to but not within the other slot the actuator cap is pressed downwardly to exert downward force on the lower hub member causing the lower hub member to separate from upper hub member and allowing the structure to assume its collapsed configuration when it is desired to convert the structure from its collapsed configuration back to its open or constructed configuration the various elements of the structure will be manipulated into a configuration whrerein the hub assembly is once again in its open or constructed configuration thereafter the actuator cap is turned in the clockwise direction causing one key to slide into locking side slot and the other key to slide into the slot thereby locking the upper and lower hub members in fixed vertical positions relative to one another and preventing the structure from inadvertently collapsing during use another alternative hub assembly is useable in embodiments where when the structure is in its fully opened or fully constructed state the internal angle a between the strut axis sa and the vertical axis va is greater than 90 u00b0 in this alternative hub assembly one or more downwardly extending legs are formed on actuator cap when the user presses downwardly on the actuator cap the legs extend downwardly into abutment with s flange on the lower hub member slots are formed in the legs and protruding keys are slidably received within the slots thereby guiding the up and down motion of the actuator cap although exemplary embodiments of the invention have been shown and described many changes modifications and substitutions may be made by those having ordinary skill in the art without necessarily departing from the spirit and scope of this invention specifically elements or attributes described in connection with one embodiment may.