the second connecting end 122 of the second bracket 12 and to the second connecting end 142 of the second sliding bracket 14 respectively the second connecting end 42 of the second linking rod set 4 located on the upper ends of the pillars 1 is provided with a cross bar 43 with a hole 44 at one end thereof there are two third linking rod sets 5 each third linking rod set 5 is also formed by a pair of rods 51 and is connected in an x shaped type the connecting area of the two rods 51 has a third connecting end 52 the third connecting end 52 of the upper third linking rod set 5 is formed with hooks 53 at respective ends as shown in fig5 the four ends of the third linking rod set 5 are connected to the four first connecting ends 32 of the first linking rod sets 3 respectively in operation as shown in fig6 the four posts 2 are expanded outwardly at four corners whereas the first sliding brackets 13 on the four pillars 1 will slide upward along the pillars 1 to the underneath of the first brackets 11 and the second brackets.