface carried on said rigid element to locate said strut in its extended position, and releasable means for causing said surfaces to be held in contact with each other.